Secondary air relay fault — very common on BMW N42/N46 and Mercedes M112/M113 engines.
P0418 — also written PO418, P 0418, 0418 — is the OBD-II code for secondary air injection relay a circuit. Same code, same fix: this page covers every spelling of it.
This code means the brain of the car can't talk to the relay that fires up your air pump. It's a classic headache on early 2000s BMW 3-Series and older Merc V6s.
P0418 is a emissions fault code, typically costing £90–£600 to put right in a UK garage.
What P0418 actually feels like to drive
- Engine sounds like a vacuum cleaner for the first 60 seconds after a cold start.
- The EML (check engine light) stays on, even though the car drives perfectly fine.
- A high-pitched whine or screeching sound on frosty mornings.
- Sudden silence from the front of the car when cold (pump not kicking in at all).
- Slightly lumpy idle for the first minute after starting from stone cold.
What causes P0418?
- Failed relay
- Failed air pump
- Corroded pump connector
UK vehicles that throw P0418 most often
- BMW 3 Series (E46) 318i/320i N42/N46 engines 2001-2006
- Mercedes-Benz C-Class (W203) C240/C320 M112 engines 2000-2007
- Mercedes-Benz E-Class (W211) E500 M113 engines 2002-2009
- BMW Z4 (E85) 2.0i N46 2004-2008
- Volkswagen Golf Mk4 1.6/2.0 petrol 1999-2005
How P0418 is fixed
- Replace relay
- Replace pump if it's seized
The P0418 misdiagnosis that wastes money
I see a lot of lads jump straight to swapping the pump itself because it’s the expensive bit, but more often than not, it's a £15 relay or a blown 40A fuse. The real trap is replacing the relay, seeing it work for a week, and then it pops again. Why? Because the pump is starting to seize and drawing too much current, or water has got into the housing and frozen. Don't just throw a pump at it without checking the fuse box for heat damage first.
What a UK garage should charge for P0418
A basic diagnostic plug-in will run you £50-£80 at an indie. If it's just the relay, you're looking at about £90 including the part and a brew's worth of labour. If the pump is shot, a Bosch or Pierburg unit will set you back £250-£350, plus an hour's labour (£60-£100). If you go to a main dealer, expect a bill north of £600 because they'll insist on replacing the entire loom and valve 'just in case'.
P0418 repair cost in the UK
Most P0418 repairs land between £90–£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 P0418
Ask your garage to do a 'component activation' test with their scanner. If they can hear the relay clicking but the pump isn't spinning, tell them to check the connector at the pump for green crusty corrosion—common on Mercs. If you’re driving a BMW, insist they check the vacuum lines and the plastic 'kombi' valve too. If that valve stays stuck open, exhaust gas travels back up the pipe, melts the pump, and fries your new relay. Save yourself a double repair bill by checking the valve.
Can you drive with P0418?
Yes but will fail emissions.
Urgency: low.