Q: Engine idle problem but doesnt match the code on the check engine

asked by on February 12, 2018

Engine revs up and down but im getting a engine code for p0420 that says the cat is bad and or bank-1 02 sensor is bad but no codes for the idle air control valve sensor only code in the car is p0420. What should i do ?

My car has 153861 miles.
My car has a manual transmission.

The engine idle fluctuation may be caused by a number of things, bit is most commonly associated with the idle air control valve either being dirty or damaged, which may also cause the other code (P0420) to be triggered. The idle air control valve monitors the air intake as it is mixed with fuel prior to being injected into the engine at low speeds and at idle. This valve is controlled by the vehicle’s computer and will adjust idle speed based upon other measurements such as engine temperature, intake air temperature and electrical system load or voltage. This is also an important function when starting the motor as it allows the motor to run and idle on it’s own once the motor fires. When you accelerate, the engine RPM increases, and as you let off the gas, the RPM slowly returns to the normal idling speed with the help of the idle air control valve making the transition from a higher RPM back down to idle speed while adjusting the air/fuel ratio constantly to allow this to happen smoothly. When the engine RPM drops below the normal range of about 750-800 RPM, this often times will cause the engine to stall indicating a dirty or faulty idle air control valve. I would recommend having an expert from YourMechanic come to your location to diagnose and inspect your vehicle.

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