This is on a relative’s 2018 SDV8, which has only done 9k miles - it’s had a very easy life, doesn’t get driven often and is never driven hard - after any period of hard acceleration it now goes into Restricted Performance. I’ve plugged my Autel code reader in and the relevant trouble codes are P004B-85 (Turbo/supercharger boost control solenoid B - circuit range/performance) and P259B-85 (Turbocharger boost control position sensor B performance - Stuck high).

My old 4.4 TDV8 L322 had a similar problem after a long period of inactivity, and it was fixed by replacing the changeover solenoid/valve - I’m assuming this is similar and things are sticking from not being used.

Has anyone had similar on an L405?

From WS manual

P004B-85 Turbocharger/Supercharger Boost Control"B" CircuitRange/Performance- Signal above allowable range.

Causes
Electrical cause Yes
Mechanical cause Yes
Control Module Cavity Circuit reference O_T_TSV. Turbine shut-off solenoid valve
Circuit reference I_A_TSVF. Turbine intake position sensor valve
The powertrain control module has determined failures where some circuit quantity, reported via
serial data, is above a specified range
Secondary turbocharger charge air shut-off valve system mechanical integrity
Secondary turbocharger charge air shut-off valve system vacuum leakage
Secondary turbocharger charge air shut-off solenoid circuit short circuit to ground, short circuit to power, open circuit, high resistance
Connector is disconnected, connector pin is backed out, connector pin corrosion
Secondary turbocharger charge air shut-off solenoid failure

ACTION
Check secondary turbocharger charge air shut-off valve for mechanical integrity
Check secondary turbocharger charge air shut-off valve system for leakage
Refer to the electrical circuit diagrams and check the secondary turbocharger charge
air shut-off valve circuit for short circuit to ground, short circuit to power, open circuit, high resistance
Inspect connectors for signs of water ingress and pins for damage and/or
corrosion,
Install a new secondary turbocharger
charge air shut-off valve, only when diagnosed as failed
Using the Jaguar Land Rover approved diagnostic equipment, clear the DTCs and retest

Use WSM and check the other DTC 2018 SE SDV8 4.4 Byron Blue

Thanks, I’ll have a look - suspect it’s a sticking actuator.