The main current relay — sometimes called the main supply relay or engine management relay — controls the primary power supply to the fuel injection system, engine control unit and ancillary electrical consumers that must only be energised when the ignition is switched on. When this relay fails, the consequences can resemble a wide variety of other faults: the engine cranks but will not start, cuts out unexpectedly at speed, or refuses to restart after reaching operating temperature when the relay contacts open under heat. Intermittent failure is the characteristic behaviour, because the relay's coil or contact set often works reliably when cold but becomes resistive as it heats up, producing symptoms that disappear after the car cools down. This pattern leads to many misdiagnoses as a fuel pump fault or crank position sensor failure before the relay is identified. Bosch and Hella are the dominant original-equipment relay manufacturers and supply the majority of relay specifications across VAG and European platforms. Valeo covers a range of French applications. Matching the OE part number is important because relay coil resistance, contact rating and connector pinout vary even between relays that appear physically similar. Relay replacement is one of the simplest and least expensive repairs possible when the correct unit is identified.
Main current relays are specified by coil voltage (almost always 12V), coil resistance, contact current rating and connector pinout — parameters that differ between relay positions even within the same fuse box. The relay is usually identified by a relay number printed on its housing, a slot number in the fuse box diagram, or the OE part number stamped or printed on the casing. Use your vehicle's make, model and year to filter the relevant references on this page, and cross-check the relay number on the unit you are replacing to confirm the match before ordering.
OEM relays from Bosch or Hella are produced to precise contact resistance, coil impedance and switching speed specifications and are tested to a defined cycle count. Aftermarket relays from reputable suppliers such as ERA or Valeo meet equivalent specifications and are direct replacements. Lower-cost generic relays can have inconsistent contact quality — higher-resistance contacts cause voltage drop that affects the sensitivity of the ECU power supply and can produce the same intermittent starting and stalling behaviour as the original failing relay, making it difficult to confirm whether the repair was successful.
A relay with pitted or high-resistance contacts can cause voltage spikes when switching, particularly when the contact arc erodes the contact surfaces over time. In practice, the ECU and fuel injection system have protection circuitry that limits exposure, but a relay that is fully open-circuit leaves those systems without power, which may corrupt memory in the ECU or adaptive values stored by the transmission control unit if the power is lost unexpectedly. Replacing a suspect relay proactively — once intermittent symptoms appear — is always preferable to waiting for a complete failure.
The most characteristic symptom is intermittent no-start — the engine cranks normally, the starter turns the engine, but the injection system does not activate and the engine will not fire. This symptom tends to appear after the car has been parked in the sun or driven to full operating temperature, because heat is what drives the relay's contact set into high resistance. Other signs include the engine cutting out without warning at motorway speed and restarting normally after a 15-minute cool-down, or a loss of all instrument warning lights and fuel pump priming sound when the ignition is turned on — indicating total relay failure.
Showing 100 of 1,303 Main current relay OE numbers. Enter the OE on the main OE search to jump to any reference.
