[1990C4S]

I agree with love911.

I had a car with really high NOx, everyone said change the cat. It found an open vacuum line and cut my NOx by about 80%. A smoke test on the intake system is cheap and an easy way to find vacuum leaks. Your car is lean, so I think you have a vacuum leak.

Another VERY IMPORTANT issue: does the car have the factory chip? My 89 will not pass emissions with a Steve Wong chip installed, I have to go back to the factory chip to pass the NOx test. In fact Steve suggested I remove it when I told him I failed on NOx.

Also, I would pull the O2 sensor off the car and do the propane torch bench test. If it passes that test re-install it.

The unplugged injector won't work here, your NOx is too high, you are 50% over on the low speed test.