Ignition Coil 4M5G-12A366-BA for Ford Focus Mondeo

Ignition coil 4M5G-12A366-BA, also supplied as 4M5G-12A366-BB and under the Volvo number 30711786, is the coil on plug unit for the Ford Duratec HE 1.8 and 2.0 petrol engines in the Focus II, Focus C-MAX, C-MAX, S-MAX, Galaxy II, Mondeo IV and Transit, and for the same engine in the Volvo C30, S40 II and V50, from 2001 to 2015.

The engine uses one coil per cylinder, and on the Focus and Mondeo it is a short tower with a three pin connector on the side, a metal mounting eye at the top and a boot that reaches down through the cam cover seal. The body is a plastic shell with the windings and igniter potted inside, and the boot has two ribs that press into the cover to keep oil vapour and water out. The Volvo version of the same engine uses the identical coil under a different part number, which is why one part covers both badges, and on the five cylinder Focus ST and S-MAX the same coil is used five times.

How they fail. A Focus or C-MAX with a coil going down runs unevenly at idle, feels flat under acceleration and puts the engine light on with a single cylinder misfire. The most common physical fault is a boot that has hardened and split, letting water into the plug well so the spark tracks down the outside; the well then rusts the plug hex and the plug seizes. On the turbocharged EcoBoost versions the coils run hotter and fail at the potting shoulder. A coil that has been in service with a worn plug will often still test good on resistance while missing in use, so the only reliable test is to swap it to another cylinder and watch whether the misfire follows.

Fitting notes. Remove the engine cover, release the connector clip and undo the coil bolt. Pull the coil out by the body, and if the boot comes off in the well, retrieve it before anything else. Dry and clean the well, then take the plug out and inspect it: a wet or rusty plug confirms water ingress. Fit new plugs across the whole engine if they are due, apply dielectric grease to the boot, seat the coil and torque the bolt to 8 Nm. On the five cylinder engine check all five wells, since water usually gets into more than one. Clear the codes and confirm the misfire counters stay at zero on a road test.

Supplied as one coil with boot and spring contact. Twelve month warranty.