Rick's Free Auto Repair Advice

Block heater on GM vehicles

GM has installed temperature sensing circuitry into the plug on factory installed block heaters. It prevents the heater from operating unless it is colder than 0°F or -18C. It may seem insane, but it really does make sense if you understand why. If you install an aftermarket block heater without this circuitry, the PCM will see warm coolant, but a cold engine and cold air coming into it. Remember, it’s the PCM’s job to determine the correct air/fuel mixture. A cold engine block and cylinder head act as a fire extinguisher when you first start up the engine. So the computer commands a very rich mixture. How does it know what temperature the engine is? It used to look at only the coolant temperature. But now it looks at both coolant temp and the temp of the metal itself. If it see a discrepancy between the two numbers, and the outside air temp coming in through the intake is 0° or above, it assumes there’s something wrong with the coolant temp sensor and sets a trouble code.

Bottom line, if you add an aftermarket heater, it must have the temperature sensing circuitry in it or you WILL set a trouble code. Also, if you ever have to replace the power cord to the factory block heater, you must use a new GM power cable.

For more information on this repair or any others for your vehicle, buy an online subscription to either Alldatadiy.com or eautorepair.net. Click on this link to compare the two services: Compare Alldata and Eautorepair.

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat

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