Hey Scotty, I have a 1993 Plymouth Colt GL 1.8l 4cyl. recently we have been trying to get it to ass emissions and we took it to a Mechanic at before we took it to them there was a new o2 sensor put in and the only thing the car keeps failing the NOx ppm part of the test is the only thing that keeps failing and the reading was for the 25/25 test was 2755 and what is allowed 878 on the 50/15 test it read 3148 and what is allowed is 970. Since we didn't know how to lower it we took it to the mechanic and the results of the test went up by 1000 for the 25/25 and 500 for the 50/15 but the other 4 test went down even though they didn't need to. What the mechanic did was set the timing for the distributor and idler back to factor settings and re-gaped the spark plugs as well as put on a new catalytic converter. what can i do to lower the NOx ppm.

well, replace the egr valve and if it's not that, replace the catalytic converter

@scottykilmer it doesn't have an egr and it already has a new cat

then do a wet and dry compression test of the engine FIRST.Could be that the engine is just flat worn out and will never pass without rebuilding

@scottykilmer it has a strong engine the compression is at 180 to 182