Emission test failure 91 Regal

Recently I purchased a 91 Regal for $200.00. this car has been sitting for approximately 3 months. It failed emission test in MA for high NOx (read 4.40 test requires 2.50). I was told by the guy at the garage that it was the C. converter and it would take 450.00 to fix. I decided to accept the rejection sticker and drive it to see if a code would surface. Eventually the code identified the EGR. I replaced the valve and went back to the shop to test emission. The NOx still failed (read 2.94 test requires 2.50), but was significantly lower. Still runs a little lean. Not sure what to do now. Can the C converter be the other problem? I just tried the gas additive, but it hasn't run through the system yet. Could the injectors possibly be the problem?

You are not that far off in NOx readings. The CAT only works efficiently when it is up to operating temperature so make sure your car is fully warmed up before going back in for the emissions test.

The CAT convertor would be the next likely choice. The first stage is called the reduction catalyst which reduces NOx. Other possibility since it is running lean would be to try a new 02 sensor. Its important to ensure fuel injection system is at 14.7:1 air/fuel mixture.

$450 is too expensive for a CAT and labor, even a OEM GM. You should consider going with an aftermarket obd-i/ii compliant CAT convertor which should be a lot less. Good luck!

If your cat was defective most of the time you would smell rotten egg like ordor and all your emission factors would fail.Usually NOX failure is defective EGR , clog egr line ports due to carbon built up and the main trouble maker car is running too hot usually 220 degrees ++.Make sure your car thermostat,cooling fan,radiator and all the cooling components are working.