P0420 catalyst efficiency below threshold — can I just ignore this?

P0420 means the downstream O2 sensor is seeing too much fluctuation — the cat isn't cleaning exhaust efficiently. But you're right, it's not always the cat itself. Before spending $1,200: 1. Check for exhaust leaks before the cat (a leak introduces oxygen and fools the sensor) 2. Compare front vs rear O2 sensor waveforms with a scan tool — rear should be nearly flat if cat is working 3. Check if the downstream O2 sensor itself is lazy (slow switching = false P0420) 4. Look at fuel trims — if the engine is running rich, it's overwhelming the cat If you want to trace the O2 sensor wiring, the [2012 Malibu wiring diagrams](/wiring/2012/Chevrolet/Malibu) have the full exhaust sensor circuit. Can save you a lot of diagnostic time.

I cleared mine 6 months ago and it hasn't come back. Sometimes it's a fluke from bad gas. If it comes back within 2-3 drive cycles though, it's a real issue. Also — if you're in a state with emissions testing, you can't ignore it. In Texas we can't pass inspection with any CEL.

Try an Italian tune-up first. Get on the highway and run the car hard for 20 minutes — sometimes carbon buildup on the cat causes P0420 and a good hot run burns it off. Free to try.