Q: Rough/ low idle, smells rich, and has a knocking sound near rear (like exhaust is rattling).

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Had a local auto store put their computer thing on it. They said the O2 sensor and the knock sensor codes were appearing. Replaced the O2 and spark plugs. Now it has an even worse rough idle and won't keep running or start unless i press the gas pedal.

My car has 200790 miles.
My car has an automatic transmission.

A: Hello. This may be potentially related to t...

Hello. This may be potentially related to the mass air flow sensor or throttle position sensor. Initially, it may have been a faulty O2 sensor, but now since that has been replaced, that part of the system should work correctly.

However, when replacing a part like that, it can cause something else related to the incorrect amount of oxygen being injected into the motor. This is what the mass air flow sensor does. There is also something called an idle air control module that regulates the air flow into the motor at idle. When this is faulty, it can cause the car to also idle roughly. For a more in-depth inspection, I would recommend having an expert from YourMechanic come to your home to diagnose and repair the rough idle.

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