Odd problem - my son's check engine light comes on - (had it checked its related to one of the cat sensors - P0420) - he says car looses power, behaves differently when shifting, hesitates, has a rough idle - check engine light goes out - stays off for a couple of days - car runs as normal. Light comes back on - same behavior. is the Cat sensor or some other sensor messing with fuel/air ratio? Do I clean sensors - on another vehicle I had similar behavior and had to buy a special cleaner for a MAP sensor - cleaned it engine ran great. Any thoughts? 

Thanks, 

Milty 

 

milty60,

Always give us the year, model and mileage of the car, as that info can help us zero in on what advice we can give you. There's a big difference diagnosing a 20 year old car with 300 000 miles versus a 2 year old one with 12 000 miles and still under warranty.

At this point the oxygen sensors seem to be the culprit, and are either defective and erratic (need replacing) or the wiring to them or their connection is frayed or corroded. Or it could be something else (bad connection at the CPU).

But don't just assume this to be the issue and throw parts at the car, and then still have the erratic  performance issue afterwards, but rather get a good independent mechanic to check the system and diagnose it properly. It may cost less in the long run.

though catalytic converter is part of the definition for P0420, many less expensive items/issues can trigger that code. Exhaust leaks, vacuum leaks - even lax maintenance items.

getting freezeframe or live data may be needed.