My son’s 2014 JKU (140k miles) wouldn’t start and left him stranded in a parking lot. We ran a scan and it showed a lot of the same codes described in this thread (P0335, P0642, P2122, U0121, U0401, etc). We had it towed to the dealer and they diagnosed multiple problems with the “no start” caused by a bad starter. They also said the two valve cover gaskets needed replacement as did the ABS module. I had them replace the starter to get the vehicle running again and planned on dealing with the rest myself. I had the ABS module repaired by a company called “Module Masters” - take the module out, send it to them, they repair it and provide 5yr warranty. If the ABS has no issues, they don’t charge you except for shipping. This is way cheaper than having the dealer replace it - they will only sell a complete ABS module with hydraulic pump assembly…requires refilling brake lines, yada. In researching what is required to replace the valve cover gaskets (quite a bit of work just to get to them), I decided that now would be a good time to take care of a bunch of maintenance items on the jeep. With the valve covers open, for a vehicle with 140k miles, I decided to change plugs, coils, plug tube gaskets, injectors, oil cooler assembly, oil temp sensor, oil press sensor, both knock sensors, all rockers/lifters and PCV and all the associated gaskets. Only things I didn’t change were items that are fairly easily accessible without pulling the intake manifold off such as cam phaser actuators, camshaft sensors, map sensor, O2 sensors. I can easily replace those in the future if they fail.
So, after changing all those parts and putting everything back together, I still had CEL with codes (P0335, P0642, P2122). The engine ran, but was kind of rough. Digging through this forum and YouTube, it appeared that I was dealing with a problem with the 5V sensor reference voltage from the PCM, since the crankshaft position sensor (CPS), throttle position sensor and throttle peddle sensor are all served from the same 5V circuit. Several comments said that if the CPS is bad, it can cause misfire. I replaced the crankshaft position sensor, but that didn’t help. So, I started tracing wiring back. I was able to find pieces of wiring schematics on jeep forums that indicated the 5V conductor is pink with a yellow stripe. I was able to trace back the crank position sensor wiring to a multi-conductor harness located on top of the engine over cylinder 3. With battery disconnected, I opened up this harness and checked the pink/yellow conductor and found it was shorted to ground between the harness and the PCM. As suggested by several videos, the short turned out to be located in the worst possible place…there was some bare copper on the oil pressure sensor connector in the harness located under the oil cooler assembly. Apparently, this is pretty common due to the heat at that location. So, after pulling everything apart again, I installed a new harness (called an oil temp/pressure & knock sensor harness). Wish I had known this harness was prone to failure. I would definitely recommend changing this harness out if you plan on replacing the oil cooler. Before putting the harness in place, I added some high temp RTV over the exposed conductors near each of the four sensor connectors to hopefully provide a little more protection from the heat. Well, I put everything back together, but was still getting a check engine light for the same 3 sensors (CPS, Throttle Posn and Throttle peddle). The weird thing was that the engine now ran pretty smooth, but I was still getting the low 5V error codes. I was also now getting a P0305 code - misfire Cyl5. I spent some time checking the cyl 5 plug and coil pack, but looked and sounded to me like the engine was running fine. Going back into the Jeep forums I found a comment by someone saying that if the CPS isn’t working right, the PCM will sometimes throw a “false misfire” code. He suggested reterminating all of the engine grounds. So, I pulled off, polished and reterminated all of the ground straps in the engine bay. That appeared to fix the problem. The low voltage errors went away along with the misfire code. So, I think I had two issues. The harness under the oil cooler was definitely bad. That was causing a hard ground of the 5V signal. But, I also had a bad ground somewhere in the engine bay that was causing the voltage at the 3 sensors to be less than 5V. I suspect the bad ground may have been at either end of the black ground strap in that connects the PCM to the engine bay.
Anyway, I appreciated all the info posted by other jeep owners in this forum…that helped me solve my issues, so I figured I should post my saga to hopefully help out other Jeep owners.
Another tip - to measure a certain conductor voltage or check for a ground, I found that the easiest thing to do is strip the ends off of a short piece of 18ga wire and slide one end into the harness where the problem conductor is terminated. That way, you can easily check for voltage even with all the harnesses connected and the engine running.
