Secondary air solenoid electrical fault.
P0412 — also written PO412, P 0412, 0412 — is the OBD-II code for secondary air injection solenoid 'a'. Same code, same fix: this page covers every spelling of it.
This code means the little electric valve that controls the fresh air injection during a cold start has given up the ghost, very common on older German and Vauxhall petrol engines.
P0412 is a emissions fault code, typically costing £100–£400 to put right in a UK garage.
What P0412 actually feels like to drive
- The EML (Engine Management Light) glows like a Christmas tree on the dash.
- A weird 'vacuum cleaner' whirring noise for about 60 seconds after a cold start.
- Engine feels slightly 'flat' or sluggish during the first mile of driving.
- Increased fuel consumption because the car stays in 'open loop' longer.
- A failed MOT on the emissions test if the light is on during the inspection.
What causes P0412?
- Solenoid failure
UK vehicles that throw P0412 most often
- Vauxhall Astra G/H 1.8 (Z18XE) 1998-2005
- Volkswagen Golf Mk4 2.0 (APK/AZJ) 1999-2004
- Audi TT Mk1 1.8T (BAM/AUQ) 1998-2006
- Mercedes-Benz C-Class (W203) C180/C200 Kompressor 2000-2007
- BMW 3 Series (E46) 318i (N42/N46) 2001-2005
- Vauxhall Vectra B 2.2 (Z22SE) 2000-2002
How P0412 is fixed
- Solenoid replacement
The P0412 misdiagnosis that wastes money
I see folks constantly throwing money at the air pump itself or the big one-way valves because they hear a noise or see 'emissions' on the dash. In the real world, the pump is usually fine; it's the solenoid that's gone open-circuit or the vacuum lines have perished to nothing. I've had lads spend £300 on a Pierburg pump when a £40 vacuum solenoid and five minutes of routing some fresh silicone hose would’ve put the light out for good. Check the resistance on the solenoid pins before you buy the big bits.
What a UK garage should charge for P0412
You're looking at £50 to £80 for a proper diagnostic plug-in to confirm the solenoid's dead. A quality Bosch or Pierburg replacement part usually sits between £45 and £90 from a factor like Euro Car Parts or GSF. Labour is rarely more than an hour—say £60 to £100 depending on where in the UK you are. Total bill usually lands around £180-£250. Steer clear of the main dealer; they'll charge you £150 an hour just to look at it and likely double the part price.
P0412 repair cost in the UK
Most P0412 repairs land between £100–£400 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 P0412
If your local garage says you need a new air pump, ask them specifically: "Have you tested the solenoid resistance and verified it's getting a 12v signal?" If they look at you blankly, take it elsewhere. These solenoids often fail because moisture gets into the vacuum system and rots the internals. Also, check the fuse! On many Vauxhalls and VWs, this circuit shares a fuse with other sensors—if one shorts, it takes 'em all out. A decent independent should be able to 'actuate' the solenoid using a Snap-On or Autel scanner to hear if it's actually clicking.
Can you drive with P0412?
Drivable.
Urgency: low.