I've been trying to figure this one out for days searching the car and forums. It runs perfect except when in drive at idle(at a stop light, drive thru, etc). The idle drops after about 8 seconds of sitting. It feels like it will die but never does. It just goes up and down. It sounds like it's being choked. I've done a full tune(OEM plugs, wires, fuel and air filter, checked all vaccum lines, etc) The CEL doesn't come on but it will pull the code p0420(Catalyst system efficiency below threshold) when its doing the funny idle. I saw that one guy wrote that the P0420 means it's time for a break. I believe it. This keeps me up at night just trying to think through it. I've seen similar posts from years past but have never seen anyone come to a conclusion and solve the problem. I've found no solutions. Please Help.

I've been trying to figure this one out ...

Did you try to clean you TB? I had problems with idle dropping very low right after hot start. Cleaned TB and it stopped. Not sure if it might help you, but its cheap to try...

I think PO 420 might come up for a few things that make the engine run poorly. In my case replacement of the front oxegen sensor cured it. So I can only speak for my car.

Not completely convinced the P0420 is relevant to the idle problem, but hey, what do I know.

 

Does your car have an idle air control valve? It might be worth your time to clean it, if so.

I would second looking at the IACV

 

I had the same symptoms and giving the IACV a good cleaning fixed it.

 

My car was throwing a different code than PO 420 but it is still worth a shot.

frankly if a 96 has not changed the front ox sensor it's time, and if the IAC has never been cleaned it's time for that too.

I think you're probably right about the front ox censor. I have 130 k and have never done it. I'll let you know if it works.

Thanks

I also agree on the fact that the front o2 sensor is old and probably could be replaced, however be aware that the P0420 code relates specifically to the rear sensor.

My stuff says it means the cat is operating below efficency. A lot of mechanics change the cats for this, but the cats were very expensive on my Forester.

I reckon anything that causes the cat to operate below efficiency can cause this. The front ox sensor is taking much more heat to control the mixture. The rear is just monitoring how the cat is doing and is in a cooler location.

I bought both front and rear sensors but have not installed the rear to see how long it will last.

Makes sense.

 

Thanks for the perspective.

Oh, one other thing I have heard is that an exhaust leak can throw it off and cause PO420 also. Usually it is pretty easy to tell if you have a leaking exhaust though.

If I recall my front sensor lasted about 110,000 miles or so before it first gave me a PO420. I cleared the code and in a year it came back a couple times before I cahnged the sensor.