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P0422 and P0304 Fault Diagnosis for 1999 VW GTI with 222,000 Miles

Model: 1999 VW GTI Fault Code: P0304 Posted: 2018-08-01 10:03

I own a 1999 Volkswagen GTI 2.0L engine with 222,000 miles. The vehicle consistently displays diagnostic trouble codes P0422 and P0304. I have already replaced both the catalytic converter and oxygen sensors, and performed a full tune-up. However, the check engine light remains on, and I am unable to pass smog testing. The original catalytic converter and resonator failed, which is why they were replaced. Despite these efforts, the codes persist. What could be causing this issue? Any guidance or troubleshooting steps would be greatly appreciated.

Related fault codes

P0304P0422

Comments (6)

Anonymous 2018-08-02 09:56

A P0304 code typically indicates a problem in cylinder #4 and may stem from one or more of the following: faulty spark plug or wire, defective coil pack, malfunctioning oxygen sensor(s), fuel injector issues, burned exhaust valve, failing catalytic converter, running low on fuel, poor engine compression, or a defective engine control module. For detailed information, see: P0304 code explanation.

Anonymous 2018-08-02 15:35

I have replaced all components listed above with brand-new parts, but the P030-4 code continues to appear.

Anonymous 2018-08-02 16:15

If spark and fuel delivery are confirmed as good, the next step should be a compression test or leak-down test on cylinder #4 to identify any internal mechanical issues.

Anonymous 2018-08-02 16:36

Thanks for the helpful input — this is very informative.

Anonymous 2018-09-21 20:47

The P0422 code has now become the only one present after a recent tune-up. This suggests an issue with the catalytic converter or exhaust system efficiency.

Anonymous 2018-09-22 11:57

Potential causes of a P0422 code include: a damaged or non-functional catalytic converter, engine operating outside optimal air-fuel ratio, or insufficient exhaust gas flow. For more details, refer to: P0422 code explanation.