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P0421 Diagnostic Trouble Code Discussion for October 2004

Warmup Catalyst Efficiency Low (Bank 1)

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Trouble Ticket Help Requests for DTC P0421
Help Request Posted
1999 Mazda Miata 10th Anniversary 1.8L Misfire, MIL Lamp on VIN: JM1NB3537X
P0421: bank 1 cat efficiency. This car came in about 2 months ago for an MIL issue. Found 2 codes PO300 and PO421. Inspected plugs and wires and found carbon track marks on number 2 wire. We replaced plugs and wires with OEM parts and found quite a few posts about bad converters in the arc...
October 26, 2004