Computer Diagnostics and Tuning Technical discussion on diagnostics and programming of the F-body computers

LT1 will not enter field service mode

Old 03-04-2019, 10:04 AM
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LT1 will not enter field service mode

For some reason my 97 m6 lt1 will not enter field service mode. I placed a paperclip jumper between pins 5 and 6 of the OBD2 connector and turned the key to ON position. Fans do not come on. Only lights on dash are the check engine, check gauges, brake light, and seat belt.

Checked out the following pins on the DLC with volt-meter:
4 to 5: 0V
6 to 5: 4.89V
16 to 5: 11.9V

Also checked the egr purge solenoid and egr vacuum solenoid and both are disabled. Anyone know what could cause this or what else I can check?
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Old 03-05-2019, 09:37 AM
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Re: LT1 will not enter field service mode

What are you trying to accomplish by entering field service enable?
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Old 03-05-2019, 10:20 AM
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Re: LT1 will not enter field service mode

My main problem is I have a P0400 code, indicating the EGR is not functioning properly. I've replaced the EGR valve and the vacuum solenoid. I also pushed the EGR valve diaphram with my fingers while the engine was running and the RPM dropped significantly. I checked vacuum entering the solenoid and its 20" Hg, even at 2k rpm. I also checked the vacuum lines and they don't appear to be leaking.

I was hoping to use field service mode to engage the vacuum solenoid thru the PCM. If not then is there another way to engage it? Been battling this code for months now.
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Old 03-05-2019, 02:10 PM
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Re: LT1 will not enter field service mode

If all you are trying to do is open the vacuum solenoid, to verify vacuum is reaching the EGR valve - unplug the solenoid, to avoid chance of backfeeding the PCM (probably overly cautious), apply 12 volts to pin A of the solenoid and a ground to pin B.

The P0400 diagnostic opens the EGR valve, and looks for a predetermined amount of change in the MAP reading. Have you made any changes to the exhaust system recently, specifically something that would reduce back pressure?
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Old 03-05-2019, 09:01 PM
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Re: LT1 will not enter field service mode

Originally Posted by Injuneer View Post
If all you are trying to do is open the vacuum solenoid, to verify vacuum is reaching the EGR valve - unplug the solenoid, to avoid chance of backfeeding the PCM (probably overly cautious), apply 12 volts to pin A of the solenoid and a ground to pin B.
It's not overly cautious at all. I made that mistake by back-driving the 12v fuel pump relay output from the PCM. Fried it. I had to disassemble the PCM and find a replacement for the driver that was damaged, Fortunately it wasn't terribly hard or expensive (for an EE;-).
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Old 03-05-2019, 09:38 PM
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Re: LT1 will not enter field service mode

Originally Posted by Injuneer View Post
If all you are trying to do is open the vacuum solenoid, to verify vacuum is reaching the EGR valve - unplug the solenoid, to avoid chance of backfeeding the PCM (probably overly cautious), apply 12 volts to pin A of the solenoid and a ground to pin B.

The P0400 diagnostic opens the EGR valve, and looks for a predetermined amount of change in the MAP reading. Have you made any changes to the exhaust system recently, specifically something that would reduce back pressure?
Have not made any changes to the exhaust system. Running an aftermarket exhaust for several years now.

You gave me an idea Injuneer. Hooked up the mityvac to the vacuum solenoid input and applied 12V. Was able to control the egr valve with the mityvac.
I think the problem is either in the harness or the PCM. Figured I could use field service mode to activate the solenoid thru the PCM. For some reason it won't enable.

BTW GaryDoug, do you think the solenoid vacuum driver on the PCM is blown? Is there a way to check that?
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Old 03-06-2019, 08:28 AM
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Re: LT1 will not enter field service mode

Originally Posted by Injuneer View Post
If all you are trying to do is open the vacuum solenoid, to verify vacuum is reaching the EGR valve - unplug the solenoid, to avoid chance of backfeeding the PCM (probably overly cautious), apply 12 volts to pin A of the solenoid and a ground to pin B.

The P0400 diagnostic opens the EGR valve, and looks for a predetermined amount of change in the MAP reading. Have you made any changes to the exhaust system recently, specifically something that would reduce back pressure?
Installed an aftermarket catback exhaust several years ago. Everything else is stock. Is there a way to check for backpressure?

Hooked up the mityvac to the input of the vacuum solenoid and applied 12V to the pins. Was able to control the EGR valve with the Mityvac. I'm thinking there is an issue with the PCM turning on the vacuum solenoid. Was hoping to use field service mode to enable the solenoid but for some reason that won't work. Is there another way to force the pcm to drive the solenoid?

BTW GaryDoug, is it possible that the vacuum solenoid driver is blown on the PCM? Is there a way to check that?
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Old 03-06-2019, 11:42 AM
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Re: LT1 will not enter field service mode

Am running an aftermarket catback exhaust, but everything else is stock. I've had it for several years so surprised it would cause issues now. Is there a way to check the back pressure?

Tried another test with the mityvac hooked up to the egr vacuum solenoid inlet. Disconnected the harness and applied 12V to the solenoid pins. Was able to control the EGR valve with the mityvac as expect. This leads me to think there's an electrical problem with the harness or the PCM signal. Was hoping to use field service mode to get the PCM to engage the solenoid but for some reason it wont work on my car. Do you guys know a way to get the PCM to engage the vacuum solenoid?
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Old 03-06-2019, 03:08 PM
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Re: LT1 will not enter field service mode

For some reason the system is flagging your posts and putting them in the moderator approval queue. I only noticed just now there were two unapproved posts. I'll approve them, and maybe when they show up, you could edit each post and delete the duplicate info.

i've been working on these cars since I bought mine brand new, 25 years ago. I have never seen anyone use field service mode. And a quick flip thru the factory service manual didn’t turn up any info. I would assume you need a bi-directional scan tool like the GM Tech-2 (or the much less expensive Chinese clone that GaryDoug has used). GaryDoug has also written OBD-2 scan software, but I don’t think he ever got around to adding the bi-directional capability.
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Old 03-07-2019, 10:41 AM
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Re: LT1 will not enter field service mode

Was wondering why some of my previous posts weren't posting. Its ok Injuneer, you can just ignore them.

[EDIT by Moderator] - If anyone is wondering why there appear to be multiple versions of the same post, it is NOT the OP's fault. The system was flagging some of the posts and putting them into the moderation queue for approval. But the company that runs the site had moved the "approve" button, without telling us, and I couldn't find it.

Last edited by Injuneer; 03-07-2019 at 02:22 PM. Reason: Site problems were flagging posts for moderation, for aome reason
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Old 03-07-2019, 10:50 AM
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Re: LT1 will not enter field service mode

Saw the write up on field service mode from shbox's site and was hoping to use it on mine. I'm guessing field service mode was included in 95 cars but removed in 97. Bummer, guess i'll look into bidirectional scan tools.

In the mean time, I have a hunch that either the PCM is not sending the command to enable the vacuum solenoid, or there's an issue with wire harness. I'm thinking about splicing the wire harness and putting an LED w/ 1k resistor in parallel. I'll run the wires to the dash board and use the LED as a visual indicator. Hoping that under the right driving conditions, the PCM will activate the vacuum solenoid and the LED should light up. Do you guys think that could work?
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Old 03-07-2019, 02:16 PM
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Re: LT1 will not enter field service mode

I'm going to approve the post just to try it out. The company that owns this site makes "improvements" from time to time. They are really good about adding the latest features, etc. Problem is, they don't tell us when they make what they feel are "minor" changes. I thought I was having a "senior moment" looking for the post tool button, but they had to show me where it is now, because they agree it is difficult to find.
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Old 03-07-2019, 08:52 PM
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Re: LT1 will not enter field service mode

Originally Posted by starcheese View Post
Saw the write up on field service mode from shbox's site and was hoping to use it on mine. I'm guessing field service mode was included in 95 cars but removed in 97. Bummer, guess i'll look into bidirectional scan tools.

In the mean time, I have a hunch that either the PCM is not sending the command to enable the vacuum solenoid, or there's an issue with wire harness. I'm thinking about splicing the wire harness and putting an LED w/ 1k resistor in parallel. I'll run the wires to the dash board and use the LED as a visual indicator. Hoping that under the right driving conditions, the PCM will activate the vacuum solenoid and the LED should light up. Do you guys think that could work?
Should work. Just be aware that the LED has polarity. Make sure the negative end goes to the grey wire and the positive end goes to the brown wire.

I assume you mean to put the resistor in series with the LED and the combination in parallel with the solenoid.
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