P0115 ECT sensor bad or harness short? (1 Viewer)

[gsi18867]

1997 FZJ80. I've spent half the day in the manual, searching and reading posts, inspecting visual and using OBD 2 reader. Here's the symptom: P0115 continues to come back after clearing. The thermostat, sensor and the harness (wrapped with heat tape) were replaced 20K miles ago with new OEM during engine rebuild. Blue fan clutch.

OBD 2 readings (2 different phone apps):
181-184 F then peg out at +280 F for a couple seconds, then return to 181-184 F.
Dash gauge consistent around middle of gauge
Indicates short circuit per FSM page DI-31, skip to step 4

Disconnected ECT sensor per FSM and read -40 F
Dash gauge consistent around middle of gauge
Replace ECT sensor per FSM page DI-32

Before I replace the sensor, I'd like to figure out the jumping back and forth from correct to +280 F. Would a bad sensor jump in values or stay pegged at max, or could it do either one? If I understand the mechanics of the sensor correctly, it should stay maxed if bad. I know the FSM doesn't point to this but it acts like its an intermittent short somewhere deep in the harness. If that's the case, would I need to replace the sensor to properly diagnose?

 

[Tools R Us]

All of the bad sensors that I have seen showed a consistent 284F, that isn't saying one couldn't be inconsistent? The dash gauge is a separate sender/circuit, so has nothing to do with it. Reading -40F disconnected is correct, could try jumping the pins at the connector, should read constant 284F, if so, most likely the sensor.

 

[gsi18867]

All of the bad sensors that I have seen showed a consistent 284F, that isn't saying one couldn't be inconsistent? The dash gauge is a separate sender/circuit, so has nothing to do with it. Reading -40F disconnected is correct, could try jumping the pins at the connector, should read constant 284F, if so, most likely the sensor.

Thank you. That's my next step. After changing out both O2 sensors today (involved welding new studs where the old ones snapped off), I'm so close to having no CEL.

Something strange a month ago. I went to crank it after it sitting more than 2 weeks, it hesitated and I heard a loud pop under the hood followed by a little smoke coming out. Popped the hood and could smell electrical burn. Checked everything I could in the dark but found nothing. Cranked it again and it drove fine but held a burning electrical smell for a while. Then it went away. No new DTCs, no visible damage anywhere in the daylight with closer inspection, no fuses blown. I'm hoping this might be it. An intermittent short makes sense if there's burnt wire insulation that grounded and blew the sensor. Can inspect closer once it's out.

 

[NorCalFJ100]

@gsi18867 did you end up swapping the sensor and get this fixed? I am getting an intermittent P0115 code. Sometimes it goes away on its own, others I have to clear it. But it keeps coming back. Started last few weeks. I have a Scangauge II so I am constantly monitoring temps. I have yet to see my temps spike to the 280 you mention though. Also not driving it extensively.

 

[leonelcanut]

Hi guys! Any solution to the intermittent fluctuation in temperature ? I am having the same, even with different ECT sensors...sometimes even the speedo needle bounces with the fluctuation...any pointers ? Thanks in advance