Hi guys, new here obviously. Just picked up a 99 xj with 128k on it, first time owning a Jeep or any truck for that matter. Seemed to be pretty solid, however maybe 15 miles into owning it it threw a CEL literally 500 feet from the place I was going to have it inspected (Massachusetts requires inspection within 7 days of registration). Codes p0340 and p0138 - camshaft position sensor and high voltage o2 sensor.

The PO tells me it had no issues before but it sat for a few weeks at least before I bought it so he though it might have been due to sitting. The only way I can justify that would be if the connections somehow corroded while it sat, in which case I'm hoping I can just clean at least the camshaft sensor if not connections for both.

Seems to run and drive fine, however when at idle it seems to go between high and low revs as if it were cycling ac on and off or something. Its a little weird. Idle rpm also fluctuates a little.

In any case, I can't seem to find the camshaft position sensor, the FSM I have makes no mention anywhere.

Any thoughts? I really would like to get this thing fixed and running properly but if its going to mean problems I'll probably end up taking it back to the guy (I don't want to be an ******* but if I need to the lemon law in Mass is very customer friendly, all I need to do is get a reject sticker slapped on it and he either needs to pay for repairs or buy it back). For the record I've done a fair amount of research and not turned up much. The intermittent crank/cam pos sensor seems much more common and most symptoms involve not starting or shutting off randomly.

Thanks everyone!

 

First thing is to check/replace the O2S heater fuse in the PDC. See pic.

This may fix the P0138 code.

 

The Camshaft Position Sensor is covered in Section 8D of the FSM.

It rarely goes bad but you can change it easily. Chrysler calls it a Switch Plate if you have trouble finding one.

 

Thanks CCKen, the fuse appears to be fine. Found the camshaft sensor section in the FSM, that thing is really handy.

Does anyone know if there's a way to check functionality of the camshaft sensor? i.e. a resistance across two pins that should be present if it functions properly or something like that? Similarly for the o2 sensor, once I find that I'd like to be able to check it before just picking up a replacement.

Thanks for the help!

 

Cam sensor can be tested with a meter and a manual; isn't there a procedure in your manual?

The oxygen sensor can be tested, but not so easily. P0138 is usually caused by the oxygen sensor itself. I just changed mine for a P0138 on my 99 at 150k. Replace BOTH oxygen sensors if you plan on keeping the vehicle as if one is shot, the other one is likely not far behind. I highly recommend using NTK for replacement O2 sensors. Widely available, inexpensive and they are the OEM supplier for your vintage so you'll be getting high quality.

 

I didn't see one in section 8D of the FSM, there was a page or so talking about removal but I didn't see anything about testing it. I'll check again when I get home. Thanks for the advice on the o2 sensors, I'll probably just do both of them.