Desperately in need of help with a P0178 code

[CONKLE73]

So I needed to chase down an oil leak on my 2003 Z71 Tahoe with the 5.3 Flex engine...

I used some foaming degreaser and a hose and was able to isolate the oil leak...


unfortunately I got a little too overzealous with the hose because the next day I get a knock sensor code...


apparently I got some water in the valley and flooded one of my knock sensors... no big deal... the Tahoe has 170k on the clock and I have been meaning to replace the intake manifold gasket anyway...


so I get the parts, watch a few dozen YouTube videos and go at it... everything came a part and went back together without too much difficulty...


I cleared the codes and drove around and the knock sensor code did NOT come back... I think success!


That was until the next day... my wife drove the Tahoe to work and texted me that the check engine light had come back on... when I got home I plugged in the scanner and I had a P0178 code (fuel composition sensor circuit low input)


after some research I removed and replaced the sensor, cleaned the connector plug and replaced the fuel filter... I cleared the codes and the P0178 code came right back...

I took the sensor back and replaced it a second time... same deal... after clearing the codes the P0178 code came right back.


I did a little more research in order to test the voltage to the plug... there are 3 wires A, B & C

The A wire is a signal wire that is supposed to carry 5v, the B wire is a power wire that is supposed to carry 12v and the C wire is ground.




When I tested the A wire it read 11.2v with the motor off and 13.2v running, the B wire read 13.3v with the motor off and 14.2 running...


I suspected that maybe the A& B wires somewhere between the sensor plug and the PCM so I pulled the PCM fuses and did a continuity check between the A &B wires and they tested as an open circuit so I don't believe that they are shorted to one another...


I'm kind of at a loss now and don't know what to check next.


Any input would be greatly appreciated

 

[sumo]

A is a signal reference voltage sent from the pcm. It should be 5v. If your reading battery voltage at a there's a problem. Sounds like it shorted somewhere with the b wire or somewhere else is feeding it that's probably the cause of the high voltage. Take a close look at the connector. Make sure you don't have any misaligned terminals or bent pins in the sensor.

What's weird is the code is for a low volt. Are you testing the right component?

 

[retiredsparky]

Have you seen any critters around? Mice like wire insulation for dessert.

 

[CONKLE73]

I don't think any critters got in the wiring... I tried to trace the loom back to the PCM but I lose it when it goes over the transmission...

Should I try to test the circuit at the PCM if that's possible?

 

[CONKLE73]

A is a signal reference voltage sent from the pcm. It should be 5v. If your reading battery voltage at a there's a problem. Sounds like it shorted somewhere with the b wire or somewhere else is feeding it that's probably the cause of the high voltage. Take a close look at the connector. Make sure you don't have any misaligned terminals or bent pins in the sensor.

What's weird is the code is for a low volt. Are you testing the right component?

A is a signal reference voltage sent from the pcm. It should be 5v. If your reading battery voltage at a there's a problem. Sounds like it shorted somewhere with the b wire or somewhere else is feeding it that's probably the cause of the high voltage. Take a close look at the connector. Make sure you don't have any misaligned terminals or bent pins in the sensor.

What's weird is the code is for a low volt. Are you testing the right component?

As far as I know I'm testing the correct component... it's the sensor hooked to the fuel line just forward of the fuel filter on the driver's side in between the frame and the transfer case...

 

[CONKLE73]

UPDATE:


It did throw a new code today P0651 (Sensor reference voltage “B” Circuit/Open)

 

[Justinj360]

UPDATE:


It did throw a new code today P0651 (Sensor reference voltage “B” Circuit/Open)

Did you get it figured out? I'm having a similar problem.

 

[CONKLE73]

Yes... actually I did...

The problems were caused by corrosion in the PCM plug.

After the pins and the plug were thoroughly cleaned all the codes went away and the Tahoe is running great.

 

[Justinj360]

Yes... actually I did...

The problems were caused by corrosion in the PCM plug.

After the pins and the plug were thoroughly cleaned all the codes went away and the Tahoe is running great.

Great to hear you found it. I on the other hand checked my PCM connections (which were fine) and now it acts like it needs to be relearned. It was my understanding that you could just disconnect the battery, check the connections, put them back, then reconnect the battery and you would be fine. Any ideas?