I have a 2003 cavalier 75,000 miles on it the check engine light came on and here is what the problem is DTC P0420 Catalyst System Low Efficiency what does this mean i think it means something about the cat sensor in the muffler system maybe im running lean dont know hope some one can help thanks alot
Could be a number of things, bad cat (most likely), bad primary sensor, or the after-cat sensor is going bad. How that part of the system works, is the after-cat measures the O2 in the exhaust after its run thru the cat, if its out of its programmed spec, the PCM will trigger that code letting you know that theres a problem, if you are running lean, the car would run a bit hot and probably not run very well, and that would also overheat the cat causing the guts to breakdown. If rich, it'll run poorly, have lots of black smoke issuing from the exhaust, and the cat will eventually plug up. Test your primary sensor, if its good, then the problem is almost certainly the cat is bad.
You say you have a hi-flow cat in your registry, that is your answer. The exhaust flows too quickly through the cat and the after-cat O2 sensor thinks something is wrong with the cat. Get an O2 simulator and it will fix the code.
Can use the simulator, or get a pair of sparkplug anti-foulers drill and tap one out to fit the O2 sensor, screw that one into the other, screw the thing into the cat, then screw in the sensor. Gives the sensor enough time to get a reading, plus keeps it cleaner.
