P0505 > Bad ECM

I got a check engine light a couple weeks ago. Scan tool showed P0505 Idle Air Control Circuit. It had a low idle (400-500 rpm) when cold but when warm it idles fine. Once it got colder it began stalling. Had to use the throttle to keep the idle up to warm up the engine before driving.

I swapped in two known good sensors. Problem got worse. I found out that that the computer is not controlling the IAC. Took it to a mechanic and learned that my IAC was bad however so is the ECM...

Just a heads up for a P0505 code. If you swap the sensor its either wiring, or the ECM. My mechanic said he has seen the ECM go bad in a lot of Cherokees before any wiring.

A re-manufactured or refurbished ECM is about $200... just spent $250 on diagnostics and a sensor... ARGH!

I would get a second opinion on this one. Did you test for voltage at the sensor connection? If present, the PCM is working. It is also possible that a blown fuse is present, often due to shorted wiring.

Please keep us posted.

It has been my experience that the XJ PCM is very rugged, and that mechanics with poor diagnostic skills will blame the PCM for their lazy detective work. Wiring is simple and cheap to inpect and diagnose, the PCM not so much. Always start any diagnostics with the basics, grounds, wire continuity, and wire connections first.

Around here at the Pick-n-Pull junkyard good used PCM's are $20, I have several spares and they all work perfectly.

Low idle, stalling, and having to use the gas pedal to start all point to low voltage.

For 1996+ Jeeps, the OBD-II engine computer (ECU/PCM) re-boots, and the idle settings are deleted, when you: (1) let the battery run down by leaving the lights or radio on, (2) disconnect the battery for an extended period of time, (3) when the battery goes bad and needs replacing, (4) when the alternator is not properly charging the battery, (5) or when poor battery connections result in a voltage drop at the PCM or poor battery charging.

Loose, corroded, or damaged battery cables or ground wires may also cause or contribute to the problem.

Your symptoms are: (1) having difficulties starting the engine without depressing the gas pedal, and (2) the engine will have a low idle and probably stall unless you keep your foot on the gas pedal. (3) The ECU will relearn the idle settings after a short period of driving and the engine will start and idle normally.

Begin with basic trouble shooting of the start and charge systems. Remove, clean, and firmly reconnect all the wires and cables to the battery, starter, and alternator. Look for corroded or damaged cables or connectors and replace as needed. Copper wires should be copper color, not black or green. Do the same for the grounding wires from the starter to engine block, the ground wires at the coil, and the ground wires from the battery and engine to the Jeep's frame/body. You must remove, scrape, and clean until shiny, the cable/wire ends, and whatever they bolt to. Jeeps do not tolerate low voltage, bad connections, or poor grounds.

If the symptoms reoccur, you need to have your battery and/or the alternator load tested at a repair shop or auto parts store. This is usually a free service they offer to bring in potential customers. Even with low battery voltage, the engine will start, but the ECU can re-boot multiple times and a new battery may be required.

Cleaning the Idle Air Controller (IAC), regular tune-ups, and using fuel injector cleaner should be part of your routine preventative maintenance for smooth starts.

I've seen that code a time or two and if a new IAC doesn't fix it, the PCM is bad as well. The old IAC has burnt out the drivers in the PCM.

I clarified that my new IAC 'wants to move' but it 'doesn't appear to be getting a good signal'. Basically its getting an intermittent signal. I'll check the battery cables as their not in the best shape.

Thanks for all the good advice. If the wiring doesn't fix it I'll be ordering a reman-ECU. I've seen several sites to order them. Any recommendations?

Replaced battery cables (corroded). CEL still on. Replaced the ECM. CEL gone!!! Runs great now and always holds proper idle.

I ordered a new ECM from NEAT computers https://www.neatcomputers.net/. It was about ~$160 and came repainted looking good. The ECM was the fix. Thanks for all the help.

Glad you got it fixed! Thanks for the update.

Did the computer come preflashed with the latest updates in it? Or did you have to take the truck in after install to get it programmed.

I had a '98 xj in the shop a while back, it had o2 sensor circuit codes. I followed a diag flow chart, it led me to a bad pcm. Further inspection i found the wiring harness that runs parallel to the fuel rail was melted and all the wires in the loom were melted solid to each other. Just a bit of info i thought id share..

four_shot said:

Did the computer come preflashed with the latest updates in it? Or did you have to take the truck in after install to get it programmed.

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I plugged it in and it runs great. It was preprogrammed with the VIN but I have no idea about the software. I'm assuming it's all the latest. I have no idea how I would tell the difference.