Relais d'alimentation du calculateur défectueux

The relay that feeds a control module and the actuators hanging off it — often called the main or powertrain relay. Its failures are disproportionately confusing because one relay serves many circuits, so when it degrades, a group of entirely unrelated components all report faults at once and none of them is broken.

The characteristic failure is not a clean break but contact erosion. Every switching cycle draws a small arc, which pits and oxidises the contact faces; over years the resistance across them climbs. A relay in that state works perfectly at low load and collapses under high load, which is why the faults appear under acceleration, with the air conditioning on, or on a hot day — and vanish before the vehicle reaches a workshop.

That is also why it is missed: swapping in a known-good relay and finding the fault gone proves nothing if the fault was intermittent anyway, and a resistance check across the contacts reads fine because a multimeter passes a milliamp where the circuit passes amps. The test that finds it is voltage drop across the relay while the circuit is loaded.

The opposite failure is contacts welding closed, leaving the circuit permanently live — which shows up as unexplained battery drain and, on heater circuits, as components powered when nothing intends them to be.

When several unrelated codes appear together, look at what they share electrically rather than mechanically. This relay is very often the answer, and it is among the cheapest parts on the vehicle.