The TSM say it's a +5 volt reference that's open. 3 sensors share the circuit, the fuel pressure sensor, oil pressure sensor, and accelerator pedal position sensor. Most of the time the truck runs fine, once in a while it has a rough idle, and the check engine light comes on. There have been no drivability problems, and it only seems to happen when it idles after warm up and in park or neutral. Everything smooths out once it's in gear.

I've reseated the connectors on all the sensors and it didn't change things. I hate to just randomly replace the sensors until it's fixed, since they're each over $100.

Anybody got any tips or tricks on this one?

TIA

Bob
tufcj

 

Anybody have any ideas?

 

get a voltage meter and check for 5 volts on said sensors. see if there is no power. sounds like it is an intermittent problem. this could make it hard to check. have the meter handy and when it happens jump in and check for the 5 volts at one of those sensors.

if there is no power then it looks like you need to find the break in the wire. i am guessing it may also be a bad ground too.

if someone doesn't beat me to it i will see if i can find the schematic for it to help trace the wires.

 

Very intermittent. I've tried shutting it off and checking voltage when I get the rough idle, but everything has always checked out. According to the TSM, it only takes .5 second of voltage loss to trigger the check engine light. I don't know if I'd even notice a bump if I was cruising down the road if it happens for that short (pun intended) of an interval.

I did find one reference to the P0641 in a google search. That person replaced the oil pressure sensor and it cleared the problem. That was after the dealer diagnosed it and told him the ECM needed replacement. I might just try it.

Bob
tufcj

 

Heres A google clip i found for ya... might help..

"The Troubleshooting chart I have for the code P0641 said to disconnect the Fuel Rail Pressure (FRP) sensor and then check the reference voltage at the Accelerator Pedal Position sensor (APP). I did that and nothing changed, still low voltage. The same troubleshooting chart showed that the ECM supplies 5 volts to sensors 1 and 3 on the APP, the FRP sensor, and the Oil Pressure Sensor. The chart doesn't say anything about disconnecting the OPR to see if that remedies the problem, but I thought I would check anyways since it comes from the same supply and seemed as if it could just as easily be that sensor as any other from the same supply. Pain in the butt sensor to get to, but sure enough, as soon as that sensor was disconnected I got 5 volts at both reference points at the APP. Plugged the OPR back in and everything was still fine. My guess at this point is that the connection at the OPR was bad or that I may have a short in the harness. "

 

check out the schematics on this page

wire schematics for TCM, BCM, ECM, etc - Duramax Diesels Forum

see if any of them are the ones you might need

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i think i found the same thread you found: P0641 Problem diagnosed as bad ECM by GM Service SHop - Chevy and GMC Duramax Diesel Forum

 

well something doesn't add up. check out these diagrams. oil pressure and throttle (motor?) use 5 volt reference from pin J1-8 but the rail pressure sensor seems to use J1-48, and i dont see any others that are on J1-8

if your are sure that the 5 volts is used on the throttle position sensor and not this motor listed in the diagram then that could help and i can find the other items on the line