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my brothers 2005 scion tc was having MPG issue. i checked the code and got P0420 Catalyst System Efficiency Below Threshold (Bank 1).
i checked the O2 readings on my scan gauge and got for upstream a steady 3.1 mv and for the downstream isolating from 0.1 to 0.7 mv.
does this mean my O2 upstream sensor is bad? since it shld be reading from 0.1 to 0.7 and the downstream also bad since its not hold steady?
or has the cat gone bad from running rich for so long.
(i still have not checked the cat temperatures at the inlet and outlet)also on a side note his k&n filter was very very very dirty, i got that all cleaned up and took the o2 sensor readings after and the results are the same.
any input? im leaning towards replacing the o2 sensors. car has 157,100 miles.
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There are a number of things that could be causing this p0420 catalyst efficiency code. The most common thing is the catalytic converter itself is no longer functioning properly. The other likely thing is the rear O2 (oxygen) sensor is no longer working properly. Other things could include exhaust leaks, damaged exhaust pipes, damaged O2 sensor wiring/connectors, plugged catalytic converter, etc.
The easiest thing to do first is a visual check of things. Visually inspect the exhaust system for leaks, check the catalytic converter for dents, holes, severe discoloration, and check for a rattle inside. If any of those syptoms are there, the converter likely needs replacement. Then, visually inspect the downstream O2 sensor (behind the converter). Check for broken wires, obvious faults, etc. If all that checks out, you’ll want to check the operation of the O2 sensor. To do that, you’ll need access to a scan tool or oscilliscope. Check that the waveform is pretty steady. If the reading fluctuates then the sensor is likely bad and will need to be replaced.
never mind i just learned a lesson, about the latest o2 sensors called air fuel ratio o2 sensors. which is why im reading a 3.1 mv. its a whole new ball game for diagnosing it. my scanner wont read the new wideband styles.
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