Bank 2 running lean — same causes as P0171 but on the other cylinder bank.
P0174 — also written PO174, P 0174, 0174 — is the OBD-II code for system too lean (bank 2). Same code, same fix: this page covers every spelling of it.
Your engine's Bank 2 is getting too much air and not enough petrol, usually caused by a sneaky air leak on big V6 or V8 engines like the Jaguar AJ-V6 or BMW's straight-six.
P0174 is a fuel & air fault code, typically costing £80–£600 to put right in a UK garage.
What P0174 actually feels like to drive
- The 'check engine' light is on, but the car feels almost normal until you stop at the lights.
- A lumpy, hunting idle that feels like it might stall when you're waiting at a roundabout.
- A faint whistling or 'hissing' sound from under the bonnet when you blip the throttle.
- Hesitation or a 'flat spot' when you pull away from a junction when the engine is cold.
- The car passed the MOT on everything but failed the 'Fast Idle' emissions test on high Lambda.
What causes P0174?
- Vacuum leak on bank 2 side
- Weak fuel pump
- Failed MAF
- Cracked intake boot
UK vehicles that throw P0174 most often
- BMW 3 Series (E46/E90) 325i/330i (M54/N52 engines) 2000-2012
- Jaguar S-Type/XF 3.0 V6 (AJ-V6) 2002-2011
- Range Rover Sport/V6/V8 Petrol (Supercharged/NA) 2005-2013
- Ford Mondeo MK3 2.5/3.0 V6 (ST220/Duratec V6) 2000-2007
- Audi A6 3.2 FSI (CALA/BKH engines) 2005-2011
- Mercedes-Benz C-Class (W203/W204) C240/C320/C350 (M112/M272 engines) 2000-2014
How P0174 is fixed
- Smoke-test for leaks
- Test fuel pressure
- Clean or replace MAF
The P0174 misdiagnosis that wastes money
I've lost count of how many times I've seen customers come in having already spent £150 on new O2 sensors or a MAF sensor because they saw this code on a cheap scanner. They think the sensor is 'wrong' because it's reporting the lean condition, but the sensor is usually doing its job perfectly. On the E90 BMWs and Range Rover L322s, nine times out of ten, it’s a perished vacuum hose or a split PCV diaphragm behind the intake manifold. Swapping sensors before doing a proper smoke test is just throwing good money after bad.
What a UK garage should charge for P0174
A standard diagnostic and smoke test usually runs about £60 to £90. If it’s just a split vacuum hose, you’re looking at £120 all-in. However, if the PCV valve (oil separator) has failed, expect £250 to £400 depending on how much of the intake manifold has to come off. Mercedes or Jag main dealers will easily double these prices with their £150+ hourly rates. I always use Bosch or Pierburg parts for these because the cheap 'no-name' intake components from eBay usually fail within a month.
P0174 repair cost in the UK
Most P0174 repairs land between £80–£600 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 P0174
If you're taking your car to a garage for P0174, don't let them just read the code and guess. Ask them specifically, "Can you do a smoke test on the intake system?" It’s a 20-minute job that uses a smoke machine to find tiny cracks in rubber pipes you’d never see with the naked eye. If they say they don't have a smoke machine, find another garage. Also, check your freeze frame data; if the fuel trims look normal at 3,000 RPM but sky-high at idle, you’ve definitely got an air leak, not a fuel pump issue.
Can you drive with P0174?
Yes but risk of piston damage long term.
Urgency: medium.