Hi everyone. I am new to this site and new to Honda's, but not new to automobile repair. My son has a 91 civic DX 1.5 Liter. He is failing emissions with high NOx on the 25/25 test with 1569 reading and 1123 allowed and the 50/15 test with 1873 reading and 1242 allowed. He had higher readings earlier and I changed the O2 sensor because it was the cheapest place to start. He then failed with the above readings. I have not changed the cat yet because the previous owner welded everything past the manifold. Is there any other checks I can make before I change the whole exhaust system? I am running out of time to get tags. Thanks in advance for any help!

 

If it's a California model with EGR system, clean the EGR valve and the passages in the manifold. (The other 49 states model does not have an EGR system).

Very good chance the cat has been hollowed out.

 

This can be caused by running hot (overheating? thermostat stuck closed?) or lean (fuel filter clogged; malfunctioning FPR?). Also check the ignition timing.