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Faulty Upstream Sensor Cause Error CEL?

If the upstream pre-cat sensor is bad could it cause a P0421 CEL error?

Not very likely…P0421 is pointing to a problem with the downstream oxygen sensor or the CAT itself…

Year, make, model, mileage is always helpful if you want a meaningful answer…

2001 Mazda Protege L4 2L 116,800

You can simply change the rear sensor and hope that cures it or have a shop with a scan-tool monitor the function of the sensor to verify it’s failure. Oxygen sensors have a design life of 100K miles. They cost $65-$85 dollars. Some are easier to change than others…

Look at the signals coming from the O2 sensors with a graphing scan tool or a scope.
Do a little research on how to interpret them.
That will reduce the guesswork.

My Mazda passed smog with one “Not Ready”. Happy, happy!!!