Hi everyone,

I'm reaching out for help with a 2018 Range Rover 4.4 SDV8 (L405) that’s been stuck with a persistent issue for months – the message “Reduced Engine Performance” appears as soon as the vehicle moves under load. I’ve performed extensive repairs and replacements, yet the following DTCs remain:

P00BF-00 – Mass or Volume Air Flow "B" Too High
P004B-85 – Turbocharger/Supercharger Boost Control "B" – Circuit high
P259B-71 – Turbocharger "B" Control Position Sensor – Stuck high
U0080-81 – LIN Communication Bus fault


Here is everything I’ve done so far in detail:

Rebuilt the complete engine after a failure:
New crankshaft, pistons, connecting rods
New valve stem seals
Both turbochargers replaced twice (second time to rule out any manufacturing defect)
DPF and exhaust system replaced completely

MAP sensor “B” (secondary side) was initially damaged during engine install
Replaced with correct OEM sensor based on VIN
MAF sensor still under suspicion – open to suggestions on this
Engine wiring harness replaced completely
Both turbo actuators replaced (left = pneumatic, right = electric)
Battery voltage and grounding checked
Secondary battery replaced
PSDB (Power Supply Distribution Box) replaced with a known good part

Swapped entire engine into an identical vehicle → Same DTCs immediately returned
This confirms that the fault “follows the engine,” not the chassis
The CSUV has been fully tested, verified functional, and was not part of the engine swap

Any help, insight, or weird edge cases you’ve run into would be massively appreciated. I'm at the point where only something obscure—possibly a software calibration issue or something really deep in the engine bay—could explain this.

Thanks in advance!

Last edited by enzo0611 on 29th Apr 2025 1:24pm. Edited 1 time in total

Turbocharger "B" Control Position Sensor suggests that the DTC is referring to the turbine control valve for the secondary turbo, as the control for that valve includes reporting the position of the valve.
Was the valve replaced when the turbo was replaced?
Some instances have been reported of this valve being sticky, not to confuse it with a sticky compressor shut-off valve (CSOV).

Edit: If the valve has been replaced, has the ECM turbine position calibration process been done? L405 D350 SE Carpathian Grey D5 19" rims

Thank you very much for your response and suggestions.

When I initially replaced the engine, I also installed brand-new turbochargers.
After noticing the persistent faults, I swapped the actuators on the new turbos, but unfortunately, the errors (P00BF, P004B, P259B, U0080) still remained.

Following that, I removed the engine again and replaced both turbochargers a second time, but the same faults persisted.

Regarding your question about the CSOV:
Could you possibly provide a picture or explain exactly where the CSOV is located?
I want to make sure I fully understand and check this component as well.

Also, to answer your last point:
No – the ECM turbine position calibration process was not done after replacing the turbo actuators.

Thanks again for your help!

EDIT:
The CSOV in the identical vehicle was fully functional and had no issues.
The identical vehicle ran perfectly without any DTCs before swapping engines.

However, when I installed my rebuilt engine into the donor vehicle, the exact same fault codes appeared immediately.

This confirms that even with a known good and working CSOV still fitted to the car,
the problem followed my engine, not the vehicle.

The issue must be related to something attached to the engine itself.

Any further ideas are highly appreciated!

Presumably you will do a TSOV ECM calibration just to rule out the possibility that your original TSOV was unusual, noting that the new TSOV was acceptable to the other vehicle's ECM calibration. L405 D350 SE Carpathian Grey D5 19" rims