I have a 2003 Ford Escort ZX2. Recently, I’ve begun to notice issues with the RPM. When I’m not actually giving it gas, such as idling or coasting in neutral, I notice that one of four things start happening: 1) the car stalls outright, 2) the RPM drops from 1000 to around 500 and stays, 3) drops to 500ish then recovers after a few seconds, or 4) acts normal. Which one is completely random. I am getting OBD codes p0171 and p0420, which points towards either oxygen sensors or the catalytic converter. However, I’ve been told that it might be the idle air control valve too. Which seems more/less likely? I don’t have a lot of money to commit to repairs at the moment, so I am leery of playing trial and error.
You might or might not be due for a cat converter, but that’s not causing your wandering idle.
The oxygen sensor could, but the P0171 code simply says there’s a lean condition. An O2 sensor is only one possible cause. But I don’t think that’s your wandering idle problem.
I think your IAC is the most likely cause.
Also look and listen for a vacuum leak.
Good point, MG.
A vacuum leak can be verified by spraying starter fluid in likely leak areas. If the engine revs in response to the spray, you have a leak.
If it does end up being the IAC, you can often clean it by removing it from the car and cleaning it with some throttle body cleaner. They very rarely actually go bad but very often do clog up.