HoratioTheJeep
NAXJA Forum User
- Location
- Texas
I've got what might be a unique problem. If it weren't raining I'd be out trying to fix it, but I might as well get some opinions from you folks just in case someone has had this problem before or might know the fix for it.
Specs: 1996 4.0L XJ, auto, 2WD, currently at 114k miles (purchased Nov. 2003 with 95,500 miles on it). This is an OBDII XJ.
Six months ago, at 108,000 miles on the odometer, I thought I'd replace the original O2 sensor, not because I had to, but just because it was old & I hoped to gain some more fuel efficiency. I was getting 17mpg in city driving, 19-20mpg hwy.
I bought & installed myself a new Denso OE-style sensor, upstream only. I reset the computer The new sensor was fine for more than 100 miles, maybe as many as 250 miles. Then the CEL came on. I used the key-on key-off trick to get the code (21) which told me that the O2 signal was bad.
Since my original sensor had been OK--& since I had kept it just in case--I reinstalled the original, & reset the computer. Drove about 30 miles, & again the CEL came on. Checked the codes, & this time the code 21 came up twice, & I thought 'that's weird'. On a hunch, I bought a downstream Denso O2 sensor, & reinstalled both Denso sensors, reset the computer, tried it again, & again, after 30 miles or so, the CEL came on.
Frustrated, I took it to a repair shop. They printed the OBDII diagnostic codes for me, so I have the whole sheet in front of me now. Codes were PO132 (O2 sensor circuit high voltage [Bank 1 Sensor 1]), PO135 (O2 sensor heater circuit malfunction [Bank 1 Sensor 1]), PO138 (O2 sensor circuit high voltage [Bank 1 Sensor 2]). So literally everything is screwed up. I figure there is a short in the wiring somewhere. The mechanic tells me to run some fuel system cleaner. Whatever. I ran the fuel system cleaner, &, as I expected, no improvement, same codes. I took apart the plug on the harness side, cleaned it in case the connectors or wires were wet, then put it back together, cleared the codes & tried again. CEL light came back on regardless.
My Jeep was running badly, so I unplugged both the upstream & the downstream sensors. Now it runs fine, but gas mileage has dropped to a rock-steady 15 mpg in city driving.
So I checked the downloadable FSM (factory service manual), the one from Pirate4x4.com, for a _1994_ model. The FSM for that year says the voltage going from the XJ's harness to the O2 sensor should be 1.2v. I used my multimeter to check the harness, & it is putting out 1.62v. So clearly there's a fault somewhere in the wiring causing the high voltage, which in turn causes the O2 sensor to give bad readings back to the computer, causing the CEL to come on.
I tried swapping out various relays in the PDC to see if a bad relay was pumping too much power to the O2 harness. No improvement. I've checked all my fuses, too, & they're all OK.
So my question is, where should I look for the wiring problem? My plan was to follow all the O2 wires on the harness side all the way to their origin, looking for frayed wires, a wet connection, or something. A guy who restores CJ's told me he thinks it's a bad voltage regulator--but wouldn't I have other problems besides this, if it was a bad voltage regulator? The voltage guage in the dash usually reads constant, though sometimes it is above the 14v mark, usually slightly below the 14v mark, but always constant. Could it be the voltage regulator? Could it be bad wiring? Has anyone else had this problem?
Any help appreciated. Sorry for the long post, but I wanted to be specific about what I've done so far. Thanks,
Specs: 1996 4.0L XJ, auto, 2WD, currently at 114k miles (purchased Nov. 2003 with 95,500 miles on it). This is an OBDII XJ.
Six months ago, at 108,000 miles on the odometer, I thought I'd replace the original O2 sensor, not because I had to, but just because it was old & I hoped to gain some more fuel efficiency. I was getting 17mpg in city driving, 19-20mpg hwy.
I bought & installed myself a new Denso OE-style sensor, upstream only. I reset the computer The new sensor was fine for more than 100 miles, maybe as many as 250 miles. Then the CEL came on. I used the key-on key-off trick to get the code (21) which told me that the O2 signal was bad.
Since my original sensor had been OK--& since I had kept it just in case--I reinstalled the original, & reset the computer. Drove about 30 miles, & again the CEL came on. Checked the codes, & this time the code 21 came up twice, & I thought 'that's weird'. On a hunch, I bought a downstream Denso O2 sensor, & reinstalled both Denso sensors, reset the computer, tried it again, & again, after 30 miles or so, the CEL came on.
Frustrated, I took it to a repair shop. They printed the OBDII diagnostic codes for me, so I have the whole sheet in front of me now. Codes were PO132 (O2 sensor circuit high voltage [Bank 1 Sensor 1]), PO135 (O2 sensor heater circuit malfunction [Bank 1 Sensor 1]), PO138 (O2 sensor circuit high voltage [Bank 1 Sensor 2]). So literally everything is screwed up. I figure there is a short in the wiring somewhere. The mechanic tells me to run some fuel system cleaner. Whatever. I ran the fuel system cleaner, &, as I expected, no improvement, same codes. I took apart the plug on the harness side, cleaned it in case the connectors or wires were wet, then put it back together, cleared the codes & tried again. CEL light came back on regardless.
My Jeep was running badly, so I unplugged both the upstream & the downstream sensors. Now it runs fine, but gas mileage has dropped to a rock-steady 15 mpg in city driving.
So I checked the downloadable FSM (factory service manual), the one from Pirate4x4.com, for a _1994_ model. The FSM for that year says the voltage going from the XJ's harness to the O2 sensor should be 1.2v. I used my multimeter to check the harness, & it is putting out 1.62v. So clearly there's a fault somewhere in the wiring causing the high voltage, which in turn causes the O2 sensor to give bad readings back to the computer, causing the CEL to come on.
I tried swapping out various relays in the PDC to see if a bad relay was pumping too much power to the O2 harness. No improvement. I've checked all my fuses, too, & they're all OK.
So my question is, where should I look for the wiring problem? My plan was to follow all the O2 wires on the harness side all the way to their origin, looking for frayed wires, a wet connection, or something. A guy who restores CJ's told me he thinks it's a bad voltage regulator--but wouldn't I have other problems besides this, if it was a bad voltage regulator? The voltage guage in the dash usually reads constant, though sometimes it is above the 14v mark, usually slightly below the 14v mark, but always constant. Could it be the voltage regulator? Could it be bad wiring? Has anyone else had this problem?
Any help appreciated. Sorry for the long post, but I wanted to be specific about what I've done so far. Thanks,
