Q: Rough idle, thermostat code

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My vehicle is rough when idling, even when it's warm. The RPMs jump between 0 and 1 on the tachometer. A few times, it has dropped all the way to zero and died. I also noticed some bad hesitation when I'm speeding up. When I had the throttle body cleaned, it worked better for a week, but the problem came back. One mechanic has suggested replacing the throttle body, and that I just have a "common issue". Maybe related: The check engine light is on, but it's not throwing a code on the throttle. The code is for the thermostat. Could a thermostat cause my issue, or would it be something else?

I’m guessing the codes is P0128 Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature).

The code and the symptoms you are experiencing may or may not be related. The powertrain control module (PCM) determines operating temperature via the signal from the engine coolant temperature sensor (ECT). A problem with the ECT or its circuit can set a code P0128 and could also cause the drivability problems you are experiencing. However, there are dozens of other possible causes for your symptoms. Anything from a vacuum leak to a faulty idle air control valve could be the cause. A dirty throttle body is a very common problem on vehicles such as yours, but you said your mechanic already cleaned that. I suggest you have a trained professional diagnose your trouble codes and inspect your vehicle firsthand to find the right fix for your rough idle.

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