I have a 1996 Jeep Grand Cherokee 4.0 4WD Auto trans with 185K on it. I had to change the computer and ever since it throws a code of P0420 every 80 miles which it never did before. It runs fine. I erase the code but it always pops up again. I wonder if it is due to the computer being flashed with the latest software and the Jeep is not meeting emission standards under certain driving conditions as in 96 Jeep had to seriously tweek the engine in order to pass emission standards. The freeze data shows short term trim of -2.2, -7.8, -0.8, -3.4 and long term trim of -2.2,-6.3,-5.1,4.9. I am getting about 23mpg overall combined. In 96 Jeep did not have a MAF sensor, only a MAP sensor so maybe it is the MAP sensor ? The 02 sensors are good. I really think it is that the flashing of the computer causing this. Please let me know your thoughts. Thanks.
Well I would check for leaks both fluid and vacuum leaks and test the fuel pressure and check the injectors you have a decent bit of miles of her so if the fuel filters never been changed you might as well. It could be something like the MAP sensor or bad wiring but it's gonna take a mechanic you trust with a high level scan tool to check for anything further like that. GL on making it 200k miles
Have you inspected the catalytic converter? Scotty has a video on how to do this. A reflash generally does not cause the problems you are describing. The ECU is still designed to operate within certain parameters and to some degree it thinks for itself. A reflash does not install fixed parameters in the way you are describing. I would recommend checking out Scotty's video on catalytic converter issues.
Thanks guys.