code p0420
wdy
06-21-2005, 08:10 PM
I have a code P0420 coming up on my 97 s-10/4.3 liter. I know it is a 02 sensor but I'm not sure which one it is. the reader I used read Catylyst Effeciency Below Threshold (Bank 1). Is this the sensor before the catylyst? Thanks, woody
NoorAutoExpert
08-21-2026, 04:49 PM
No, P0420 doesn’t automatically mean the O2 sensor is bad. It means the computer thinks the catalytic converter on Bank 1 isn’t operating efficiently enough.
The system compares the O2 readings before and after the catalytic converter to determine its efficiency. A bad downstream O2 sensor can cause the code, but so can an exhaust leak or a failing catalytic converter.
I’d check the sensor readings and look for exhaust leaks before replacing either the sensor or the cat.
The system compares the O2 readings before and after the catalytic converter to determine its efficiency. A bad downstream O2 sensor can cause the code, but so can an exhaust leak or a failing catalytic converter.
I’d check the sensor readings and look for exhaust leaks before replacing either the sensor or the cat.
maxwedge
08-21-2026, 07:14 PM
Good info, but the thread is 21 years old!
silicon212
08-28-2026, 08:50 PM
Good info, but the thread is 21 years old!
off topic
It is hard to believe I have been on this forum for that long! It'd be nice to see it come back.
off topic
It is hard to believe I have been on this forum for that long! It'd be nice to see it come back.
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