Catalytic converter, or new sensors?



I have a 2000 Subaru Impreza 2.5. It has two cats coming off the y-pipe to the main exhaust. I have a CEL that is giving me code P0420 which is “Catalytic converter efficiency below threshold”. Now about a month before this code showed up I did blow a headgasket, but drove less than 10 miles to the mechanice to get it fixed.

My question is, how do I know for sure the catalytic conv. is toast, or could it just be a fouled up sensor before or after the cat?


One thing to keep in mind is that if you are within 8yrs(of purchase)/80,000 miles the cat converter is covered under emissions warranty.


You would be better off if you could find a mechanic who can use the scan tool to look at the performance of the oxygen sensors, and the health (“tune”) of the engine. Anti-freeze is a poison to oxygen sensors and catalytic converters (motor oil, and silicone are poisonous, also). Any guess, without proof, is still a guess.


If the warranty is still active, have the dealer determine whether your catalytic converters need to be replaced. If so, you should qualify for at least some part of the replacement cost. It depends whether the guarantee is for parts and labor, just parts, and if it is a limited warranty. Why use the dealer? They are an authorized representative of Subaru and the manufacturer will accept their word for it. If you go elsewhere, you need to convince Subie that the shop was correct.


Two cats, one each bank.
One of two banks head gasket failure?
Get the related O2 sensors changed.
The upstream and downstream sensors were likely contaminated with coolant.
DIY cheapest.
Unlikely warranty will cover O2 sensor failure due to head gasket faiure.