AT bucking and snorting - with engine codes

Hi all -

I've got this new thing going on with my 2001 Cherokee -

Engine light came on a few months ago - code P0456. While driving to work in the a.m. - phoenix weather, my automatic transmission wouldn't shift into overdrive. Then it just kinda stopped. Had the codes reset.

Now this last week - same code came up - and now I'm getting an occasional shifting thing - transmission bucks at the get go - a good and quick 'clank', and a noticeable drop in power - and only for a split second. Not just from the get go either, but sometimes at freeway speeds and/or driving around town speeds - being 35 - 65 mph.

Any ideas where to start? I had the code read at a local auto parts store, which read the P0456, and a list printed out that says:
1) defective, loose, or missing fuel cap
(just replaced it BEFORE this latest round of codes)
2) EVAP canister broke, hose cracked or not connected
3) Purge or vent solenoid defective
4) vacuum leak at engine.

Where to start?

Inspect the hoses to/from the Evap Purge Cannister to the Leak Detection Pump under the hood.

The trans issues seem to be an electrical problem, Test the Throttle Position Sensor (TPS) and the CrankShaft Position Sensor (CPS/CKP). If the TPS is bad you can have shifting or stalling symptoms. The CPS can cause stalling. If the engine cuts out and returns, that can feel similar to a transmission problem.

The symptom and the code may not be related. An evaporate leak would not generally give you the symptoms you describe.

So if you have NEVER had this code before you replaced the gas cap, the cap is certainly suspect and you need to get another cap on there. Because there have been problems with certain aftermarket caps and evap. codes, if this cap is not one from JEEP, buy one from the dealer. You must rule out a cap issue before moving onto the other possibilities 2-4 as you listed.

As for the transmission, what is the history of routine transmission fluid maintenance? IE: changes? Is fluid at the correct level on dipstick, it should be red in color and should not smell burnt.

Also, the neutral safety switch can be involved in a failure to shift into overdrive, as well as the TPS.

That code wouldn't cause the transmission issues. For the code, check the vacuum and emissions related hoses. For the transmission, check the TPS.