The catalytic converter has an oxygen sensor in front and behind it when the vehicle is warm and running in closed loop mode the upstream oxygen sensor waveform reading should fluctuate the downstream o2 sensor reading should be fairly steady typically the p0420 code triggers the check engine light if the readings of the two sensors are similar. However if the p0420 code is the only trouble code or if the other codes are all p042x codes the likelihood of a failed catalytic converter is much higher several things can cause a p0420 code in a toyota leaded fuel in an unleaded system will nearly always trigger this given the differing properties of leaded fuel combustion and exhaust. Have a p0420 code on 2000 celica should i replace o2 sensors or cat toyota 2000 celica question 2000 toyota celica fault code p0420 cat efficiency below threshold bank 1 i have two sensors before and after catalytic converter manuals user guides pdf manual 2000 celica manual 2000 celica. Oh yea after i replaced the mass air flow sensor to fix the running rich problem it runs perfect again so basically what im saying is that i dont notice a loss in performance from the p0420 code except when i first start the car after its been sitting overnight it seems a little boggy but doesnt bog at all after running for a minute
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