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Juan Crespo
Juan Crespo, ASE Certified Technician
Category: Chevy
Satisfied Customers: 1515
Experience:  A.S.E. Master Technician, Advanced Level, Emissions - Asian, Domestic, & European
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I am working on my 2008 duramax lmm i have a short to ground

Customer Question

i am working on my 2008 duramax lmm i have a short to ground i think somewhere but having one heck of a time finding it.
JA: Thanks. Can you give me any more details about your issue?
Customer: i have replaced cam sensor, alternator, batteries, checked grounds to motor all good. but when i disconect fuse block under hood with 2 grey handles my test light goes out. i have the neg bat cables unhooked and a test light hooked to neg post and neg cable and have power
JA: OK got it. Last thing — Chevy Mechanics generally expect a deposit of about $18 to help with your type of question (you only pay if satisfied). Now I'm going to take you to a page to place a secure deposit with JustAnswer. Don't worry, this chat is saved. After that, we will finish helping you.
Submitted: 8 months ago.
Category: Chevy
Expert:  Juan Crespo replied 8 months ago.

A few things that might help you solve your issue:

  1. What you're describing is a short to power, which is what happens when a circuit doesn't switch off and keeps draining the power source.
  2. There are computers/modules in your truck that need some current at all times. GM considers acceptable a draw of up to 30 milliamps (0.030 Amps).
  3. A test light can't distinguish an acceptable draw from one that is actually draining your batteries - only an ammeter can.

Having clarified those points, what is the issue you're trying to diagnose? Batteries going dead overnight? Not enough electrical power to start the engine? "Battery" icon lit up on the dash?

I'll be back online tomorrow after 2PM ET. I look forward to hearing from you then.

Customer: replied 8 months ago.
dtc code p0642
Expert:  Juan Crespo replied 8 months ago.

Now I see how you could have gotten confused. This issue has to do with the 5 volt reference signal the onboard computer sends to several sensors. If the onboard computer detects a sensor is shorted to ground, it will set P0642.

A scan tool such as the Tech-II, capable of reading sensor data, can quickly show you which sensor or circuit is causing the issue. Not having that kind of scan tool available makes diagnosing this issue the same as cutting down a 100 ft tree with a kitchen knife; very hard. You will have to check voltage with a voltmeter at every individual sensor until you find the guilty one - sensors to check are cam, crank, intake air flow valve, fuel tank level sensor 2 and EV fan sensor.

Please tell me how you want to proceed and how I can help.