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I have a 1996 contour 2.0L 4 cylinder that has had the check engine light on for a couple of weeks now. When it first came on i scanned to see what codes were there. The scanner picked up trouble code P0420
Catalyst System Efficiency Below Threshold (Bank 1) so i erased the code to see if it would come back again. It did after a couple days (possibly how long it would take to run the catalyst efficiency test again). The car still seemed to run fine so i left it alone for now. Yesterday i was driving it and when i got to the highway and started accelerating the engine started misfiring whenever i tried to accelerate over around 70km/hr and the check engine light started flashing. I drove it home slowly and noticed on the way that the light stopped flashing whenever i wasn't trying to accelerate. When i got home i scanned it again and it now comes back with codes P0420
Catalyst System Efficiency Below Threshold (Bank 1) and P0302
Cylinder #2 Misfire Detected.

I own the proscan software and adaptor for access to obdii live sensor info if this is useful. However I am not experienced enough to know what is normal for each sensor. I am not sure what the best plan of attack would be for this problem and any help is greatly appreciated.

JoeShmoe

Don't know what those codes mean but I would go ahead and check for a vac leak as well as changing out your spark plugs.

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'98 Contuor SE, 2.5 V6 Auto Silver
'08 Altima 2.5S, Auto Super Black

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Update: I measured the resistance of the secondary windings of the coil and they were both aprox. 12-13kohm which means that the secondary windings are good according to autozone specs. Then i pulled the plug wire off of cylinder 2(the one with the misfire) and found water laying around the spark plugs of cylinder 1 and 2.

I believe this was caused by when i changed the plug wires about a month ago the new ones were a bit taller and didn't seal perfectly on the valve cover. Once i get the water out i plan to take the spark plugs out and checking for any fouling. I will post back with results from that.

RoB51988

Misfires can be caused by a number of things. Bad compression, faulty ignition system like coil wires and plugs. Some times when this happens it can be really bad for the catalytic converter. The P0420 is obviously before or after the converter but Im not sure which one it is right now.

The misfire may be causing the O2 sensor to get faulty readings.

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Update: I dried out the space around the spark plugs that had water in it before, and made sure the plug wires had no water in them. I started it up and took it for a drive with no problems, no hesitation at all. Then I took it for a drive later on and code p0302 came back again with flashing check engine light.

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Update again: poured some injector cleaner in the gas tank and took it for a drive on the highway and car seemed to accelerate fine with no hesitation. Hopefully i just had a dirty injector in cylinder #2. Would this be plausible for all my symptoms posted before?

Davescort97

Yes. The misfire due to the water in the plug housing was causing too much raw fuel into the cat. More than it could handle. This could be the reason of the cat code. Seems to me the fuel injector was plugged up causing another misfire. The injector cleaner fixed that. If it is running fine now you have fixed the problem. You had a good grasp of the problems. Try the simplist things first and you did.