Okay, here it is in plain English…If you failed because of NOx, it’s almost guaranteed to be one of these three reasons:
- Ignition timing is advanced too far.
- EGR system is not working correctly.
- 3-way catalytic converter has failed.
Obtain an Emissions Control Systems Service Manual for your car. This is likely to be a separate manual from the regular Service Manual. It will contain instructions on how to “read the codes” and what those codes mean. It will also outline, in detail, a troubleshooting flow-chart for diagnosing high NOx emissions test failures. A step by step process of elimination.