P0615
PowertrainStarter Relay Circuit
The ECU has spotted a voltage that doesn't add up in the starter relay control circuit, the wiring that tells the starter relay to close and crank the engine. For you, that usually shows up as a car that won't start reliably, or one that fires up fine but throws the engine light and leaves you wondering when it'll let you down. The fault is in the control side of the circuit, not the starter motor itself, so the fix is normally a relay, some wiring, or a connector rather than a big-ticket component.
ⓘ Information only. This page provides general educational information about fault code P0615. 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 P0615 mean?
P0615 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Starter Relay Circuit.
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 the dash, sometimes the only sign if the car still starts
- • Long crank or a flat no-start when you turn the key or hit the start button
- • Intermittent starting, fine one morning and dead the next, which is the classic relay-circuit behaviour
- • A single click from the relay or starter area with no crank
- • Car cranks normally on some attempts and does nothing on others, with no clear pattern
- • No running problems at all once the engine is actually going
Possible causes
Causes commonly associated with P0615, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Starter relay worn out or breaking down internally, the cheapest and most common culprit. Relays have contacts that pit and stick after years of use
- 2. Corroded or loose connections in the relay control wiring, very common on older cars and anything that's seen UK winters and road salt
- 3. A poor earth or corroded ground strap dragging the relay circuit voltage off where the ECU expects it
- 4. Blown or partially blown fuse on the starting circuit
- 5. Ignition switch not sending a clean start signal, more likely on higher-mileage cars with worn switch contacts
- 6. Faulty neutral safety switch on autos or clutch position sensor on manuals, which can interrupt the start command
- 7. ECU internal driver fault, rare and the last thing to suspect once everything else checks out
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. Pull the relay out and physically check it, then swap it with an identical relay from the same fusebox (the wipers or horn relay is often the same part) and see if the fault moves or clears. This is the fastest, cheapest test and sorts a big chunk of P0615s on the spot
- 2. Read all stored codes before touching anything. Codes like P0616 or P0617 sit alongside P0615 and tell you whether the relay is sticking open or closed
- 3. Check the starting-circuit fuse for a blown element or any browning that points to overheating
- 4. Go over every connector and terminal between the battery, ignition switch, relay and starter for green corrosion, loose pins, or chafed wiring. Wiggle-test the loom while someone tries to start it
- 5. With a multimeter, confirm the relay control terminal gets voltage when the start signal is sent. You want to see it pull up properly when you crank, anything weak or absent points back up the control wire
- 6. Check earth continuity from the relay and ECU to the chassis. Resistance should be down near zero, below about 0.5 ohms. A dirty earth here mimics a failing relay all day long
Common questions about P0615
Can I keep driving with a P0615 showing? +
If the car won't start, the decision is made for you. If it's starting intermittently, treat it as a car that will strand you sooner or later, probably in a car park or on your driveway when you're already late. It won't suddenly cut out while you're moving, the starter circuit only matters when you're trying to crank. But an intermittent relay fault always gets worse, never better, so book it in rather than gambling on the next cold morning.
Is this going to fail the MOT? +
P0615 is a starting-circuit fault, nothing the emissions or safety side of the MOT directly tests. The catch is the engine warning light. If the MIL is glowing on the dash when the tester looks at it, that's a fail on its own under the current rules. Sort the underlying cause and let the light clear, or get it cleared, before you take it in.
What's the damage cost-wise? +
Depends entirely on what's actually wrong. A new relay is a tenner or so and a five-minute job. A blown fuse is pennies. If it's corroded wiring or a poor earth, an independent garage will usually have it traced and repaired for around £60 to £150 depending on how long the fault-finding takes. An ignition switch fitted is typically £150 to £300. A main dealer will charge noticeably more on labour for the same diagnostic work. ECU faults are rare but can run into four figures, so that's the worst case, not the likely one.
How do I work out which cause it actually is on my car? +
Start with the relay swap, because if the fault clears or moves to whatever circuit you stuck the old relay into, you've found it and saved yourself a load of testing. If swapping the relay does nothing, the problem is in the wiring, the earth or the trigger signal, and that's where the multimeter comes in. An intermittent fault that comes and goes when you wiggle the loom or in damp weather screams corroded connector or earth. A dead-consistent no-start with a good relay and good earths points more at the ignition switch or the safety switch. Leave the ECU until everything else has been ruled out.
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 →