Yo Guys, I just recently picked up a 1993 Jeep Grand cherokee Laredo, 5.2L V8 with only 66000 miles on it(original). Just the other day the engine started to surge from about 1000rpm to 1500rmp just back and forth while parked. Then the check engine light came on. I took it to my friends shop where we found a Code 21, O2 sensor high circut. We found a leak at the PCV valve and replaced that along with the PCV hose. All was good until yesterday when the check engine light came back. I think I need to replace the O2 sencers which I am going to do today but is there anything else I should be looking at?

Thanks for your help.

recheck the codes.
heres how to do it........

Turn the key ON, OFF, ON, OFF, ON - leave ON this time, the CHECK ENGINE light will come on then go off shortly. Then start to flash the codes, count the numbder of flashes, each code will be in double digit form with a short pause between digits and a long pause between codes.

example: | = 1 CHECK ENGINE light flash
| pause | | = 12,
pause pause pause pause pause
| | | | | pause | | | | | = 55,

Code 12 - Power to PCM was lost within the last 50 ignition cycles, Battery was disconnected.

Code 55 - End Of fault codes.

***NOTE: On Jeeps equipped with DIGITAL ODOMETER the codes will appear in the odometer. P****

Example:
P0300 - Multiple Cylinder missfire.

recheck the codes.
heres how to do it........

Turn the key ON, OFF, ON, OFF, ON - leave ON this time, the CHECK ENGINE light will come on then go off shortly. Then start to flash the codes, count the numbder of flashes, each code will be in double digit form with a short pause between digits and a long pause between codes.

example: | = 1 CHECK ENGINE light flash
| pause | | = 12,
pause pause pause pause pause
| | | | | pause | | | | | = 55,

Code 12 - Power to PCM was lost within the last 50 ignition cycles, Battery was disconnected.

Code 55 - End Of fault codes.

***NOTE: On Jeeps equipped with DIGITAL ODOMETER the codes will appear in the odometer. P****

Example:
P0300 - Multiple Cylinder missfire.

Did recheck today, came back with the same O2 sensor code 21. I replaced the sensor and found a faulty PCV valve which I will replace tomorrow morning. All in all everything is working wonderfully! :smile:

Thanks for your input.

OK CODE 21, I just had this happen to me, it wasn't the O2 Sensor tho, it was a bad wire at the PCM was broken,

It was the BLACK/ORANGE (K41 wire), PIN #41 (1st wire on the bottom in from the right)... I didn't notice the wire was broken until I disconnected the connector from the PCM and flipped it over (I was trying to check for resistance at the wire but it was too hard to get too without flipping it over)..
Check that wire at the PCM, if it is OK, then check for resitance from that PiN on the connector to that PIN in the O2 Sensor Connector.,

OK CODE 21, I just had this happen to me, it wasn't the O2 Sensor tho, it was a bad wire at the PCM was broken,

It was the BLACK/ORANGE (K41 wire), PIN #41 (1st wire on the bottom in from the right)... I didn't notice the wire was broken until I disconnected the connector from the PCM and flipped it over (I was trying to check for resistance at the wire but it was too hard to get too without flipping it over)..
Check that wire at the PCM, if it is OK, then check for resitance from that PiN on the connector to that PIN in the O2 Sensor Connector.,

Ok, Well the O2 sensor was bad (readings all messed up), so I did replace that and I fixed the PCV leak. Instead of using the plastic one I matched it up with a nice metal one (heavy duty). Anyway everything checked out Ok and I was nice and happy unitl the Check engine light came back on today!! I hooked it back up and there were no codes everything was fine all the readings were accurate (go figure?), I did however find that when you hit/move the little computer thing it sends the engine all out of wacked (engine surges to about 1100rpm in Idle) so I have a feeling I'm going to need to replace the "brains" of the car or there is something wrong with the wiring, either way still more work I have to do. Any Ideas would be greatly appreciated.

Ok, Well the O2 sensor was bad (readings all messed up), so I did replace that and I fixed the PCV leak. Instead of using the plastic one I matched it up with a nice metal one (heavy duty). Anyway everything checked out Ok and I was nice and happy unitl the Check engine light came back on today!! I hooked it back up and there were no codes everything was fine all the readings were accurate (go figure?), I did however find that when you hit/move the little computer thing it sends the engine all out of wacked (engine surges to about 1100rpm in Idle) so I have a feeling I'm going to need to replace the "brains" of the car or there is something wrong with the wiring, either way still more work I have to do. Any Ideas would be greatly appreciated.

Most likely bad wires, remove the connector from the PCM ("BRAIN") and check all the wires, more then likely there are a couple bare/broken wires in there...