code 42
12-13-2014, 08:40 PM
#1
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code 42
mechanic already changed the ignition module,2 ground wires are tight they were loose but that didnt clear the code,so im gonna get the opti changed out (new cap and rotor)will that be the problem???????
12-13-2014, 10:30 PM
#2
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Re: code 42
You have to clear the code. It will remain as a stored code for a significant number of drive cycles. Pull the PCM BAT fuse (end of dash panel on a 94) for 30 seconds.
Does the engine start and run OK?
Does the engine start and run OK?
12-13-2014, 10:35 PM
#3
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Re: code 42
The distributor cap and rotor have nothing to do with this failure. This code appears when the pcm sees too much load on the ICM input from the PCM. Normal input resistance of the ICM is about 10 kohms. When the pcm sees much less resistance to ground than that, it sets code 42 again. If the code is erased but comes back, check the ICM connector for damage or dirt or grease that could short out the signal.
12-15-2014, 06:18 PM
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Re: code 42
The distributor cap and rotor have nothing to do with this failure. This code appears when the pcm sees too much load on the ICM input from the PCM. Normal input resistance of the ICM is about 10 kohms. When the pcm sees much less resistance to ground than that, it sets code 42 again. If the code is erased but comes back, check the ICM connector for damage or dirt or grease that could short out the signal.
12-16-2014, 05:15 PM
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Re: code 42
I decided to dig into this DTC42 issue deeper. I set up my 94 LT1 PCM with switches for the Low-res and High-res signals so I could control them (on/off). When I rapidly switched on and off the high res pulse line, DTC42 set every time. I used a scan tool to watch for the codes continuously, since DTC42 does not light the MIL/SES. When this happened, DTC36 did not set. And DTC36 would not set unless I switched the high-res signal off for more than about 2 seconds.
So it appears there are at least 2 other causes of the DTC42: high load on the output and/or an intermittent high-res signal from the distributor.
Doing the same with the lo-res signal did not produce any results unless I switched the signal off for more than 1 second, then DTC16 set. DTC42 never set while interrupting the lo-res signal.
So it appears there are at least 2 other causes of the DTC42: high load on the output and/or an intermittent high-res signal from the distributor.
Doing the same with the lo-res signal did not produce any results unless I switched the signal off for more than 1 second, then DTC16 set. DTC42 never set while interrupting the lo-res signal.
12-16-2014, 07:42 PM
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Re: code 42
I decided to dig into this DTC42 issue deeper. I set up my 94 LT1 PCM with switches for the Low-res and High-res signals so I could control them (on/off). When I rapidly switched on and off the high res pulse line, DTC42 set every time. I used a scan tool to watch for the codes continuously, since DTC42 does not light the MIL/SES. When this happened, DTC36 did not set. And DTC36 would not set unless I switched the high-res signal off for more than about 2 seconds.
So it appears there are at least 2 other causes of the DTC42: high load on the output and/or an intermittent high-res signal from the distributor.
Doing the same with the lo-res signal did not produce any results unless I switched the signal off for more than 1 second, then DTC16 set. DTC42 never set while interrupting the lo-res signal.
So it appears there are at least 2 other causes of the DTC42: high load on the output and/or an intermittent high-res signal from the distributor.
Doing the same with the lo-res signal did not produce any results unless I switched the signal off for more than 1 second, then DTC16 set. DTC42 never set while interrupting the lo-res signal.
thank you so much!!!!!!!!!
12-16-2014, 08:07 PM
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Re: code 42
Well, the bad news is: If that is the case here, you would need to check the high res signal wires from the distributor to the pcm. If that yields no solution, the next step would likely be replacement of the whole distributor.
12-17-2014, 03:19 PM
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Re: code 42
The difficult part to understand is how this contradicts what the manual indicates.
It says the engine can run without the high res pulse, but loss of high res pulse is setting DTC 42 and shutting the engine down. The PCM should be able to run the engine strictly with the low res pulse.
And are we looking at two different sources for code descriptions? The 94 manual indicates DTC 42 sets when there are 84 crankshaft revs w/o IC activity. At 1,200 RPM, that would require 4.2 seconds. It doesn't indicate that resistance is a factor, but apparently it interprets very low resistance as "no IC activity".
DTC 36 sets when there are 40 low res pulses ( = 10 crank revs) with no high res pulse. At idle (600 RPM) that's 1.0 seconds. If you hold the circuit open for more than 1 second, the code should set.
DTC 16 is what is supposed to shut down the fuel system, and that sets when there are 720 high res pulses with no low res pulse. That would be two cam rotations = 4 crank rotations, an even shorter period. But how can DTC set when there is no high res signal?
What used to seem very straight forward is now very confusing.
It says the engine can run without the high res pulse, but loss of high res pulse is setting DTC 42 and shutting the engine down. The PCM should be able to run the engine strictly with the low res pulse.
And are we looking at two different sources for code descriptions? The 94 manual indicates DTC 42 sets when there are 84 crankshaft revs w/o IC activity. At 1,200 RPM, that would require 4.2 seconds. It doesn't indicate that resistance is a factor, but apparently it interprets very low resistance as "no IC activity".
DTC 36 sets when there are 40 low res pulses ( = 10 crank revs) with no high res pulse. At idle (600 RPM) that's 1.0 seconds. If you hold the circuit open for more than 1 second, the code should set.
DTC 16 is what is supposed to shut down the fuel system, and that sets when there are 720 high res pulses with no low res pulse. That would be two cam rotations = 4 crank rotations, an even shorter period. But how can DTC set when there is no high res signal?
What used to seem very straight forward is now very confusing.
12-17-2014, 09:01 PM
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Re: code 42
We have lately seen several occurrences of people having the DTC42 with no real idea what to do about it. Replacing the ICM didn't always cure that. Wires weren't usually the problem. So there had to be something else going on. With only the DTC42, the engine sometimes shut down but other times did not. So my question was: What do those have in common? We know about the shorted circuit/ICM issue causing it, but there had to be something else going on.
It isn't a loss of the high res pulse that is one cause of the DTC42. It is an intermittent condition. Just a regular cessation of the signal does not cause a DTC42, but rather the DTC36 as expected. I see no setting of DTC16 or DTC36 while causing only an intermittent state of either signal. On a related subject, I also see no cessation of the injector signals after DTC42 sets. They keep on going as long as the low res pulse is still active. So I have problems with the timing and the effects noted in the FSM description of DTC42.
I have no issues with the descriptions for DTC16 and DTC36 although I have not verified the timing of either.
It appears to me that what is happening is that the Opti outputs become intermittent shortly before failure. That sets the DTC42. Then both signals stop completely. Without the other signal for comparison, neither DTC16 nor DTC36 sets. Then we are left with a stalled engine showing only the DTC42.
The attached image shows a sample test with several interruptions of the high res signal for under 2 seconds total time. It set DTC42 only and the injectors kept running.
As for the resistance effects, they seem to have two failure modes. When a resistance is connected from the ICM input to ground, a resistance value of 17k Ohms or less sets DTC42. When the resistance is placed in series with the input, a resistance value of 58k Ohms or more sets DTC41. In both cases, the peak voltage of the signal is reduced to a lower value than normal, although there is no correlation of the two DTC's (different voltage levels). I suspect some tricky sampling is being done to differentiate the two.
Regarding the timing of the setting of DTC42, another quick test grounding the input to the ICM caused DTC42 to set in about 2 seconds. "Engine" RPM was about 900.
It isn't a loss of the high res pulse that is one cause of the DTC42. It is an intermittent condition. Just a regular cessation of the signal does not cause a DTC42, but rather the DTC36 as expected. I see no setting of DTC16 or DTC36 while causing only an intermittent state of either signal. On a related subject, I also see no cessation of the injector signals after DTC42 sets. They keep on going as long as the low res pulse is still active. So I have problems with the timing and the effects noted in the FSM description of DTC42.
I have no issues with the descriptions for DTC16 and DTC36 although I have not verified the timing of either.
It appears to me that what is happening is that the Opti outputs become intermittent shortly before failure. That sets the DTC42. Then both signals stop completely. Without the other signal for comparison, neither DTC16 nor DTC36 sets. Then we are left with a stalled engine showing only the DTC42.
The attached image shows a sample test with several interruptions of the high res signal for under 2 seconds total time. It set DTC42 only and the injectors kept running.
As for the resistance effects, they seem to have two failure modes. When a resistance is connected from the ICM input to ground, a resistance value of 17k Ohms or less sets DTC42. When the resistance is placed in series with the input, a resistance value of 58k Ohms or more sets DTC41. In both cases, the peak voltage of the signal is reduced to a lower value than normal, although there is no correlation of the two DTC's (different voltage levels). I suspect some tricky sampling is being done to differentiate the two.
Regarding the timing of the setting of DTC42, another quick test grounding the input to the ICM caused DTC42 to set in about 2 seconds. "Engine" RPM was about 900.
Last edited by GaryDoug; 12-17-2014 at 09:14 PM.
12-17-2014, 10:06 PM
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Re: code 42
There have been many threads over the years that I have seen the two opti codes (either 16 and/or 36) combined with a 42 when scanned. The 42 only seemed to be caused by the accompanying opti fault and not a fault related to the ICM circuitry, since the usual fix was to replace the opti and the 42 never came back. This seems to relatively follow to what you are saying, Gary (if I am following you correctly ).
).
12-17-2014, 10:34 PM
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Re: code 42
I suspect it just depends upon the timing and sequence of events that determines which of the 3 DTC's are actually set. An example would be if the two Opti outputs fail at slightly different times. Then the first one to go would get the code.
It does happen that the ICM circuit can be at fault by simply opening or being shorted to ground as in an internal short of the ICM. Those two modes are the first two for which I tested, but that didn't explain all the failures we are seeing lately.
Another thing I don't get in the manual is where it states that the DTC42 will only set while cranking. I have repeatedly seen that code set under normal operation and have not been able to duplicate it while cranking.
It does happen that the ICM circuit can be at fault by simply opening or being shorted to ground as in an internal short of the ICM. Those two modes are the first two for which I tested, but that didn't explain all the failures we are seeing lately.
Another thing I don't get in the manual is where it states that the DTC42 will only set while cranking. I have repeatedly seen that code set under normal operation and have not been able to duplicate it while cranking.
12-20-2014, 08:54 PM
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Re: code 42
I decided to dig into this DTC42 issue deeper. I set up my 94 LT1 PCM with switches for the Low-res and High-res signals so I could control them (on/off). When I rapidly switched on and off the high res pulse line, DTC42 set every time. I used a scan tool to watch for the codes continuously, since DTC42 does not light the MIL/SES. When this happened, DTC36 did not set. And DTC36 would not set unless I switched the high-res signal off for more than about 2 seconds.
So it appears there are at least 2 other causes of the DTC42: high load on the output and/or an intermittent high-res signal from the distributor.
Doing the same with the lo-res signal did not produce any results unless I switched the signal off for more than 1 second, then DTC16 set. DTC42 never set while interrupting the lo-res signal.
So it appears there are at least 2 other causes of the DTC42: high load on the output and/or an intermittent high-res signal from the distributor.
Doing the same with the lo-res signal did not produce any results unless I switched the signal off for more than 1 second, then DTC16 set. DTC42 never set while interrupting the lo-res signal.
The difficult part to understand is how this contradicts what the manual indicates.
It says the engine can run without the high res pulse, but loss of high res pulse is setting DTC 42 and shutting the engine down. The PCM should be able to run the engine strictly with the low res pulse.
And are we looking at two different sources for code descriptions? The 94 manual indicates DTC 42 sets when there are 84 crankshaft revs w/o IC activity. At 1,200 RPM, that would require 4.2 seconds. It doesn't indicate that resistance is a factor, but apparently it interprets very low resistance as "no IC activity".
DTC 36 sets when there are 40 low res pulses ( = 10 crank revs) with no high res pulse. At idle (600 RPM) that's 1.0 seconds. If you hold the circuit open for more than 1 second, the code should set.
DTC 16 is what is supposed to shut down the fuel system, and that sets when there are 720 high res pulses with no low res pulse. That would be two cam rotations = 4 crank rotations, an even shorter period. But how can DTC set when there is no high res signal?
What used to seem very straight forward is now very confusing.
It says the engine can run without the high res pulse, but loss of high res pulse is setting DTC 42 and shutting the engine down. The PCM should be able to run the engine strictly with the low res pulse.
And are we looking at two different sources for code descriptions? The 94 manual indicates DTC 42 sets when there are 84 crankshaft revs w/o IC activity. At 1,200 RPM, that would require 4.2 seconds. It doesn't indicate that resistance is a factor, but apparently it interprets very low resistance as "no IC activity".
DTC 36 sets when there are 40 low res pulses ( = 10 crank revs) with no high res pulse. At idle (600 RPM) that's 1.0 seconds. If you hold the circuit open for more than 1 second, the code should set.
DTC 16 is what is supposed to shut down the fuel system, and that sets when there are 720 high res pulses with no low res pulse. That would be two cam rotations = 4 crank rotations, an even shorter period. But how can DTC set when there is no high res signal?
What used to seem very straight forward is now very confusing.
We have lately seen several occurrences of people having the DTC42 with no real idea what to do about it. Replacing the ICM didn't always cure that. Wires weren't usually the problem. So there had to be something else going on. With only the DTC42, the engine sometimes shut down but other times did not. So my question was: What do those have in common? We know about the shorted circuit/ICM issue causing it, but there had to be something else going on.
It isn't a loss of the high res pulse that is one cause of the DTC42. It is an intermittent condition. Just a regular cessation of the signal does not cause a DTC42, but rather the DTC36 as expected. I see no setting of DTC16 or DTC36 while causing only an intermittent state of either signal. On a related subject, I also see no cessation of the injector signals after DTC42 sets. They keep on going as long as the low res pulse is still active. So I have problems with the timing and the effects noted in the FSM description of DTC42.
I have no issues with the descriptions for DTC16 and DTC36 although I have not verified the timing of either.
It appears to me that what is happening is that the Opti outputs become intermittent shortly before failure. That sets the DTC42. Then both signals stop completely. Without the other signal for comparison, neither DTC16 nor DTC36 sets. Then we are left with a stalled engine showing only the DTC42.
The attached image shows a sample test with several interruptions of the high res signal for under 2 seconds total time. It set DTC42 only and the injectors kept running.
As for the resistance effects, they seem to have two failure modes. When a resistance is connected from the ICM input to ground, a resistance value of 17k Ohms or less sets DTC42. When the resistance is placed in series with the input, a resistance value of 58k Ohms or more sets DTC41. In both cases, the peak voltage of the signal is reduced to a lower value than normal, although there is no correlation of the two DTC's (different voltage levels). I suspect some tricky sampling is being done to differentiate the two.
Regarding the timing of the setting of DTC42, another quick test grounding the input to the ICM caused DTC42 to set in about 2 seconds. "Engine" RPM was about 900.
It isn't a loss of the high res pulse that is one cause of the DTC42. It is an intermittent condition. Just a regular cessation of the signal does not cause a DTC42, but rather the DTC36 as expected. I see no setting of DTC16 or DTC36 while causing only an intermittent state of either signal. On a related subject, I also see no cessation of the injector signals after DTC42 sets. They keep on going as long as the low res pulse is still active. So I have problems with the timing and the effects noted in the FSM description of DTC42.
I have no issues with the descriptions for DTC16 and DTC36 although I have not verified the timing of either.
It appears to me that what is happening is that the Opti outputs become intermittent shortly before failure. That sets the DTC42. Then both signals stop completely. Without the other signal for comparison, neither DTC16 nor DTC36 sets. Then we are left with a stalled engine showing only the DTC42.
The attached image shows a sample test with several interruptions of the high res signal for under 2 seconds total time. It set DTC42 only and the injectors kept running.
As for the resistance effects, they seem to have two failure modes. When a resistance is connected from the ICM input to ground, a resistance value of 17k Ohms or less sets DTC42. When the resistance is placed in series with the input, a resistance value of 58k Ohms or more sets DTC41. In both cases, the peak voltage of the signal is reduced to a lower value than normal, although there is no correlation of the two DTC's (different voltage levels). I suspect some tricky sampling is being done to differentiate the two.
Regarding the timing of the setting of DTC42, another quick test grounding the input to the ICM caused DTC42 to set in about 2 seconds. "Engine" RPM was about 900.
There have been many threads over the years that I have seen the two opti codes (either 16 and/or 36) combined with a 42 when scanned. The 42 only seemed to be caused by the accompanying opti fault and not a fault related to the ICM circuitry, since the usual fix was to replace the opti and the 42 never came back. This seems to relatively follow to what you are saying, Gary (if I am following you correctly ).
).I suspect it just depends upon the timing and sequence of events that determines which of the 3 DTC's are actually set. An example would be if the two Opti outputs fail at slightly different times. Then the first one to go would get the code.
It does happen that the ICM circuit can be at fault by simply opening or being shorted to ground as in an internal short of the ICM. Those two modes are the first two for which I tested, but that didn't explain all the failures we are seeing lately.
Another thing I don't get in the manual is where it states that the DTC42 will only set while cranking. I have repeatedly seen that code set under normal operation and have not been able to duplicate it while cranking.
It does happen that the ICM circuit can be at fault by simply opening or being shorted to ground as in an internal short of the ICM. Those two modes are the first two for which I tested, but that didn't explain all the failures we are seeing lately.
Another thing I don't get in the manual is where it states that the DTC42 will only set while cranking. I have repeatedly seen that code set under normal operation and have not been able to duplicate it while cranking.
12-21-2014, 10:11 PM
#13
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Re: code 42
DAMN now we have another issue now the car is getting hot its going to the 3/4 mark,and before this code 42 i never seen the car get past 180-190 it always stayed in that range. My mechanic told me he bled all the air out,i bought a new t stat using a 180 one,water pump is new too,fans are coming on they are programmed to come @ 180 to 190 thats why my temp never got past that range.brothers i dont know what else to do,and he cant figure out why my cars getting hot just had an idea could they have forgotten to put the water pump driveshaft in when they did my distributor cap?
just had an idea could they have forgotten to put the water pump driveshaft in when they did my distributor cap?
Last edited by 94Z28LS1toLT1; 12-21-2014 at 10:14 PM.
12-22-2014, 07:32 AM
#14
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Re: code 42
Did you get the t'stat that is specifically for the LT1?
http://shbox.com/1/tstat.jpg
Seems odd that he said "it was 95% bled out got alot of air pockets in there.".... this is actually a very simple procedure, and shouldn't take him two days.
4th Gen LT1 F-body Tech Articles
http://shbox.com/1/tstat.jpg
Seems odd that he said "it was 95% bled out got alot of air pockets in there.".... this is actually a very simple procedure, and shouldn't take him two days.
4th Gen LT1 F-body Tech Articles
12-22-2014, 06:05 PM
#15
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Re: code 42
Did you get the t'stat that is specifically for the LT1?
http://shbox.com/1/tstat.jpg
Seems odd that he said "it was 95% bled out got alot of air pockets in there.".... this is actually a very simple procedure, and shouldn't take him two days.
4th Gen LT1 F-body Tech Articles
http://shbox.com/1/tstat.jpg
Seems odd that he said "it was 95% bled out got alot of air pockets in there.".... this is actually a very simple procedure, and shouldn't take him two days.
4th Gen LT1 F-body Tech Articles
