Out for a pub lunch and our trusty Focus starts misfiring, flashing orange engine light. Limp back 20miles on 3 cyls to indy, attaches computer diagnostic screen, 'lean mixt 4 cyl', told to bring back tomorrow and he'll try and fix it. Said it might be a duff plug.

Limp home, extract 23,000m old plugs;1,2 & 3 look grey and dry, 4 darker, slightly oily. Replace with old 50,000m plugs (yes, I hoard things which might be useful..) which were working perfectly at 102,000m when I replaced them - same misfire.

Pull off HT leads, rest them back on plugs, start engine, lots of spark from 1 2 and 3 with engine slowing when pulled up away from plug, no spark from 4 and no change in engine speed. Replace the very short no.4 HT lead with a spare one I have knocking around - still no spark.

And there's the puzzle - I thought if a coil pack failed it would be either 1 and 4, or 2 and 3 with no spark, not just one lead, because I thought there were only two coil packs which fire in turn to 'their' two plugs, one of which will ignite the mixture.

Any thoughts from the experts. please?