I took my Z28 to an automotive repair today expecting to have some O2 sensors replaced. Instead of any repair I got a list of codes set in the PCM. Three weeks ago the SES light came on and I had Autozone check the codes. They were the: P0171(lean bank1), P0172(rich bank2), P0174(lean bank2) and P0175(rich bank 2). I called an auto repair shop and the manager told me that with both banks coding I most likely had a vacuum leak. He was correct, the PCV tubing had a hole and was sucking air. I replaced it and the SES light extinguished. I have also noticed that my voltage gauge drops into the red (low volt) range intermittently and is pronounced when I use the AC. I suspect my alternator is fading. The codes the repair shop gave me to today are:


P0122 tps lo
P0123 tps hi
P0171
P0172
P0174
P0175
P0463 fuel level sensor circuit hi input
P1121 tps intermittent hi
p1122 tps intermittent lo
P1415 sec air inject sys bank 1
P1626 theft deterrent sys fuel enable circuit
P1635 5Volt reference low (PCM ref)


I suspect the PCM is running in limp mode. Runs great on the highway. gas mileage is fair. Idles a little rough. If I have the AC on the voltage gauge drops low at idle. Intermittent at above idle RPMs. I have replaced the TPS before. I will recheck the connection. I have coded for that before. Being an electronic technician and knowing what low reference voltage and low supply voltages can cause in a CPU based system I am wondering if the low voltage indication I see on the gauge, while at idle, could cause the PCM to generate false codes. Like I have stated a week ago there were know codes and SES light was extinguished.


I would concur that the alternator is probably going bad or the control wiring to the alternator needs to be checked for corrosion and conformity but all these codes suddenly appearing blows me away.


2002 Z28 w/79K miles. Automatic Tranny.


Does anyone concur or have additional info or advise. I sure could use a second opinion. Thanks so much. Dave

 

I would clear the codes, and buy a scanner so you can read them yourself. Then see what codes if any, come back, and how soon. OBD-2 scanners are very inexpensive.

You aren't in "limp mode".... that condition only exists when the PCM has lost most of the sensor inputs, and it attempting to keep the engine running so you can get it somewhere safe. If would be running extremely poorly in limp mode.

DTC 1635 is the 5V reference for the TPS, MAP and EGR pintle position sensor, That would be of the most concermn, if the voltage problem was due to the internal PCM circuit voltage control.

 

Thanks Fred. It runs well. Just rough idle. I am going to check a few things tomorrow. I agree the 5V reference is critical. If I am clear about your reply the 5V reference is internal to the PCM which only makes since. If the battery voltage does intermittently drop to below 9V then is it possible the regulator circuit internal to the PCM drops out. Most regulator circuits require some headroom to maintain regulation. At least 3V. Internal sensor references could be affected too. Does this make since to you?

 

Easiest way to find out would be to have AutoZone bench-test the alternator.

If it's not that, I'd start checking the connection quality of the various grounds.

 

There is a code for low system voltage to the PCM (P0562) and it does not set until the voltage drops below 8.0V for 5 seconds or more. That would seem to imply that 9V is not a problem with regard to the PCM.

 

The internal 5v reference generator circuit actually boosts the voltage, if needed, from the applied ignition voltage, then regulates it down to 5.0v. The 5v generator circuit picks up and starts making the 5v at about 6 vdc ignition voltage and up. It drops out (turns off) at about 3 vdc or lower. So, for example, if the ignition voltage should drop to only 4vdc after the ignition was higher than 6vdc for at least a few seconds, the 5vdc ref output would still output 5.0vdc.

 

The 5v ref output is current-limited to about 200ma (1/5 amp) maximum. If the load current increases due to a short or low resistance load, the voltage will fall to a level that produces only 200 ma current. For a hard short to ground, that could be as low as nearly 0 vdc. The 5v ref circuit should recover without damage after the short or heavy current load is removed. I suspect you have a short somewhere. It should be a simple test to disconnect each sensor while monitoring the 5v ref.