I have a 1999 chevrolet cavalier 2.4L DOHC 4cyl, that has failed an emissions test twice. I've changed the air filter, and spark plugs. Also I connected an OBD2 reader and it came back zero codes, and the check engine light is not on. So if anybody has any info on why it failed.

Why did it fail ? When they give you results, what readings were out of standards

Exactly... nobody can offer a guess without knowing what it failed for... what its readings were measured at and what the acceptible range is for where you live. You would have been given a printed report by the inspection station.

I'll take a guess (since I've actually had to deal with these issues)
You probably had to sit in line waiting for your turn and the catalytic converter cooled down enough to lose efficiency. Next time, when you see it's nearly your turn up, rev the engine to about 15-1800 RPM and hold it there. That high-idle should be enough to 'wake' the converter and then you should pass your test.
Let me know what happens, k?

The catalytic converter is likely marginal and needs to be replaced. It may have been destroyed by a failing Ignition Control Module (ICM), which allowed raw gas to contaminate the converter. I recommend replacing ICMs every 125,000 miles or 10 years, whichever comes first. Failing ICMs often don't throw a code.

First emission test: 15mph NO(ppm) MAX 459, AVE 67, MEAS 897 (FAIL)
25mph NO(ppm) MAX 746, AVE 62, MEAS 167 (PASS)

Second emission test:
15mph NO(ppm) MAX 459, AVE 67, MEAS 587 (FAIL)
25mph NO(ppm) MAX 746, AVE 62, MEAS 14 (PASS)

So with this info can anyone tell me what the problem might be?

Among the potential causes of non-compliant NOx readings are:

. Misfire condition--think plugs, plug gap, and ICM failure.
. Malfunctioning or improperly adjusted EGR valve.
. Failed oxygen sensor.
. Leak in exhaust tubing upstream of converter.
. Excessive carbon deposits in combustion chamber.
. Improper spark advance.
. Blocked coolant passage.
. Overly lean air-fuel mixture.
. Damaged cold air duct.
. Failed or malfunctioning catalytic converter.
. Corroded or damaged engine sensor electrical connections.

First emission test: 15mph NO(ppm) MAX 459, AVE 67, MEAS 897 (FAIL)
25mph NO(ppm) MAX 746, AVE 62, MEAS 167 (PASS)

Second emission test:
15mph NO(ppm) MAX 459, AVE 67, MEAS 587 (FAIL)
25mph NO(ppm) MAX 746, AVE 62, MEAS 14 (PASS)

So with this info can anyone tell me what the problem might be?

So how does the emission test fail at 15mph and pass at 25mph?

A failing ICM, which shows up at low speeds in your case, could explain it.

Pvc,catalytic converter,fuel injection,vacuum line connections,fuel cap functional,MIL,ignition spark controls,O2 sensors and connectors,other emission related,fuel evaporative controls,fuel tank cap visual,wiring of other sensors or switches,liquid fuel leaks.ALL OF THESE PASSED


(N/A:)VISUAL EGR,FUEL EVAP TEST,THERMOSTATIC AIR CLEANER,AIR INJECTION SYSTEM,FUNCTIONAL EGR,FILL PIPE RESTRICTOR, CARBURETOR,IGNITION TIMING:TDC
SO IT COULD BE THE( IGNITION CONTROL MODULE?

Reflect on whether any of this fits concerning your ICM:

When not replaced, vehicle could experience no- or hard start, long crank time, misfire, lack of power, inconsistent transmission shift points, spark plug fouling, catalytic converter failure, poor fuel economy or CEL/MIL illumination.

Make sure the intake manifold gasket does not leak. This could cause a lean air-fuel mixture.

Personally, I would make sure the ignition system is perfect. This involves the ICM, coil packs, plugs (AC Delco only), plug gap, and spark plug wires. I would replace these items, if they have been in for 125,000 miles or 10 years. Only you can tell if the car is worth replacing these items.

Another area you definitely want to check out is the Exhaust Gas Recirculation (EGR) System. This system keeps NOx levels lower by reducing the temperature of the fuel mixture. Consider replacing the valve, gasket, and hoses.