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Blown Head Gasket or needs new ECM/engine wire harness/thermostat?

I’m at a loss as to who to believe now. Recently, my 2013 Chevy Sonic (approximately 140,000 miles on the car, recently new water pump and thermostat and housing) began beeping with an oil pressure light on constantly. I took it to my regular mechanic, where the oil filter and oil pressure sensor were both replaced, twice, to no avail. Then, I took the car to a dealer. There, they diagnosed a coolant leak originating in the thermostat and housing that had gotten coolant into the wiring and the ECM (electronic control module). They quoted me over 2,000 dollars to replace the thermostat and housing, replace the engine wire harness (which they told me was completely fine, just potentially wet), and to replace the ECM. I did not let them do these things.

I took it back to my regular mechanic to ask them to check the thermostat and housing for a leak and clean up the wire harness if possible. They then told me that the thermostat and housing looks completely fine, that the wire harness replacement is unnecessary, and that they thought what the problem actually was. A blown head gasket, sending oil up into the coolant.

Now, these things do not seem related to me. Just looking for some thoughts from the community, since I have no idea who to believe at this point. Thanks!

Believe neither if neither shop tested the actual oil pressure before just replacing senders. Believe neither if they didn’t tell you if there was coolant in the oil - the milkshake oil becomes when water enters.

I don’t think you are giving us the complete picture. Head gasket leaks can be tested for - not rocket science. Paying $2k to replace a wet wiring harness is just ridiculous. Since the ECU is sealed and potted, coolant can’t enter, that is ridiculous, too. The dealer’s shop is incompetent.

Have your regular mechanic test for a blown head gasket before you start replacing it. If he doesn’t know how, find a better mechanic.


I agree with Mustangman. Diagnosing a head gasket is not a guessing game at all.


I will also agree with the gents here thusfar… This does not sound like a logical flow of diagnostics to me… not in any way. As the guys have also picked up upon…

I think I will just let Mr @Mustangman speak for me… he pretty much has it covered here. How these two entities could be so far off the mark is troubling… Neither should be so lost in this situation.

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How does the engine oil look to you?

When coolant or water wicks though wiring and gets into connector or modules it can cause a lot of damage and erroneous behavior, like an oil pressure light staying on. Did they show you the fluid in the connectors and ECM? They may have taken pictures of the damage that they can show you.

For example, this is water damage on a BCM in a 2018 Lexus LC500;

Thank you @Mustangman. You are probably right about me not giving you a complete picture, since I don’t know much about car repair. I am just giving you the information they gave me!

@Nevada_545, the engine oil looks dirty to me, but again I know very little about this topic. And they did not show me photos at the dealer when mentioning coolant getting in. The shop we regularly visit tested the pressure in the coolant system and saw oil in the coolant, and then told me it was a blown head gasket.

Bingo… blown head gasket.


Engine oil will turn white/brown when mixed with water. If coolant in the engine oil is causing the oil pressure warning light to illuminate then the engine is damaged and will need to be replaced.

If you didn’t ask to see the damage then they likely won’t offer to show it to you.

In case any of you are interested, no one was correct. It was a bad oil cooler.

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