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Contributed by Laurence Chapman

I plan to use my GTM as a daily driver so I have been trying to retain as many “luxury” features as possible. One of the features I wanted to retain was the Malfunction Indicator Light (MIL), or check engine light as it is commonly called.

This may also be an issue for builders in states that require a smog inspection, if that inspection requires the technician to see the check engine light or query its status through the OBDII port.

I wanted to know if an O2 sensor stopped working, or if the MAF sensor voltage was out of range etc. etc. I needed to eliminate “expected” codes, so that if a real problem developed it would show through.

So, I have documented what it took to get the MIL light to operate, and then what I did to eliminate some of the OBDII codes that I got. This is a royal pain, but since I was waiting on parts, I did it anyway. If other builders find the solution to other codes, maybe they will be willing to post them.

On page 338 of the build manual, FFR has you connect the OBDII serial bus to the dark green wire of the OBDII connector. I did find that for my OBDII reader to work, I had to also connect the purple wire of the OBDII connector to the same dark green wire. I guess it depends how your OBDII reader works.

I use an OBDII reader from OBD-2 . What they sell is a connector for Ford, GM or foreign cars (you choose which you want, or all three), which links to your laptop. You can then download their latest Windows program and use it to view OBDII codes, including codes from auxiliary computers such as the ABS computer on the corvette. It is a one person business and the documentation is almost non-existent, but for the price I am very pleased with what it can do.

The MIL light is operated by fusebox connector C2 providing a ground signal on pin C5, which is a brown and white wire. So all you need to do is take an ignition voltage source, through your 12V indicator bulb, and let the PCM ground it through C2, pin C5.

The light is supposed to come on when your turn on the ignition (as a bulb check), and then turn off when the vehicle runs. If it stays on, you have an OBDII code which has commanded the light on.

My light stayed on at first, so I went on a hunt to eliminate codes, until the light stayed off. Most of the codes I eliminated are not supposed to turn the light on, but for some reason mine stayed on until I had eliminated them.

The codes you can expect to find will depend on your donor year, and if it is automatic or manual. This is what I found.

P0412 AIR solenoid error
P0441 EVAP purge flow error
P0449 Evap canister vent valve
P0463 Fuel level sensor error
P0522 Engine oil pressure sensor
P0801 Reverse inhibit solenoid error
P0803 1-4 shift solenoid error
P0804 Skip shift lamp error
P1433 stuck fuel level sensor

Only the first 3 were supposed to turn the MIL on.

P0412 is because the AIR solenoid wire at Fusebox C3, pin D3 is expecting to find a 12v signal that has run through the AIR solenoid. Since I was not connecting the AIR system, I had to fool the PCM by running 12V through a resistor (5K ohms), into this pin. Once done, the code went away.

P0441 and P0449 went away when I connected up the EVAP canister.

P0522 went away when I used two resistors to take the 5V signal voltage on the grey wire at the EOP sensor connector, and dropped it to 2.5 volts and fed it back through the Tan/White sensor wire at the same connector.

P0801 and P0803 require 12v through a resistor to connector C142 (the round connector that has the two VSS wires (yellow and purple)). Pin A is grey wire and is the 1-4 shift solenoid control, and Pin T is light green and is the Reverse inhibit solenoid control signal.

P0804 is a bulb error and also needs 12v through a resistor to fusebox C2, pin F10.

P0463 and P1433 are because the PCM is no longer getting fuel tank sensor input. They do not turn on the MIL light so I am just going to ignore them.

I wish I could offer to help other builders eliminate their various codes, but that would probably be a full time job, and I am still working hard on my build. However, maybe other builders would be willing to post the codes they have found and eliminated.

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