NOx Failures

Most NOx (OXIDES of NITROGEN) failures are due to inoperative components such as leaking vacuum diaphragm on the EGR valve, broken vacuum lines, clogged passages in the intake manifold, misadjusted ignition timing, engine temperature or a bad catalyst converter. The only purpose of the EGR (EXHAUST GAS RECIRCULATION) system is to keep temperature in the cylinder from getting too high. If the temperature is too hot, it will cause the cat converter to overheat and melt the catalyst material and besides causing the emissions output to go up it will also restrict the exhaust to the point of causing a drivability problem.


Below are three examples of clogged EGR ports on a Honda 4cyl engine,



This car had been taken to a shop that had diagnosed it as having a bad EGR valve, so $200.00 later it failed it's retest, Proper and thorough diagnosis would have saved this customer a lot of time and money. Here is a file that you can download pertaining to EGR issues on Honda's:

EGR Technical service bulletin

Here is another example of why not only ignition timing is very important for smog or NOx emissions but also for the protection of the engine as well


As you can see it's very important  that all of the systems are working properly.


When do I change my oil

Freon changeover

Why did I fail my emission test

Wiring Diagrams 

TSB's and Recalls


Oxygen Sensors

Timing Belts_ 1970-1997 Domestic & Imported Cars

Brake Squeals



Last Updated on 3/15/2006
by Scott Throneberry