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Q: Idles very rough when heater/defroster is on

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It idles very rough (5k to 7k) when we attempt to warm it up on mornings when it's cold. We had a $800 sensor installed last year, but the "Service engine soon" light remains on consistently, and when we do not have the heater or defroster turned on, it idles very nicely.

My car has 217000 miles.
My car has an automatic transmission.

Hello, thank you for writing in. The vehicle is put under extra load when the climate control is turned on. The engine is impacted different ways by the different systems. Typically when the defroster is turned on, it uses the air condition system. The heater obviously uses a different system. The one component they share is the blower motor. This motor does pull its power from the engine and causes an extra load. The extra load is what is bogging down the rest of the engine’s functions. The load may be having an extra effect on the engine because the blower is pulling an excessive amount of power, or because the engine is not in a good enough condition to support everything it needs to. Inspect the blower motor, and if it is working correctly, you will want to look into tuning and updating your engine. This typically includes spark plugs, filters, and fluid changes. For more help, contact our service department.

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