If you have check engine light on you need to have the Diagnostic Trouble Codes read and this will give you a clearer idea of where the problem may lie. A faulty O2 sensor could certainly lead to an over -rich mixture but here is another possibility: Faulty coolant temperature sensor...leads to ECU not being told when engine has reached operating temperature ..... ECU instructs injectors to maintain open cycle condition and keep the mix rich as if engine is still cold. So the causes of rich mixture and lumpy idle can be varied. The DT codes really help in focussing area for examination and ultimately will save time and money. Autozone do DT code reading as a service.
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