Q: Engine code says my thermostat is bad but nothing seems wrong with my car.

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I have a code of P0599 saying my thermostat heather control circuit high. But when my car is operating it is completely fine and nothing overheats or shows signs of error.

My car has 57000 miles.
My car has an automatic transmission.

Your vehicle is probably covered by GM’s 5 year, 100,000 mile Powertrain Warranty. Consequently, you may be entitled to a free warranty repair of this thermostat issue, which issue by the way is not particularly unusual and so the dealer service department will have already encountered it and be familiar with it. Thermostat replacement may resolve the problem. However, a loose or corroded connector on the thermostat is another cause. Other reported causes are shorts or opens in the wiring harness between the computer and thermostat and rarely the control computer will fail. The existence of diagnostic trouble code P0599 on your vehicle does definitely mean there is a fault in the system but the fault does not always immediately result in a visible malfunction. Nevertheless, the efficiency of the engine is degraded simply because there is some loss in the electronically controlled thermostat’s ability to precisely control coolant flow at part throttle versus high engine load. GM should repair this for and you should definitely get it resolved as I have indicated that the fault DOES negatively affect engine performance. If you run into difficulty you can request diagnosis and repair though YourMechanic and the service to request would be a check engine light diagnostic. Please let us know if you have further concerns.

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