PO420.....so now what? - Toyota Yaris Forums

FunctionSpec · 03-29-2009, 06:32 PM

Air/fuel ratio sensor(AFS B1 S1) voltage should be around 2.8V to 3.8V idling. Stoichiometry is at 3.3V. A voltage of 3.44V means a lean condition (A/f ratio sensor output is the opposite of what happens in a oxygen sensor).

I'm assuming it may be the catalytic converter because of your previous problem. Since you accidentally ran diesel and gas, it didn't burn completely when trying to drive the car which dumped a bunch of raw fuel onto the Cat causing it to overheat and slowly deteriorate. Also some diesel fuel was probably still in the system after draining it plus mixed with higher octane fuel(resists detonation) caused the engine to slowly dump unburned fuel onto the cats. Which is probably why it took awhile for a CEL to come on.

I would test out the O2 sensor(sensor 2) to check for proper operation since it is cheapest to replace and then replace the cat if all sensors check out correctly.

03-29-2009, 06:32 PM	#20
FunctionSpec Super Newbie View User's Garage Drives: 2007 Yaris Liftback White Join Date: Jul 2007 Location: 626, Southern California Posts: 180	Air/fuel ratio sensor(AFS B1 S1) voltage should be around 2.8V to 3.8V idling. Stoichiometry is at 3.3V. A voltage of 3.44V means a lean condition (A/f ratio sensor output is the opposite of what happens in a oxygen sensor). I'm assuming it may be the catalytic converter because of your previous problem. Since you accidentally ran diesel and gas, it didn't burn completely when trying to drive the car which dumped a bunch of raw fuel onto the Cat causing it to overheat and slowly deteriorate. Also some diesel fuel was probably still in the system after draining it plus mixed with higher octane fuel(resists detonation) caused the engine to slowly dump unburned fuel onto the cats. Which is probably why it took awhile for a CEL to come on. I would test out the O2 sensor(sensor 2) to check for proper operation since it is cheapest to replace and then replace the cat if all sensors check out correctly.