Troubleshooting P0135 and high short term fuel trim (+30ish at idle)

jcormier

jcormier

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Joined
Apr 14, 2024
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21
Location
Phoenix, AZ
2002 Wrangler X
4.0L with California emissions setup
Engine is basically stock minus a few replacement parts that are fairly trivial (coolant/heater hoses, couple of wires replaced, a few vacuum lines, etc.).
The P0135 code just won't go away and bank 1 fuel trim is crazy high (>+30 at idle; drops to 0 when held at about 2000 RPM for a few minutes) but bank 2 looks fine.
I'll list the things I've done so far:
1) Replaced 1/1 sensor (yes, NTK) as the heater element had slightly out of spec resistance; seemed like it's the most likely culprit but didn't change anything else.. Cleared codes, it came back.
2) Traced all wires from PCM to all 4 sensors; all 4 sets of wires had less than 1 ohm resistance between when appropriate and were otherwise not shorting to ground, etc. (per service manual 8W-30-13). A couple of the wires on C2 (black PCM module) looked damaged, but it was superficial to the coating, nothing exposed; rewrapped the harness)
3) Checked spark plugs. Spark plugs 2 and 3 were pretty dirty, probably from running rich, replaced them. Cleared codes, P0135 came back
4) Compression check of the engine; all 6 cylinders were fine.
5) Checked other O2 sensors; 1/2 sensor is basically still the OEM one, so I'm replacing it, new one on order; figured may as well replace all 4; went ahead and ordered the other 3. Should be here this week, but still no clue on how that might impact the p0135 code.

Next docket item:
Replacing sensors 1/2, 2/1, 2/2 (they're all old, probably due anyway), but I don't see how that could cause a P0135 code unless 2/1 upstream was shorting to ground on the F142 wire (which it wasn't).
Per other forum threads, I've surmised a vacuum leak may be the culprit and will replace all the vacuum lines anyway, but I have yet to find a leak and I also don't see how only bank 1 would be impacted.

Any ideas on this one would certainly help as I'm about tapped out on them.
 
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Not a vacuum leak; spent the better part of the afternoon checking all the vacuum lines and replacing any crumbly ones. None of them were leaking (at least, not until I broke one trying to remove a different one). 1/2 O2 was replaced, will be waiting to start up and recheck after 2/1 and 2/2 are on.
 
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Good morning in Maricopa county,

I can test your ecm in my Jeep if you would like to send it into me. The P0135 does come back as a possible ecm failure so I think you're on the right path. You can call me to discuss in further detail if you'd like.

Thanks,

Mark
 
Wranglerfix said:
Good morning in Maricopa county,

I can test your ecm in my Jeep if you would like to send it into me. The P0135 does come back as a possible ecm failure so I think you're on the right path. You can call me to discuss in further detail if you'd like.

Thanks,

Mark
Click to expand...

Sent it off today, should arrive by Saturday; I'm leaning towards a catalytic converter (most likely) or valve problem (secondary possibility) at this point, but it's worth getting the PCM checked out.
 
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Pre-cat ordered since PCM checked out fine. Retired Jeep mechanic said "Was cylinder 1 spark plug clean, cylinder 2 spark plug was slightly fouled, and cylinder 3 plug was blacker than tar at midnight? If so, change the pre-cats" So, we'll see how that goes; should be here next weekend
 
Painfultruth said:
Voltage to the O2 sensor?
Ground to the sensor?
Click to expand...

The sensor is fine, The heater wiring is fine.
Edit: I checked resistance to ground on all 4 points, nothing grounded. I checked for continuity per the wiring diagram, no issues there. 4.5 ohms across the heater when cool. Voltage going across it when running and the thing actually gets hot when the engine is running. Heater circuit is actually just fine.
 
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O2 sensor isn't actually heating up. I took the previous o2 sensor I had on there which has a heater element that tested fine (4.6 ohms at ambient temp). Started the Jeep, had voltage, but shouldn't that sensor get super hot really quickly? It didn't.

EDIT: so it takes a little while longer than I thought to get hot, I was thinking it happened in seconds, but it takes a couple of minutes. That said, error code still isn't going away and the engine is running way too rich...
 
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It should warm fairly quickly, how was the warm up time compared with the other sensors? (4-5 ohms cold is good, the resistance should decrease as it warms up)

Did you put a vacuum gauge on it after changing the vacuum lines?
 
Warmup time was about the same across all the sensors including one I chopped up and hooked straight to the battery. They are PTC heaters, so resistance increases with temperature. The fact that the engine is just running super rich might be a causative agent here rather than the effect. Not sure why one bank would run rich and the other would be lean aside from the O2 sensors since all the other sensors that I know of are common between both.
 
I doubt the attachments will be of much value, but a review may help.

It appears that the sensor is conditionally working. Air leak somewhere.

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That actually does help quite a bit. I'm thinking there's a leak near bank 1 that's letting in extra air somewhere causing the voltage to be way too high on the upstream o2 sensor and resulting in a (sometimes) code being set on occasion. That could also the reason my Bank 1 fuel trim is so high at idle.
 
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