Q: engine idles rough and slow after replacing the battery

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After replacing the battery, the engine idles very rough and will die with the air on.

My car has 225000 miles.
My car has an automatic transmission.

You will need to re-learn the idle for the vehicle to get it to idle high enough. You should first clean the throttle plates and throttle body and idle control valve. When you disconnected the battery, the idle control was reset to factory and will idle differently unless you relearn the idle again.

After cleaning the parts out, then start and run the engine at 1500 rpms to warn engine for a few minutes, and then let the car idle with all accessories off for two minutes. Put the vehicle in drive and let it idle for two minutes then turn on A/C on medium blower speed, and then let it idle in gear for two minutes. Put it back in park and idle for two minutes with the A/C on. Then road test for about 5 miles in stop-and-go traffic. This should fix the idling issues.

If you need help with all of this, consider having a certified technician from YourMechanic come to your car’s location to diagnose the rough idle and perform any needed tasks to have this resolved.

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