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2004 Chevrolet Silverado 2500HD:
Intermittent Check Engine Light

Alex Steele
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seen in Truck Trend



We have a 2004 Chevy Silverado 2500HD with the LB7 Duramax. The check engine light keeps coming on intermittently and is not throwing any codes.


The truck is completely stock. We have taken it to the dealer several times, but all they can say is keep bringing it back. Could you shed any light on this situation?




Intermittent electronic faults can be a real project to diagnose. Oftentimes a shop, or a technician, will avoid investing a lot of diagnostic time until the problem becomes more pronounced, or develops into a hard failure (light is always on). But it's not always that difficult.


If the "Malfunction Indicator Lamp" (check engine or service engine soon) is lighting up on the instrument panel and no Diagnostic Trouble Code was stored in the Engine Control Module's memory, that narrows things down quite a bit. No guarantee, but odds are high that the problem is the engine control module itself.


You may not find this procedure in the service manual, or GM digital database, but sometimes you just have to hit the thing -- seriously. The control module is located under the hood, on the left side near the battery. With the hood up, engine running and someone in the driver's seat to observe the instrument panel, tap on the engine control module with the back end of a good-size screwdriver. It's the black box with three electrical connectors attached. If the malfunction indicator lamp comes on, or even flickers while tapping, there's the problem. It's an internal failure and the control module needs to be replaced, and the new unit reprogrammed.


Again, odds point that way but no guarantee. The engine control module may still be at fault but passes the "Tap Test", or there's a bad connection in a major circuit going to the control module, or it could even be a short circuit within the instrument cluster.


A considerate technician should have at least tested for codes and other irregularities with a scan tool, done a quick tap test and inspected a few of the control module circuit electrical connectors before sending you on your way.

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15 Apr 2016, 19:35
I will be putting this dailzzng insight to good use in no time.
21 Feb 2013, 10:39
What's the DTC (Diagnostic Trouble Code)?
14 Feb 2013, 13:35
I have a 2006 GMC Duramax and have had several techs look at it and attempt to fix. The issue is my engine light comes on intermittently and every time it is scanned it displays the glow plug module is bad. I have replaced it twice and 4 glow plugs. The engine light still keeps coming on. Please can anyone help? It goes back to the shop next Wednesday 02/20/2013. I would like to tell the tech what to look at. Thanks for any help.


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