Do I really need a new catalytic converter?

John Paul, AAA Northeast's Car Doctor, answers a question from a reader wondering whether a recommended repair is necessary.

–AP Photo/Gene J. Puskar, File

Q. My son was driving his 2009 Subaru Impreza when all the dash lights came on and the cruise control light started blinking. We plugged into a diagnostic device and it showed a code PO420. We brought it to our mechanic, and he replaced the O2 sensor. Everything was fine for two weeks, but then all the lights came back on. This time my mechanic put a lifter/adaptor under the sensor. Again, everything was fine for two weeks, but then the dashboard lit up. We took it to a Subaru dealership where, without much diagnostics, declared we needed a new catalytic converter that costs $1,500. I then brought it to a muffler and exhaust shop. They replaced another sensor and took the lift-device out from under the first sensor. It was near inspection time on the car, so he did an inspection and the car passed the emission test and safety portion. The PO420 code has never shown itself again since the first event. The car never functioned differently from the get-go. It runs just fine. It has been one week since the last repair and no lights are on. Do we really need a new catalytic converter? If so, does it have to be an original one as the dealership stated? We only plan on keeping the car another two years.


A. From your description, it is most likely the car needs a catalytic converter. The idea that the shop mounted the oxygen sensor out of the exhaust stream (fooling the computer by tampering) tells me the catalytic converter efficiency is not where it should be. It has been my experience that aftermarket catalytic converters don’t always work as well as the factory units. If you can save money and the repair shop will stand behind their work and the parts they install, then use the after-market part.

John Paul is AAA Northeast’s Car Doctor. He has over 40 years of experience in the automotive business and is an ASE certified master technician. Email your car question to