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Juan Crespo
Juan Crespo, Tech Trainer
Category: GMC
Satisfied Customers: 1526
Experience:  A.S.E. Master Technician, Advanced Level, Emissions - Asian, Domestic, & European
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I have a 2008 GMC Sierra 1500 SLT 4x4... It has a 5.3L

Customer Question

Hello, I have a 2008 GMC Sierra 1500 SLT 4x4... It has a 5.3L Vortec V8 with 152000 miles. Over the past 6 months it would occasionally drop into reduced power mode and throw DTC P0641. It also threw a misfire code for cylinder 4. After some troubleshooting I replaced the ignition coil for #4 and reset the PCM. That seemed to fix it until recently. It went into Reduced power mode again but this time no code. After some research I ended up turning off the AFM/DOD function. This, once again seem to work. Now it is dropping into reduced power mode and throwing the P0641 code again and at times will not even engage the starter. I've checked the electrical harness that I can see but none of them seem to be compromised. Battery and fuse box connections are good. I know it has to be a short somewhere because it will even happen when the engine hasn't even been started, just turned on (after reset). Any ideas on where else to look?
Submitted: 1 year ago.
Category: GMC
Expert:  Juan Crespo replied 1 year ago.

Hi there. Welcome to our site!


If you’re thinking on working on the vehicle yourself, all my knowledge and resources won’t help unless you…

… have basic mechanical knowledge and skills, and…

… have whatever tools and equipment might be needed for a proper diagnosis


I've attached a table showing the different possibilities for P0641 being set. A capable scan tool, such as the TechII or the MDI, would show the actual voltage data which would greatly speed-up our diagnosis. Otherwise, it's a matter of disconnecting the PCM and all the sensors affected, then running continuity tests on each circuit until the culprit is found - I would start with the oil pressure sensor.

Please let me know if you need anything else from me and I'll do my best to provide it.

Best Regards.