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AutoMD Member

AutoMD ... 10/2/2011

2003 Mazda Protege5 Base 4 Cyl 2.00L


I get PO 421 code from my scanner Warm-up catalyst efficency below threshold ( bank1 )

1 Answer

AutoMD Member

AutoMD Member 10/2/2011

P0421 means Warm Up Catalyst Efficiency Below Threshold (Bank 1). P0421 is triggered when the downstream oxygen sensor on bank 1 senses that the catalytic converter is not functioning efficiently. Bank 1 is connected to cylinder number one exhaust.

Possible Causes:

Exhaust leak
Defective oxygen sensor
Defective catalytic converter

Possible solutions:

Repair exhaust leak
Replace oxygen sensor
Repair or replace wiring harness or connector of the rear oxygen sensor
Replace catalytic converter


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