P0612
PowertrainFuel Injector Control Module Relay Control
You go to start the car and it cranks longer than it should, or fires up rough and lumpy at idle, and the engine warning light is on. On a diesel you might get black smoke and a sudden drop in power, sometimes dropping into limp mode. Behind all that, the ECU has spotted a fault in the circuit that controls the relay feeding power to the fuel injector control module. That relay is what wakes up the injection electronics, so when its control circuit plays up the injectors don't get clean, reliable power.
ⓘ Information only. This page provides general educational information about fault code P0612. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0612 mean?
P0612 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Fuel Injector Control Module Relay Control.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine warning light on, often the first thing you notice
- • Long cranking before it catches, worse from cold
- • Lumpy, uneven idle that smooths out as the revs come up
- • A flat spot or general lack of poke when you put your foot down
- • Black or grey smoke from the exhaust, particularly under load on a diesel
- • Limp mode kicking in and capping the revs to protect the engine
Possible causes
Causes commonly associated with P0612, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. The relay itself has failed, usually burnt contacts or a dead coil. This is the first thing to suspect because relays are a wear item and cheap to swap
- 2. Corroded or chafed wiring between the relay and the injector control module, common where the loom runs near heat or moisture
- 3. A dirty or loose connection at the relay socket or the module plug, enough to drop voltage even when everything else is fine
- 4. Water ingress into the relay box or harness, which corrodes pins and throws intermittent faults that come and go with the weather
- 5. A blown fuse in the injector control feed, quick to check and quick to rule out
- 6. Internal failure of the fuel injector control module, the expensive end of the scale and the last thing you should land on
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Read all stored codes, not just the P0612. Power and ground codes like P0611, P0670 or P0251 sitting alongside it tell you a lot about which way to point the diagnosis
- 2. Find the injector control module relay, usually in the engine bay fuse and relay box, and pull it out. Look at the pins for burning, green corrosion or melting around the socket
- 3. With the ignition on, put a multimeter across the relay coil terminals. You're looking for roughly 12 volts at the trigger side. No voltage means the fault is upstream in the control circuit
- 4. Swap in a known-good relay of the same part number if you have one. A surprising number of P0612s clear right there for a couple of quid
- 5. Run the loom from relay to module by hand, checking for chafing, breaks and damp connectors. Wiggle-test the plugs with the meter connected to catch intermittent drops
- 6. Confirm voltage actually reaches the module connector when the relay clicks in. If the relay and wiring are good but the module sees nothing, you're looking at a module fault
Common questions about P0612
Can I keep driving it or do I need to sort it straight away? +
You might limp it home, but I wouldn't make a habit of it. Long cranking, rough running and a possible limp mode all point to the injectors not getting clean power, and an injection system that's misbehaving can wash fuel into places it shouldn't and put extra strain on the high-pressure side. If it's running this rough you also risk being stranded when it finally won't catch at all. Get it diagnosed before the next long journey.
Is it the module that's gone or just the wiring and relay? +
Far more often it's the cheap stuff. A tired relay or a corroded connector causes the bulk of these, and a fresh relay plus a clean-up of the pins fixes a good share of them. Genuine module failures do happen but they're the rare and pricey outcome, so never let anyone sell you a control module before they've ruled out the relay, the fuses and the loom with a meter. Confirm there's no voltage reaching the module before you condemn it.
How long is it in the garage for? +
A relay swap is minutes once it's diagnosed, and most of the bill is the half hour to hour of testing to be sure that's the cause. Tracing and repairing a wiring fault is the unpredictable one and can eat an hour or two depending on how buried the damage is. A full injector control module replacement is usually half a day or so once you factor in fitting, coding and a test drive, and that's the job that pushes the cost into the three-figure or higher range.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →