The check engine light will come on and flash then go off ,no codes are stored and no freeze frame data stored.the strange part is it only happens going down hill on long grades,never comes on four wheeling or up hills.I have replaced every sensor on the jeep even the fuel pump moduel.All parts are factory parts from dealer ,the only parts left to change are the cam syncronizer and E.C.U. .no loose or broken wires ,all grounds are clean and tight all plugs and connectors are clean and making contact,new coil pac, new spark plugs good compression on all 6cyl this is on a 2002 jeep wrangler 4.0 3sp auto ,it runs like a champ except for this problem,

Is the exhaust system stock and are there any exhaust leaks?
Big JIm <img src="/forums/images/graemlins/pfft.gif" alt="" />

ALL new magnaflow cat back system with new factory California approved cat ,I had the system checked for back pressure,system had less than 1/2 psi ,im told this is good for a Cal cat.It doesnt make sense I thought if the engine light comes on it should store a code .Thanks for the help ,anything that comes to mind please let me know .

You probably have a poor connection to the instrument cluster, it's very common. If your scan tool will read the cluster module, it usually will show a "loss of communication" fault. When this happens, the gauges may drop, and the warning lights may come on. Everything will still be running normally with the engine when this happens. Next time it does it, smack the top of the dash and see it the light goes out.
The fix involves removing the cluster and cleaning the male pins in the back of the cluster. You will see little black spots on the pins where they are corroded. You need to scrape the black spots with a small screwdriver blade. There is a new repair cluster connector available at the dealer (assuming they are still in business!)
Hope this helps!

Thanks good tip ,the problem is that it only seems to flash when Im on a long down hill section of highway ,I can wheel all day on the Rubicon trail and it never comes on until I get to a long down hill section of highway .I will pull and clean the cluster plug though,I have checked the gauge function right after the light flashes and they check out fine , I do this by holding the odometer reset button in and at the same time turn the key to start without cranking the motor over and the computer goes through and activates each gauge at diffrent voltages ,they always pass with A done code showing on the mileage readout. It just might be haunted........ THANK YOU FOR THE INPUT.

Man that long downhill problem sure sounds like exhaust.. If that is the only time it happens I'm still thinking the ox are getting a sniff they don't like on that long downhill.
Big JIm <img src="/forums/images/graemlins/pfft.gif" alt="" />

There would be a stored code/s in the PCM if the sensors were faulty or the readings were out of the norm. If the PCM commands the light on, it will store a code!

Losing the buss signal from the PCM to the cluster will make the light come on, but not store a code in the PCM. By checking for codes in the cluster control module, you can tell if it lost communication with the PCM. Most of the gauge readings and lamp commands come through the PCM first, then to the cluster over the buss wires. The gauge self test will show you that the gauges and lights work, not if you are getting communications or if the cluster has any stored codes.

How do I retrieve codes from the gauge cluster? do you think a auto repair shop can do this or is it a dealer thing? THANKS FOR THE INPUT .

You need a good scan tool, a cheap one will only pull emissions related engine/transmission codes.

Can you tell me what is a good one to buy ,my scan tool is a innova from napa ,someone said I need a DRB scan tool. I would rather buy the tool than pay to have it read for the same cost as the tool. thanks Martysocal.