Question about 1999 Volkswagen Jetta
Check engine light is on and codes
po422 main caytalyst efficienty below threashold bank one
po300 ramdom mult. cylinder 1 misfire
po134 oxygen sensor bank one no activity
po303 cylinder 3 misfire
Sounds like two problems. P0300 and P0303 are possibly spark plugs. They could be a coil pack but I'm leaning toward a spark plug problem because of the other two codes - they're indicating O2 sensor problems. If the O2 sensors are screwed up, the car reverts to preprogrammed, rich fuel maps that send excessive fuel through the engine. The reasoning is, if the ECU can't trust the O2 sensors to determine how much fuel to burn optimally, it'll pump a bunch of extra fuel through the engine to keep it safe. Having too little fuel (aka running lean) can destroy an engine, so the ECU plays it safe and runs rich (too much fuel) instead. The consequences of rich running are relatively minor compared to lean running, but can and do tend to create excessive carbon buildup on the catalytic converter and O2 sensors, as well as fouling spark plugs.
First thing to do though is to swap the wires on your coil packs and see if you end up with a misfire code for the same cylinder #3. If you do, then it's the plug or wire. If it moves, it's the coilpack. Replace whichever part is faulty. You'll probably need oxygen sensors too, before you can clear all the codes.
Posted on Sep 02, 2008
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Posted on Jan 02, 2017
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