94 aspire failed e-test

O2 sensor would be a cheap try. Does the check engine light ever come on? If so get the code to see if its an EGR problem. Your problem is high Nox so the EGR system would be a likely culprit. But a "lazy" O2 sensor could do it too.

If you have no codes I would go for the O2 sensor. The CO and HC are high too. Even though they passed, it looks a little rich. Heck, on that old of car an O2 wouldn't hurt anyway :)

Only the NOX failed? Too hot, too lean,or bad cat. The HC and CO numbers would tend to tell me it's not too lean. I'd make sure the thermostat etc is working right, and replace the cat.

The only thing that causes this is EGR. Most likely the passages going from the exhaust manifold to the EGR valve and then to the intake manifold are plugged up with soot. It is of course possible the EGR valve failed but a lot of people have replaced EGR valves only to see no change, you need to check the passages first.

Ted

Add EGR to the list

too hot too lean bad cat bad EGR

If it also fails at idle, scrub the EGR, as EGR does not come into play at idle.

Also, in regards to EGR, check all the vacuum actuation devices to be sure vacuum is getting to the actuator when required.

Like I said, put in a new catalitic converter.I'd say better than 80% of vehicles with high (not borderline) NOX emissions with no driveability problems are cat problems. The O2 sensor was basically a non-player from the very beginning. And the EGR valve is VERY easy to test and eliminate without changing it.

No way, no way! Even a reducing cat doesen't drop NoX down that much.

I do agree though that the cat is probably shot. Note that when he replaced the EGR that Nox went down and HC went up. Basically, what was happening before is the EGR valve probably wasn't working at all, so the engine was getting zero exhaust gas dilution, and so was probably burning a bit more of the fuel. With the EGR valve replaced now he's getting some dilution which is reducing engine efficiency just a bit, so the HC's are going up more.

But, I also still think he's also got plugged EGR passages. If the EGR valve was the only restriction, then replacing it should have dropped the Nox down far more I think.

Yes to both. Which is why I said in my earlier post he had to check for clogged EGR passages first.

With all the EGR valves I've dealt with, the test is rediculously simple. Apply vacuum to the valve when the car is idling and the engine should stall out. If you replace the valve and this test still doesen't cause it to stall out, then the passages are plugged.

I also think that besides the cat he has other problems. There's no reason a cat should fail in a 14 year old car if the engine is in tune. But if the engine is running rich or lean a cat will fail quickly. 14 year old cars like to develop cracked vacuum lines and all sorts of other things that will throw off fuel calibration beyond to where the engine computer can compensate. In all liklihood a new cat will allow him to pass but if the underlying problems aren't corrected, the new cat will be burned out in a couple years and he will be right back to where he is today.

Ted

Just for the record.....A lazy O2 can cause high Nox..... AND, testing an EGR is not always that simple, some have to have back pressure to open even with a vacuum pump applied (Idling in park won't get it). And there is more to the EGR system than just the valve and passages. It sure looks like the car is running rich, high CO can cause high HC without a real missfire (like from a bad plug or wire-he said he replaced both). I wonder if his CEL is comming on...???I wonder if the t-stat is stuck open.... And yes a new CAT could make it pass, but I would fix the real problem too :)

You forget where that car was made. The cat is marginal on that critter to start with. Anything over 8 years is a "gift" But yes, check all other operational parameters. It's not an OBD2, so it does not monitor everything as well as later models.

Agreed - BUT this is a 1994 vehicle. No backpressure. And a lazy O2 sensor causing high NOX will also put HCs up.