Fan, A/C and fuel pump relays: P0480, P0645, P0691
The fan runs non-stop, the air conditioning stopped switching on, or the car will not start — and the list shows relay codes. Almost always this is a cheap part nobody gets to, because everyone looks for an expensive one.
A relay is a switch commanded by the control unit. The unit itself is low-current: it cannot spin a fan or pump fuel directly. It sends a small current to the relay, and the relay closes a big one.
Every code below is about that small circuit, not about the fan, the compressor or the pump. Which is exactly why they are so often «fixed» by replacing the wrong part.
Cooling fan: P0480, P0691, P0692, P0693
P0480, P0693 — the circuit of the first and second fan relay. P0691 — low signal, P0692 — high: an open and a short respectively.
This fault has two faces, and they are opposites.
The fan never switches off — a stuck relay, or the unit has gone into protection. The car drives, but the battery dies overnight and the noise drives you mad.
The fan never switches on — now that is dangerous, and the code will not appear straight away. While moving there is enough air flow and everything looks fine; the needle climbs exactly when the car stops in traffic. What to do on the road is in overheating, and the lamp itself is covered on the engine overheating page.
It is easy to check: start the engine, open coolant temperature and let it reach 100–105 °C standing still. The fan must cut in and knock about ten degrees off. If it does not — the relay, the sensor or the wiring, and this is not a «it can wait until the weekend» matter.
Air conditioning: P0645
P0645 — the A/C compressor relay circuit. The air conditioning simply stops switching on, and off the owner goes to have the refrigerant recharged.
⚠️ You can tell one from the other without dismantling anything: on a healthy system the idle revs rise by 50–100 the moment you press the button — the unit compensates for the compressor load in advance. No twitch at all means no command reaches the clutch, and the refrigerant has nothing to do with it.
Fuel pump: P0627, P0628, P0629
P0627 — the pump relay circuit, P0628 and P0629 — low and high signal. This is the case where the car cranks but will not start.
A check by ear, no instruments: turn the key to ignition without cranking and listen near the rear seat. A healthy pump whirrs for two or three seconds and goes quiet. Silence means the relay, the fuse or the pump itself.
If the pump whirrs and the car still will not start, the question moves on to fuel pressure — a different group of codes.
Why relays fail more often than you would think
Inside a relay is a mechanical contact that clicks tens of thousands of times. It burns and its resistance grows. For a while nothing shows, then comes «sometimes it works, sometimes it does not» — the most tormenting fault of all, because everything works when the workshop checks it.
On most cars the under-bonnet fuse box holds several identical relays. Swapping a suspect one with a known-good one is a free test that takes a minute and answers the question for good.
What to do, in order
- Find the relay in question in the fuse box — the layout is usually printed on the lid.
- Swap it with an identical one. If the fault moves, the relay is guilty.
- Inspect the fuse of the same circuit.
- Check the connector and the ground: relay codes often appear because of a poor ground rather than the relay.
- Clear the code and repeat the conditions — heat for the fan, the button for the A/C, cranking for the pump.
