My wife's 96 Outback threw code P0304 (Cylinder #4 Misfire) last night after sitting at the airport for 2 days. The light went off by itself today. I ran the code when I got home this afternoon. It has also been throwing code P0420 (Catalyst below efficiency) about once every 4 weeks also after sitting for extended periods.

 

Is this anything to be overly concerned about?

 

Thanks,

 

John

The cylinder misfire code could be as simple as moisture in the gas - if it reoccurs, more diagnosis is required. The catalyst below eff. code could be the oxy sensor after the cat converter (there is 2 on your car - 1 b4 the cat and 1 after). How many miles on the car and have the oxy sensor(s) ever been replaced?

The car has 110K and to the best of my knowledge has never had the O2 sensors replaced. She bought the car in 98 with 67k

I would start with replacing the oxy sensor after (downstream from) the cat and clear the codes. Drive as usual and see if any more codes pop up.

Ignore the converter code for now. First you need to determine the misfire code. I would suspect plug wires or a failing valve/guide. It is easiest and cheapest to simply replace #4 plug wire. If you have access to a leak-down tester you should check #4 cylinder.

 

As far as the converter is concerned it is common for failure. Yet you have more pressing concerns with the mis-fire.

Ordered some new wires and plugs. So far the code has not re-emerged. I put in a bottle of fuel system cleaner as well. I'll swap out and see what happens. Car seems strong, so it may have been an anomoly.

 

Thanks for all the input.

 

John