Limp mode

[Stolty9665]

I have a 2018 defender which goes into limp mode as soon as it starts up. It throws a misfire code. P0301. I have replaced plugs. Wires, ignition coil, fuel pump, cleaned fuel tank, changed out map sensor, o2 sensor, throttle body, foot pedal sensor, pulled intake manifold and resealed it, replace after market full exhaust, checked compression which is 150 both cylinders, replaced injectors, cleaned grounds, and still goes into limp mode.im about to change the harness , but before I do I want to see if anyone else has any ideas

[roaddawg]

I recently picked up a 2022 Defender Max, that on the first ride, went into limp mode.  Then it cleared, but over the next week it happened sporadically.  One day when it happened it indicated the seatbelt wasn't latched.  However it was.  I followed the wire from the seatbelt to the middle of the front seat area.  There was a connection there (where the sensor was) that I unplugged, added electrical grease and plugged back in.  a few hundred miles later and no more issues.  Maybe check that?

[CampHamp]

You've done a ton of work - it makes spitballing hard for this one!

Does the engine sound like it is actually misfiring?

Could timing issues cause the misfire reading? Did you check timing already?

I just read that the misfire sensor monitors for changes in crank shaft speed (on cars, at least). So, I wonder if friction or faulty gearing on the shaft could be interpreted as a misfire because it might cause fluctuation in the crank speed.

Is the code always ending with 01 (meaning it is always sensing shaft speed change when cylinder #1 is firing)? If it is always that code (and not 00 or 02), then could we rule out a general problem (like a harness) and conclude it must be something specific to cylinder #1?

It's inspiring to see someone able to replace all this stuff themselves. An incredibly good effort - you'll get it licked soon. Please let us know how it goes.

[ChetMotoX]

Any chance you found the problem? 

I have the EXACT same issue basically. Mine is a 2018 Defender HD10 XT. My problem originally started as the check engine light staying on and as long as I just jumped in and drove it, it wouldn't go into limp mode until I let it go back to an idle, at which point I would have to shut it down and let it completely reboot, then fire up and go again with the same results. I would get codes P0301 and P1172.

I also have drained and cleaned fuel tank, replaced fuel pump, replaced injectors, MAP sensor, O2 Sensor(twice), plugs, plug wires, coils, and filters. I pulled apart the throttle body and cleaned it and tested it, adjusted the valves, made sure the battery was fully charged and passed a load test. Compression is great, and I also threw a new belt on it while I was that deep into it. 

Now I am able to start it up, drive it and the check engine light will go out until I let it go back down to an idle, then when I begin to accelerate again, the check engine light will come on and almost instantly go to limp mode. Codes I'm getting at P0301 and P0302. 

Just looking for the next place to look. Was debating on either harness or ECU but hate to make that jump as I've already sunk almost a grand into parts for this problem alone.