My check engine light’s been on for a good year. A month after it came on, I took my KJ to Auto Zone for a quick check. I don’t recall the error code but I think it had something to do with a relay for the AC. Since the AC was working fine, I blew it off. A couple months later I drove by another parts store and had them check it out as well. They told me the same thing.

10 months later I make a 1,200 mile trip pulling a trailer. My KJ ran strong and the AC worked just fine the whole trip. The check engine light was still on. A couple weeks later the AC stopped blowing cold air for a couple minutes and I remember thinking it probably had something to do with that relay. The AC blew nothing but cold air for the next month and then, for a couple minutes one day, it stopped blowing cold air again. It started blowing cold air again after a couple minutes.

A month later I took my KJ to the dealership. The error code they gave me was P0601. The service guy types the code into his computer and tells me I need a new ECU. This really turned me off because he never asked me how long the check engine light had been on nor if my KJ was giving me trouble. Not only that, he immediately called the parts department and said he’s gonna give me an estimate. He’s yet to ask me a single question and he’s yet to take my KJ out for a spin. I politely told him not to bother with the estimate.

I drove down the street to another Auto Zone and they give me the same error code: P0601. The printout he gives me says the probable causes are:
1) A shorted input sensor
2) A shorted electrical circuit
3) Failed ECM

He asks me how my KJ is running and I tell him it’s running as strong as ever (it’s got almost 93,000 miles.) He then tells me it’s unlikely the ECU is bad because, at the least, my KJ wouldn’t be running well. I never told him about the AC because, as I type this, it’s been blowing nothing but cold air.

P.S.:

The same printout I got from Auto Zone says the definition of P0601 is "ECM internal fault." The explanation reads "ECM unable to communicate on the SPI line."

I have an '03 KJ Limited with a 3.7L gas engine.

What the fault code means is that the microcontroller inside the ECU cannot communicate with another chip on the serial bus. This could be caused by outside sources, however it is more likely an internal fault. Try clearing the code and seeing if it comes back. If it does, you will probably need the ECU replaced, however it may still be covered under an emissions warranty in some states. I will take a look at the diagnostic worksheet tomorrow and see if there is anything that you can to do make sure it is the PCM and not an outside source.

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2008 Liberty Sport
OME Lift, JBA A-Arms + Track Bar
245/75/16 Nokian Rotiiva AT Plus
Numerous junkyard upgrades

You can get a good working ECU for theses jeeps off EBay or through LKQ for $200 and all it needs is to have your program transferred on to it.

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My CRD Build Thread
Soon to Come:more lights

Thanks for your help.

There's something I should have mentioned. Back in '09 I took my KJ to the dealership for the 60k service. While I was there they told me I needed a new ECU and new coils (in addition to cleaning the fuel injectors, replacing the spark plugs, etc.) I was told the ECU wouldn't cost me anything because, as you said, it was covered under warranty because of emissions. Since the ECU wasn't going to cost me anything, I said it was OK to replace it (and so they did.)

My KJ is an '03 with 93k miles. Is this second ECU I got still covered under emissions warranty? The last thing I need right now is an expensive bill to replace it. Also, how can I try to clear the code? Should I disconnect the battery for a couple minutes and then reconnect it? Again, I appreciate your help.

From another Carolinian, thanks for your reply. I'll definitely keep that in mind but, hopefully, as a last resort.

I'd like to emphasize once again (to anyone reading this post), that my KJ still runs strong. If you didn't see the check engine light on my dashboard, there wouldn't be any clue something might be wrong. Oh yeah, and the AC blows nothing but cold air.