Catalyst efficiency below threshold bank 1 is a PO420. But i would check the oxygen sensor out first that also will cause a check engine light .A cheap one can fail but to check yours take it out and heat the end with a torch with and check it with a voltmeter. I would google that first if your not familiar with that method.Some vehicals take about 30-60 miles before the code will pop back up but with newer vehicals it will pop the code right away.
SOURCE: fault code po420 ; is problem sensors or is it the cat conv
The only thing that will set this code is the catalytic converter or the oxygen sensor behind it. Way to tell if it is the sensor is to monitor voltage reading with scan tool and see if it is fluctuating the same as the upstream oxygen sensor. If it is the converter is "free flowing" and will have to be replaced. If the sensor is "flatlined" then it is most likely bad and setting the code.
SOURCE: 2000 corolla with trouble code po420
If the code has cleared on its own then you need to disconnect the battery and re-connect it. This will clear the codes out of the memory. If you don't want to do this you can go to an auto parts strore and have them hook the little computer up to your car and they can clear out the code as well.
SOURCE: check engine light on computer codes 171 174
switch, we run off snitches. 171 174 banks 1 & 2 Lean. Most likely cause are Upper Intake Manifold Gaskets, failed seals. It's not 1990 on. It's 1996 and on OBD2.
SOURCE: 2001 VW Golf 2.0 throwing error codes P0103 & P0172
Go after the Egr valve, if its sticking then there is a vacumn leak.
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