Q: My check engine light is on , mechanic said it is catalyst converter however the car run perfectly fine , no misfire or more

The impact on engine operation (and fuel efficiency) of a "failed" catalytic converter depends on exactly what the failure mode was. In one scenario, if the catalyst within the converter is simply "poisoned" and thus there is no longer any chemical conversion going on in the converter to reduce tailpipe emissions of unburned hydrocarbons, then there is essentially no detrimental effect on engine operation or efficiency. However, there is a negative effect on air pollution. On the other hand, if the failure mode is melting of the internal structure of the catalyst, thus blocking exhaust flow out of the engine, the engine could perform poorly with other noticeable effects.

It is easy to test the efficiency of operation of a catalytic converter using a relatively basic hand held scan tool that reads real-time upstream versus downstream oxygen sensor outputs. The converter should be tested using that, or other, techniques. With regard to the check engine light, the codes that have caused the light to set should be downloaded and the fault that has caused the light to illuminate should be fully resolved. Once the check engine light has set for "one" fault, if that fault is not resolved, and the light turned off, if and as "new" faults occur (elsewhere in the engine or power train), those NEW faults become essentially invisible to you simply because the check engine light is just on continuously (because the first fault was never resolved). So, the fault at this time has to be resolved, so that the monitoring system is restored such that it is usable to you if a NEW problem with the engine arises.

The most efficient course is to request a check engine light diagnostic and the responding certified mechanic will resolve both the uncertainty with regard to the catalytic converter and find the cause(s) behind the illumination of the check engine light. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.