2003 GMC Duramax LB7
 

I am new here and trying to help my dad fix his 2003 GMC Duramax. Sporadically when the truck is cranked it misses, gets the reduced power mode indication. Generally the oil psi gauge drops to 0 and it will flash Water in Fuel and the code will be PO-641 r some research I have located a schematic and a post that indicates checking the voltage at the Fuel Rail Sensor, the Throttle Position sensor, and the Oil Pressure Sensor.

Does anyone have a picture of the LB7 engine showing sensor positions they could share.?

i would first check the large harness connectors on the driver side of the engine for pushed in pins. this is an often missed problem that could create all of the issues you described. if you find any of the pins that look shorter than the rest pull on them with some needle nose pliers back into position. i had that happen to my LB7 recently, same symptoms as you, and checked everything you listed and all looked good and could not get it to clear. then i remembered about the large connector problem and found many of the pins pushed in. after fixing that it started right up and ran great with no codes.

2003 GMC Duramax LB7
 

Thank you I will check that today and let you know what I find!

Agreed definetly electrical wiring or ecm but start cheap and simple first

LB7 P0641 REPAIRED
 

I think I may have my intermittent P0641 Error Code repaired. I could find no culprit to the low voltage on the buss, I had found repair charts that pointed to the FRP Sensor, the Water in Fuel sensor, the Throttle Position Sensor, and the Oil Pressure sensor.

To date I have replaced the H2O in Fuel sensor and gone back to a Delco filter and trashed the Wix brand. I replaced the Oil PSI Sensor only because I found a great deal on one on Ebay. Still the error, but I did notice one way this error was sure to occur was when I left the truck idling when it had the choppy idle.

After some reading I found a post someone wrote about the FPR causing a choppy idle and throwing that P0641 Code due to the actual FRP being out of spec from the desired FRP PSI. I changed the FPR which I found online for 139 bucks, and it was a genuine Bosh OEM for the LB7. The GM Dealer wanted 300 bucks for this little unit.

Immediatly the choppy idle is repaired and as of yet no P0641 error code!!!