P2138 - Replaced Acc. Pedal and Both O2 Sensors. Limp Mode

[p0isin]

I am hoping for some help diagnosing an issue with my A4. About a week ago while cruising on the highway the car went into Limp mode and the CEL, TCS, and EPC lights came on.

VCDS Scan shows 1 Fault Code:
008504 - Accelerator Position Sensors (G79) / (G185)

P2138 - 008 - Implausible Correlation

So far I have replaced the accelerator pedal and both O2 sensors but the fault code comes back immediately when cleared. All visible wiring & connectors look fine but I haven't traced them all back to their sources yet.
VCDS Block 62 shows that Pedal Position Sensor 1 (G79) reaches 76.6% max and Pedal Position Sensor 2 (G185) reaches 45.7% max.

VCDS Block 62 Accelerator Pedal No Throttle


VCDS Block 62 Accelerator Pedal Full Throttle


VCDS Block 64 Voltage Measurements are all within specification
Lower Adaptation Sensor 1 (G187)

Voltage = 0.520 V

Specification: 0.4 - 0.6 V

Lower Adaptation Sensor 2 (G188)

Voltage = 4.580 V

Specification: 4.0 - 5.0 V

Emergency Air Gap Sensor 1 (G187)

Voltage = 0.840 V

Specification: 0.5 - 1.0 V

Emergency Air Gap Sensor 2 (G188)

Voltage = 4.280 V

Specification: 4.0 - 5.0 V

I'm not sure what else could be causing the issue. Any ideas?

[p0isin]

Does anybody know what block 62 readings should be at both no throttle & full throttle?

[allstock]

The throttle sensor position readings should together add up to 100. Which your throttle pos does. I don't know off hand if the acc pedal should do the the same, but I'm guessing it should as well
What happens/ed after a throttle adaptation?

If no one beats me to it I'll go check the block on my vagcom.

[allstock]

Doesn't look too far off from what I saw. Full throttle, your acc pedal pos sensor seems to be higher on yours. Of course I am stock and don't know if that is because of your tune.

No throttle
Throttle drive angle sensor 1 16.8% Throttle drive angle sensor 2 82.8% Throttle position sensor G79 14.8% Accelerator pedal position sensor 2 7.4%

Full Throttle
Throttle drive angle sensor1 87.9% Throttle drive angle sensor 2 12.1% Throttle position sensor G79 77.3% Accelerator pedal position sensor 2 38.7%

[p0isin]

Thanks for the information. Last night I removed the ECU, checked all O2 sensor wiring to its source, and removed the dashboard to inspect the main wiring harness. After removing the dash I found a loose ground connection. I just tested Block 62 in VCDS and the new measurements are:

No Throttle


Full Throttle


I believe that Accelerator Pedal Position Sensor 2 (G185) should read half what Accelerator Pedal Position Sensor 1 (G79) reads. The values are much closer together since I fixed the ground connection issue and VCDS reports no fault code. I'll test it out soon, just have to finish putting the dashboard back in. Thanks again.

[Okedokey]

Where abouts did you fine the lose ground. I think I need to check some of these things.

[p0isin]

Your situation is probably different than mine. I repaired the main harness a few years ago due to a different electrical issue and found that a wire I had "repaired" came loose. While I had fixed most of the harness the correct way, I got lazy towards the end and used solder on a few ground connections.

3 years later and I find a bunch of corrosion and one of the ground wires detached. All the wires that weren't soldered still look fine. Definitely learned my lesson and I'm happy I didn't use solder on any of the other wires.

The red arrow in the picture below shows the portion of the harness where my issue was. Unfortunately I didn't take any pictures once everything was unwrapped but there are a bunch of grounds which connect to a larger ground and that was where my issue was.



This picture shows the location of some ground connections, maybe one of those is loose. Also check your engine bay, there are a few ground connections which could be corroded. I found mine to be in pretty rough shape a few months ago.

[vtraudt]

I believe that Accelerator Pedal Position Sensor 2 (G185) should read half what Accelerator Pedal Position Sensor 1 (G79) reads. The values are much closer together since I fixed the ground connection issue and VCDS reports no fault code. I'll test it out soon, just have to finish putting the dashboard back in. Thanks again.

Getting the code.
Sofar done: TB alignment, unplugged upper and lower O2 sensors.
What is G185 in 062/4 supposed to read compared to 062/3? Is 'half of G79' correct?

If the output of the all 4 sensors (G187, G188, G79, G185) looks right, I am not planning to change them; too many reported 'no effect'.

Is unplugging the O2 sensors a viable 'test' to rule out the O2 sensors (as reported) might mess with the sensor values and throwing the code?

What other troubleshooting can be done to locate the source problem?

[vtraudt]

Does anybody have the pin outs for the 2 sensors, and the ECU pin out?

Or wiring diagram B7 ECU for the 2 sensors and the ECU?