Question about 1998 Mazda 626
"FWIW, according to the Mazda factory service manual, the P0421 code indicates "Warm up catalyst system efficiency below threshold".
The DETECTION CONDITION for this code indicates "Number of heated oxygen sensor (rear) inversion becomes closer to that of heated oxygen sensor (front)".
POSSIBLE CAUSES are:
Warm up three way catalytic converter deterioration
Leakage exhaust system
Heated oxygen sensor (front) malfunction
Heated oxygen sensor (rear) malfunction
Mass air flow sensor malfunction
Throttle position sensor malfunction
Vehicle speed sensor malfunction"
I copied the above from a discussion group on the PO421 code. The code can be caused by any of the above listed problems. The vehicle should be further inspected and diagnosed, so you do not have components replaced that do not need to be replaced.
Posted on Apr 16, 2014
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Posted on Jan 02, 2017
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