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Oxygen sensor heater fuse keeps blowing

iroc86

NAXJA Forum User
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A few weeks ago, my '98 XJ turned on the Check Engine light and complained that the upstream O2 heater wasn't working. Assuming this was a problem with the sensor itself, I got a replacement under warranty (Denso brand), but that didn't fix the problem.

Then, I checked the fuses. (I probably should have done this first.) It turns out that the O2 heater fuse, a 15A mini, was blown. I've already went through two of them and can't figure out what's wrong with the circuit.

I checked the wires with a multimeter and verified they were going to the correct places. I also manually wired the oxygen sensor heater, which I've verified is functional. This fuse receives power through the ASD relay, so I swapped that with the one for the cooling fan to rule out any relay issues. The battery was disconnected between fuse replacements to allow the PCM to readjust the fueling. The grounds appear the check out, as well. This problem came out of the clear blue with no previous symptoms.

Short of running my own wires for the heater, are they any other things to try? I'm stumped.
 
I hate Jeeps :looney:. It turns out that the heater fuse is wired to both the upstream and downstream sensors. The wire clip for the rear sensor came undone and has been bouncing on the driveshaft yoke for who-knows-how-long. The insulation was shredded and I guess the 12 V heater wire was grounding itself on the driveshaft. Go figure.
 
iroc86 said:
I hate Jeeps :looney:. It turns out that the heater fuse is wired to both the upstream and downstream sensors. The wire clip for the rear sensor came undone and has been bouncing on the driveshaft yoke for who-knows-how-long. The insulation was shredded and I guess the 12 V heater wire was grounding itself on the driveshaft. Go figure.

The older Renix jeeps have a similar problem with unpleasant exhaust manifold encounters.
 
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