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P0505 code 2004 Jeep Rubicon

My 2004 Rubicon has been having some idling issues. It idles kind of rough (± 200 RPM) and is hard to start. I tried to start it recently and after 4 cranks, it started then the CEL came on, and has been on ever since. I have recently moved from Washington state to Houston Texas, and the problem starterd here in Texas. I checked the code and am getting a P0505 code. I had a mechanic replace the IAC valve, Checked for vacuum leaks, and cleaned the throttle body. The mechanic says the output from the main computer appears to have burned out, and needs to replace the main computer(for $1000). The mechanic can clear the CEL, but after turning the jeep off then back on it reappears. The mechanic says he verified the computer was the problem by installing a known good computer and it cleared the problem. Does this sound right to you? If my computer is bad, wouldn’t my whole operation be affected?

4.0 L ? Does The Problem Exist On Both “Cold Starts” And “Hot Starts” Or Mostly Just Hot Starts ?

Some of the 4.0L 00-04 Jeeps have this problem, but possibly set a PO303 (Cylinder #3 Misfire) in hot weather and hot vehicle. Jeep came out with a bulletin on how to shield the fuel injector for this problem.

The problem is exacerbated by stop and go, short tirps, and ethanol in gas.


2.4L ? Jeep Advises Using A New Style Spark Plug In 03 & 04 Wranglers With The 2.4L. When The Plugs Are Updated, They Recommend New Igniton Cables To Prevent High Secondary Ignition Voltage Flashover.

These Jeeps were suffering from cold engine carbon fouling.



There are testing / diagnosing suggestions here.


Ok some more information: Engine 4.0l. I had changed to Bosch platinum plugs long before this problem started. Jeep has 47000 miles. This problem mostly happens in the morning, after being shut down for 12 hours. The warning beep has started going off intermittently for 20-30 seconds with no change in performance or other indication.