P0421 Warm Up Catalyst Efficiency Below Threshold

Description and meaning of DTC p0421

This diagnostic trouble code (DTC) is a generic powertrain code, which means that it applies to OBD-II equipped vehicles. Although generic, the specific repair steps may vary depending on make/model. Basically this means that the oxygen sensor downstream of the catalytic converteron bank 1 is detecting that the converter is not working as efficiently as itshould be (according to specs). It is part of the vehicle emissions system.

p0421 diagnostic trouble code symptoms

You will likely not notice any drivability problems, although there may besymptoms. The code may be more likely to occur after the engine has had numerouscold starts in the past 1-2 days.

DTC p0421 - possible causes

A code P0421 may mean that one or more of the following has happened:The catalytic converter is no longer functioning properlyAn oxygen sensor is not reading (functioning) properlyA sparkplug is fouled up

How to fix OBD-II diagnostic trouble code p0421

Measure the voltage at the oxygen sensor on Bank 1 (the rear sensor, or thesensor after the converter). In fact, it would be a good idea to test each oxygenO2 sensor while you're at it. One thing to note is that many vehicle manufacturers offer a longer warrantyon emissions-related parts. So if you have a newer car but it's out of it'sbumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It's worth checking into.

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