Question about 2003 Mitsubishi Lancer

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2003 Lancer the service engine soon light keeps coming on, have changed out both O2 sensors and the gas cap, the code is P0421. The light comes on about 15 miles after filling up with gas

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I can see that you've done the right things, but something is still amiss... Mitsubishi says

A code P0421 may mean that one or more of the following has happened:

  • The catalytic converter is no longer functioning properly
  • An oxygen sensor is not reading (functioning) properly
  • A sparkplug is fouled
Measure the voltage at the oxygen sensor on Bank 1 (the rear sensor, or the sensor after the converter). In fact, it would be a good idea to test each oxygen O2 sensor while you're at it.

One thing to note is that many vehicle manufacturers offer a longer warranty on emissions-related parts. So if you have a newer car but it's out of it's bumper-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.

Posted on Sep 23, 2009

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