Replaced wires and spark plugs on a 99 Grand AM GT. Running rough, check engine light comes on and catalytic converter runs very hot.

The light blinks to let the driver know the problem is serious enough to damage the catalytic converter.
If you know for a fact cylinder 4 is getting a hot spark, it could be the fuel injector. Code 304 means there is no power from cylinder 4.

what mileage,spark plugs,fuel filter[,coils module,this #1]

A code P0302 may mean that one or more of the following has happened:Faulty spark plug or wireFaulty coil (pack)Faulty oxygen sensor(s)Faulty fuel injectorBurned exhaust valveFaulty catalytic converter(s)Running out of fuelPoor compressionDefective computerPossible Solutions
If there are no symptoms, the simplest thing to do is to reset the code and see if it comes back.
If there are symptoms such as the engine is stumbling or hesitating, check all wiring and connectors that lead to the cylinders (i.e. spark plugs). Depending on how long the ignition components have been in the car, it may be a good idea to replace them as part of your regular maintenance schedule. I would suggest spark plugs, spark plug wires, distributor cap, and rotor (if applicable). Otherwise, check the coils (a.k.a. coil packs). In some cases, the catalytic converter has gone bad. If you smell rotten eggs in the exhaust, your cat converter needs to be replaced. I've also heard in other cases the problems were faulty fuel injectors.

this obd2 trouble code table can help you understand the meaning of code.
and for p040,you can try:

Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
Inspect the downstream heated oxygen sensor (HO2), replace if necessary
Replace the catalytic converter
for p0306, you can try
If there are no symptoms, the simplest thing to do is to reset the code and see if it comes back.
If there are symptoms such as the engine is stumbling or hesitating, check all wiring and connectors that lead to the cylinders (i.e. spark plugs). Depending on how long the ignition components have been in the car, it may be a good idea to replace them as part of your regular maintenance schedule. I would suggest spark plugs, spark plug wires, distributor cap, and rotor (if applicable). Otherwise, check the coils (a.k.a. coil packs). In some cases, the catalytic converter has gone bad. If you smell rotten eggs in the exhaust, your cat converter needs to be replaced. I've also heard in other cases the problems were faulty fuel injectors.

P0304 Cylinder 4 Misfire Detected,A code P0304 may mean that one or more of the following has happened:

• Faulty spark plug or wire
• Faulty coil (pack)
• Faulty oxygen sensor(s)
• Faulty fuel injector
• Burned exhaust valve
• Faulty catalytic converter(s)
• Running out of fuel
• Poor compression
• Defective computer
• If there are symptoms such as the engine is stumbling or hesitating, check all wiring and connectors that lead to the cylinders (i.e. spark plugs).. I would suggest spark plugs, spark plug wires, distributor cap, and rotor (if applicable). Otherwise, check the coils (a.k.a. coil packs). In some cases, the catalytic converter has gone bad. If you smell rotten eggs in the exhaust, your cat converter needs to be replaced.

Start the engine and let it idle.Pull off the spark plug wires one at a time(wear leather gloves or use a plug wire tool to avoid being shocked).The wire that you pull off that makes no difference to the way the truck runs is the problem.If a set of plugs and wires don,t fix the problem,remove the suspect plug and check to see if it is firing by laying the metal portion against a good ground and then have someone turn over the engine while you watch for spark,if you have spark then you likely have a plugged catalytic converter,if there is no spark then you need a coil pack.

bro, check ur main relay. its under the dash. replace it, check also ur distributor(replace it). and also your ECU

The run rough problem is due a cyl.mis firing,it becaused buy a bad plug,plug wire,coil,or even the ing.module. the servive eng.light flashing is warning you that you have a cyl.misfire that can cause damaget to your catltic converter if you continue to drive it.with may lead to why you think you have a exhaust problem,it may be colgged.

your engine seems to be running rich. that may be do to a bad spark plug, spark plug wire, bad distributor cap. If you have a 22re you should check for trouble codes by jumping TE1 and E1.


You should also check your engine timing. With a timing light. The timing marks are located on or above the oil pump. keep in mind in order to get timing right you must have TE1 and E1 jumped. Set to 5 degrees advanced. If your engine is improperly timed it will accelerate slowly, and get worse mileage.

Also adjust/Check engine valves appropriately. they will cause low power.

Also does it misfire at idle?

have you the intake off? if so check for trash in the fuel ejectors. I had the same problem, I just replace the fuel meter body with one from the junk yard