Question About 1991 Corolla Excessive Nox Emissions

October 6, 2006

I have a 1991 Corolla. It failed the emission test for NOx. The CO and HC readings were fine and well below the maximum allowed. The "check engine" light has never come on and there are no error codes stored in the computer. The vacuum hoses are all good and fastened tightly and vacuum gauge readings are all normal. Spark plugs, wires, cap/rotor, timing belt, and ignition timing are all good. I tested the O2 sensor with a meter, and it fluctuates as specified between 0.3 and 0.7 volts indicating that the computer is consistently mixing the correct fuel/air ratio. There is no EGR valve, MAF, or O2 sensor after the converter on this engine (a 4AFE 1.6 liter), so that wouldn't be the problem. The car drives fine with no hesitation upon acceleration, although it idles a little rough at times and I get an amazing 36 mpg on the highway even though the EPA estimate for the highway is 29 mpg. I don't know if this is because of an overly lean conditon, but like I said, the O2 sensor is reading correctly and there are no computer error codes coming up. The car has 55,000 miles on it and this is the first time it hasn't passed the NOx test. (however,it passed 2000 miles ago... mostly highway miles) Has anyone else had this problem or does anyone know what the problem could be? I'm thinking converter, but I'd like to verify this with someone else who has had the same problem.

October 10, 2006

This sounds exactly like the problem I had with the 'O2'/Lambda Censor on a AE111 '98 Corolla.

No fault codes showed, car had covered 50,000 mile and idled rough, but fuel consumption was fine.

Replaced 02 sensor and all was well.

Taken from 'SENSOR MANUAL'

"Replacing Oxygen Sensors / Lambda Sensors

Everybody knows that spark plugs have to be replaced periodically to maintain peak engine performance, but many people don't realise the same goes for oxygen sensors. As long the lambda sensor is working properly, there's no reason to replace it. But after 30,000 to 50,000 miles of being constantly bathed in hot exhaust gas, a build up of deposits on the sensor tip can make it sluggish. If there's enough clinker on the sensor tip, the sensor may produce little or no voltage at all. This produces a false "lean" signal that makes the computer think the engine needs more fuel, which it doesn't but gets anyway. This creates a rich fuel condition that kills fuel economy and sends carbon monoxide and hydrocarbon emissions soaring. The engine may also experience additional drivability problems such as surging or hesitation.

Code fault recording ?

If you don't find any codes, that doesn't necessarily mean the lambda sensor is okay. In many instances, a sluggish sensor may not be bad enough to record a fault code but will still be causing an emissions or drivability problem."

Just an idea !?

But please beware as I tried to help someone out with a simlar problem 2 months ago on this Forum and it appeared fault was MORE than just the lambda / C02 sensor; hence I am answering your question from MY own personal experiance.

Also I believe there are slightly different 'Smog' laws in America to my home country (Great Britain).