How to pass nitrous oxide emission testing?

We drove our '92 Honda wagon 6 miles to the emission testing station, then waited in line an hour, with our engine idling, before getting tested and barely failing the nitrous oxide emission. (The others - CO and particulates - passed with no trouble, as usual.) Any suggestions for just retesting and passing? If the car doesn’t really need work, I would prefer not driving it 25 miles to the garage and paying lots of money.

NOx is created when combustion temperatures get too high. The component that’s suppose to keep combustion temperatures under control is the EGR valve and it’s circuit. There are other things that can cause excessive NOx emissions such as an engine that’s running too hot, an engine that’s running too lean, or an engine where the ignition timing is off. But the EGR valve and it’s circuit are the first things to check when failing an emissions test for excess NOx.


Thank you, tester. Someone I know suggested that it would be good to run a tank of high octane gas before taking it back for retesting. I always use the lowest octane. Is that likely to have any effect? I can imagine that sitting idling for an hour might have run up the temperature, too. It is a little overdue for an oil change. Would taking care of that possibly help, too?
Forgive my ignorance. I really appreciate knowledge being shed on my problems.

No. Running a higher octane fuel won’t help. And in fact may raise the hydrocarbon emission level due to incomplete combustion.

You need to have someone check to make sure that the EGR valve is functioning, that the EGR valve is receiving control vacuum, and that the EGR valve is receiving and controlling the inert exhaust gasses going into the engine.


I had a 1989 Mazda. It’s systems are much like your 1992 Honda. It failed NOx emissions. The checks on the egr valve control were inclusive. Realizing that ignition timing has an effect on NOx emissions, I loosened the distributor and retarded the timing a few degrees. Long story short, it passed emissions retest with flying colors.