My daughter’s Jeep check engine light is on. Trying to get the car inspected so cannot be ignored. I think the code wasP0422, indicating it needed a new gas cap. We bought a new gas cap, cleared the code and it went away for a few days but before she could get it inspected, the light came on again. I saw similar problems suggestions were to get a better gas cap (we got one at autozone), or it could be something like a pump problem. We haven’t taken to a dealer - it’s a 2001
So what’s your question? Seems like you’ve done the research. The cheap Autozone gas cap kind-of worked. Maybe a trip to the dealer to get a better quality gas cap is in order. If that doesn’t fix it, you have a leak in the vapor recovery systems - not unusual on a 15 year old car. That leak would need to be located and repaired.
Take it to a good independent mechanic and be prepared to pay for his time to locate the leak.
Or search online forums to see if there is common leak point for these cars. I’ve made my repairs MUCH easier at times doing a little web research for things like this. See below for an answer to a P0422 code on a Jeep forum;
Hey, I have an 02 4.7 I had a po422 code, had major problems trying to find it. It turned out to be a leak off of the evaporator canister. You have to take off the rear bumper and you will see it on the drivers side there is the box with a few hoses coming off of it. I have 3 hoses that were cracked. I just cut them back and put hose clamps on them and no problem since
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I had a EVAP code on my 4Runner. Could have been a number of issues including gas cap. So I went to ADAP and bought a new cap. Code went away for a week. Went to dealer and paid 4 times the money for a new cap and never had the problem again.
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ok, thanks awesome. I just watched a youtube video and the hoses were the problem. Hopefully it will be one of the simpler fixes.
ok thanks. will try the gas cap solution, someone else suggested checking the hoses too.
There is a standard test, called a “smoke test” that can find leaks in evaporative emissions systems. Basically, a smoke machine pumps smoke into the EVAP system and it comes out where the leak is. It isn’t infallible, but it’s the best bet I know of for finding a hard-to-find leak. Many if not most shops are equipped to do this.
Any rust or roughness on the lip of the gas filler tube? That will prevent the cap, cheap or expensive, from sealing properly.
However, given the age of the vehicle, i suspect the problem is elsewhere.
See if there is a recall to reprogram the computer.
If you don’t want to spend any money . . .
I agree with the others . . . check those hoses
I believe the canister and leak detection pump is located at the left rear of the vehicle
On a relative’s 2004 Grand Cherokee 4.0, same body style as yours. ALL of the hoses back there were rotten and split. Yup. Every single one. You could actually hear a whooshing sound
Another possibility . . . and I’m no Jeep expert, so I’m not sure if this applies . . . even if the cap and hoses are good, if the purge valve can’t pull a sufficiient vacuum on the system, you’ll get that code
Am I reading this wrong. I see P0422 which is main catalyst efficiency below threshold. Did you mean P0442?
Keith is right , the PO422 is a catalyst code .
That reads to me like the OP doesn’t remember the exact code for certain, which suggests that it should probably be P0442. These errors are very easy to make. I’d make them all the time if I didn’t write everything down. I think he/she did an excellent job describing the problem, however. 
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P0422 does not apply to this vehicle, it must have been P0442, evaporative emissions system small leak.
The rubber hoses on Chrysler products during that period were terrible, I replaced many evap. harnesses under warranty (3 years). It is 110F here so perhaps we see these problems more frequently.
Yes, it was P442. I got a Mopar gas cap from the dealer and we went back to autozone so they could clear the code. The hoses will be next thing to check if that doesn’t do the trick.
Yes you were right, 442. Waiting to see if the new gas cap will work at least long enough to get it inspected…
Usually, you’re only allowed to have one incomplete readiness monitor, when you take the vehicle to be inspected
So don’t bother heading to the inspection immediately after clearing that code
Hook up the code reader/scanner and confirm the vehicle is indeed ready
[quote]… we went back to autozone so they could clear the code. [/quote]Well, shame on them for violating corporate policy. We are NOT permitted to clear codes.
Ok thanks for the advice!