My service engine light keeps coming on.

Service Engine light will come on for a variety of issues. Can you post the OBD trouble codes, someone might be able to point you in the right direction if we know what's triggering the service light. Pep Boys, AutoZone, and a few other chains will read the code for you for free.

P0420 is the code. It says the catalytic converter efficiency is low. Probable cause: cat converter defective (meineke says it isn't); engine misfire (I have new plugs and wires - unless the gaps weren't set properly); large vacuum leak (not sure how to tell); eng oil leak into exhaust-valve guide seals, piston rings. The two mechanics I've taken it to don't seem to think this is an issue. I won't go back to the dealer because they said the cause was the transmission. If you have any ideas of the cause and if there is a way I can look for signs before taking it to service, I'd appreciate it. Thanks.

Find the large vacuum leak. A vacuum leak will cause unmetered air to enter the engine, which causes a lean condition. The ECM will try to compensate by dumping additional fuel into the system, but you're actually running very lean. This will trigger the P0420 code and call up the "defective cat" code because the exhaust O2 sensors are reading the lean condition as a Cat issue. Meineke is probably right. I would also check the MAF sensor, make sure it isn't dirty. It can also throw a P0420 code. Once you've solved the vacuum leak, clear the code, and run the engine at 3000 rpm for a few minutes to blast out the Cat.

Thanks for the help.