1991 infiniti G20 failed smog test on the CO & NOx

Yes, it could cause a problem for you. The timing, however, is set by the computer. Do they feel there is a timing belt issue? Basically, the engine is firing too early.

they did not say anything about timing belt but do you think being the timing is off could cause it to fail on the co and nox

Yes, I do.

well i had the timing set by a smog and repair shop and it still did not pass it lowered it some then they wanted to put it on a diagnostic for $90 after they just charged me $70 for timing it and a retest can u help .

Now it comes down to sensors, converter... and will need to be analyzed. I hate to say that, but with today's cars, that is how it is.

my girl friend told me that she had put some fuel injector cleaner in the gas tank the same day i took the car in to get smog could that have anything to do with not passing the test .

I believe it could have something to do with it. Run that tank of fuel out and then have it checked again. You may want to run a couple tanks through it first.