So after a couple of weeks with the car parked up I returned home to a Christmas tree of warning lights and a juddering brake pedal with almost no servo assistance. Having read an old post from a fellow P400e owner (Adara), I knew it was the 12v batteries playing up. Sure enough, the tester showed the auxiliary at almost 0% SoH and 0% SoC. The main 12v was surprisingly at near 90% SoH with plenty of charge. Given that it’s a 2018 car and both batteries JLR branded, I can only conclude that previous owner must have replaced the main battery at some stage.

So I promptly swapped out the auxiliary with a new non-JLR Exide and warning lights gone, firm brake pedal, and happy days. Well that’s what I thought until I realised that the car refuses to go into EV mode. Not on start up, not when stopped at traffic lights, not in reverse, never. This is all happening with the cabin heating off and the car being relatively warm.

Is it likely that the car just needs time to learn its new battery (no tool or reset required for the auxiliary battery) or is it something unrelated that needs fixing? 2018 P400e AB LWB

What is the temperatura outside? Below 5 degrees Celsius, the ICE engine always kicks in.
Other possible causes are of course the HV heater failure, or the O2 sensor on the exhaust. The last one should trigger an error on an OBD reader (type: P2275-00 Oxygen sensor signal stuck rich. Bank 1 - sensor 3).

The temperature was around 8-9 Celsius last I tried so not too cold. Am also running the car with the HVAC off to remove the HV Heater variable. I’m also getting zero DTCs when I’m scanning.

I could swear that not only is it failing to go into EV mode, but that there isn’t even any assistance from the electric motor. The car feels like it lacks immediacy and is a bit sluggish. 2018 P400e AB LWB

When you press the EV switch what happens? Do you get the grayed out EV logo on the dashboard?

In my case the O2 sensor stopped the EV mode from starting and later even triggered a check engine error. But the Autel AP200 found the error...

Yes I get the EV light on the button itself and it appears in the instrument display greyed out.

Autel isn’t showing any errors 2018 P400e AB LWB

Sorry I can't be of help, I'm actually worried about the cause, if no error is displayed. Maybe try to disconnect the main 12V battery and reconnect? It worked once for me, after the car stayed in a parking lot for 1 month and at start myriads of errors popped out... I carry a wrench in the door pocket ever since...

Thanks for trying adara.

This morning the car decided to fix itself it would appear. EV works as it should so perhaps it just needed some time to realise it has a new auxiliary battery 2018 P400e AB LWB

That's great news. A helpful member (aljo) said at some point that his PHEV does this "every time jlr connect the diagnostic equipment , goes back too normal after 40-50 miles".