* P0422: Main Catalyst Efficiency Below Threshold (Bank 1): This code indicates a problem with the catalytic converter on the exhaust system's bank 1 (usually the side with cylinder #1). Several things can cause this:
* Failing Catalytic Converter: This is the most common cause. The catalytic converter is breaking down and not efficiently converting harmful exhaust gases. Age and mileage heavily contribute to this.
* Exhaust Leak Before the Catalytic Converter: A leak upstream of the catalytic converter allows unprocessed exhaust gases to bypass it, triggering the code. This leak could be anywhere from the exhaust manifold to the cat itself.
* Oxygen Sensor Issues (Bank 1 Sensor 2): The oxygen sensor after the catalytic converter (sensor 2, bank 1) monitors its efficiency. A faulty sensor could misinterpret the converter's performance and throw the code, even if the converter is fine. A sensor contaminated with fuel or oil could also be a culprit.
* Air/Fuel Ratio Problems: An overly rich or lean air/fuel mixture can stress the catalytic converter and lead to failure. This might be related to the P0303 code.
* P0303: Cylinder 3 Misfire: This code means that cylinder number 3 isn't firing properly or consistently. Possible reasons include:
* Spark Plug: A worn, fouled, or damaged spark plug in cylinder 3 is the most likely culprit.
* Ignition Coil: A failing ignition coil, specifically the one for cylinder 3, can prevent the spark plug from firing correctly.
* Worn or Damaged Valve: Problems with the intake or exhaust valve in cylinder 3 can lead to a misfire.
* Injector Problems: A clogged or malfunctioning fuel injector for cylinder 3 won't deliver the necessary fuel for combustion.
* Low Compression: Low compression in cylinder 3 indicates a problem with the cylinder's sealing (rings, valves, head gasket).
The Relationship: While seemingly unrelated, the misfire (P0303) can *cause* the catalytic converter inefficiency (P0422). Unburnt fuel from a misfire passes through the catalytic converter, potentially damaging it and causing it to become less efficient. Therefore, fixing the misfire is crucial, and it *might* resolve the P0422 code as well.
Troubleshooting Steps:
1. Check Spark Plug and Ignition Coil for Cylinder 3: This should be your first step to address the P0303. Inspect for wear, damage, or fouling.
2. Scan for Further Codes: A more sophisticated OBD-II scanner might provide additional codes or data that can narrow down the problem.
3. Visual Inspection of Exhaust System: Look for leaks or damage in the exhaust system, especially upstream of the catalytic converter.
4. Check for Fuel Injector Issues (Cylinder 3): A fuel pressure test or injector flow test might be needed.
5. Compression Test: To rule out a mechanical issue in cylinder 3 (valves, rings).
6. Oxygen Sensor Check (Bank 1, Sensor 2): This should be checked *after* addressing the misfire.
It's highly recommended to take your 2000 VW Jetta to a qualified mechanic for proper diagnosis and repair. Attempting to fix these issues without proper tools and knowledge could lead to further damage or incorrect repairs. They'll have the equipment to perform the necessary tests and pinpoint the exact cause of the problems.
