Upstream O2 sensors appear connected to the wrong bank.
P0040 — also written POO4O, P 0040, 0040 — is the OBD-II code for o2 sensor signals swapped (bank 1 sensor 1 / bank 2 sensor 1). Same code, same fix: this page covers every spelling of it.
This one is almost always 'human error' where someone has swapped the oxygen sensor plugs after a big job, very common on V6 Jaguars and older Range Rovers.
P0040 is a sensors fault code, typically costing £40–£200 to put right in a UK garage.
What P0040 actually feels like to drive
- Engine light (MIL) glowing bright on the dash usually right after a big repair.
- The car feels 'held back' or hesitant when you're pulling away from a roundabout.
- Thick black soot on the tailpipes that wasn't there last week.
- Abysmal fuel economy—you'll see your MPG drop from 45 down to 25 instantly.
- A distinct smell of unburnt petrol or 'rotten eggs' from the catalytic converter getting roasted.
What causes P0040?
- Sensors fitted wrong way
- Wiring error from previous repair
UK vehicles that throw P0040 most often
- Jaguar XF 3.0 V6 (2008-2015)
- Range Rover Sport 3.6 TDV8 / 4.2 V8 (2005-2013)
- Vauxhall Insignia 2.8 V6 Turbo (A28NER)
- Audi A6 3.0 TFSI (C6/C7)
- Volkswagen Touareg 3.2 / 3.6 V6 (2003-2010)
- BMW 335i (N54/N55) E90/E92
How P0040 is fixed
- Swap sensors back to correct bank
The P0040 misdiagnosis that wastes money
I’ve seen dozens of lads throw a pair of brand new Bosch sensors at this, thinking both have gone belly up because the readings are 'all over the shop.' It’s rarely the sensors themselves. Usually, it's after a clutch job or a gearbox swap where the loom has been pinned back out of the way and the technician has blindly plugged them back into the wrong grey and black connectors. It looks like a sensor failure because the ECU is adding fuel to Bank 1 but seeing the result on Bank 2, so it panics.
What a UK garage should charge for P0040
If it’s just a case of swapping the plugs back over, you're looking at half an hour's labour, maybe £40 to £60 at a local indie. If the wiring has been butchered or melted against an exhaust manifold, you might need a loom repair or a new Denso sensor costing £80-£120 plus fitting. A main dealer will likely try to sell you the 'official' loom overlay for about £400, but a decent sparky can solder it for a fraction of that.
P0040 repair cost in the UK
Most P0040 repairs land between £40–£200 including parts and labour. The spread is wide because the cheap fix (a sensor, a hose, a leak) and the expensive fix (a major component) throw the same code — which is why a proper diagnosis before parts are bought saves the most money.
Bob's tip on P0040
If you've just had major engine or gearbox work done and this pops up, don't let a garage charge you for parts. Insist they perform a 'live data pin-out check.' We do this by unplugging one sensor and seeing which bank on the scanner goes to a flat 0.45V. If you unplug Bank 1 but Bank 2 goes dead on the screen, they've crossed the looms. It’s a ten-minute fix on a ramp, so don't be paying for an hour's diagnostic and two new sensors you don't need.
Can you drive with P0040?
Drivable but flagged.
Urgency: low.