p0420 Error code. Oxygen sensor?

P0420 means "Catalyst Efficiency Below Threshold". This CAN be caused by a bad O2 sensor. It is also the code that results from a bad catalytic converter. Given the low gas mileage, I suspect the Cat. You can' always try replacing the O2 sensors. You can by both O2 sensors for about $100 and should have no problem changing them yourself. A catalytic converter will run around $350 for the part. Labor is extra.

If the dealer did the previous repairs it seems that they should be trustworthy repairs. Oxygen sensors fail often, sometimes due to sensor failure and sometimes due to their internal heater failing. Nobody uses the wrong sealer anymore, those types of sealers have been off the market for years. Since all cars now have oxygen sensors those old style sealers are now obsolete. While the oxygen sensor probably is the problem you may have a different problem the computer reads as an oxygen sensor failure, like a blown fuse to its heater circuit or a broken wire. I have found on a GM car once where it had a throttle position sensor code where the real problem was a bad oxygen sensor fuse. Crazy, but that's what the diagnostic trees are for. I would try the sensor and clear the codes, then hope for the best. If it doesn't fix the problem, have it diagnosed by a qualified shop.

in case you have a p0420 it incredibly is brought about via countless issues. #a million undesirable downstream O2 sensor and #2 defective downstream O2 sensor. I had a 2000 Toyota Camry with over 312000 miles on it and the unique catalytic converter nevertheless working precise. First you will desire to purpose to apply catalytic converter purifier. 2d, if that doesn't artwork and the catalytic converter is nice, replace the downstream O2 sensor and it fixed it. to income for a nasty catalytic converter degree the ameliorations of the inlet and outlet of the catalytic converter to work out if that's working precise. stable success!

if you recently had your head gasket replaced. this tells me that coolant was buring inside the clyinders and that coolant aslo went through your exhaust and catalytic converter. having a blown head gasket would make you converter fail. you need a new catalytic converter. possabily new oxygen sensors also they were contaimated by the coolant