The ECU's main power relay isn't holding — leads to no-start or intermittent shutdown.
P0685 — also written PO685, P 0685, 0685 — is the OBD-II code for ecu power relay control circuit open. Same code, same fix: this page covers every spelling of it.
This is the ECU losing its main power feed, usually because the 'heartbeat' relay has given up the ghost. It's a classic plague on older Golfs and Astras where a £15 bit of plastic stops a two-ton car.
P0685 is a electrical fault code, typically costing £40–£350 to put right in a UK garage.
What P0685 actually feels like to drive
- The car just dies while driving, like you've turned the key off at 60mph.
- Engine cranks over perfectly but won't fire a single shot.
- Dashboard lights up like a Christmas tree with U-series communication codes.
- Intermittent 'No Start' when the weather is damp or particularly cold.
- The cooling fans suddenly run at full speed even though the engine is cold.
What causes P0685?
- Failed main relay (very common on VAG)
- Corroded fuse box connections
- Failed ignition switch
UK vehicles that throw P0685 most often
- VW Golf Mk5/Mk6 1.9 & 2.0 TDi (2004-2012)
- Audi A3 2.0 TDi (8P chassis) 2003-2013
- Vauxhall Astra J 1.7 CDTi (A17DTR) 2009-2015
- Ford Focus Mk2 1.6 TDCi (DV6) 2005-2011 (Corroded GEM modules)
- Seat Leon 1.9 TDi (2005-2012)
How P0685 is fixed
- Replace main relay
- Clean fusebox contacts
The P0685 misdiagnosis that wastes money
I’ve seen dozens of lads waste money throwing a new ECU or a £200 ignition switch at this, when 9 times out of 10 on a VW or Audi, it’s just the '100' or '458' relay in the engine bay fuse box. People assume the ECU is dead because their scanner can't talk to it, but it’s usually just a bit of internal arcing in the relay or a dry solder joint. Don't go buying a brain for the car until you've verified the simple 4-pin switch is actually clicking over.
What a UK garage should charge for P0685
A basic diagnostic plug-in will cost you £50 to £80 depending on your local rates. If it's just the relay, a genuine Bosch or Hella part is usually £15-£30, plus maybe half an hour of labour, so you're looking at under £100. However, if the fuse box (like the under-bonnet units on VAG cars) has suffered water ingress and the pins have rotted off, you're looking at £250-£350 for a replacement unit and the time to swap it over. Main dealers will easily double these prices.
P0685 repair cost in the UK
Most P0685 repairs land between £40–£350 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 P0685
If your car won't start, don't just keep cranking the battery flat. Pop the bonnet and find the engine bay fuse box (the E-Box). Give the main relays a firm wiggle or a gentle tap with the handle of a screwdriver. If it fires up after that, you’ve found your culprit. When you take it to a garage, insist they do a voltage drop test on the relay's supply side rather than just 'scanning for codes'—any half-decent sparky should be able to prove the relay is failing under load in ten minutes.
Can you drive with P0685?
Car may cut out.
Urgency: high.