On starting my car from cold the engine management warning light comes on, The engine lacks power, but once warmed up if switched off and re started the light goes out and full power is restored. A diagnostic test showed an error code indicating a faulty glow plug. As these plugs are only on for a few minutes from start up , why should it cause the lack of power until the system fault is cleared by re starting the engine?

5 minutes ago, Invicta said:

As these plugs are only on for a few minutes from start up , why should it cause the lack of power until the system fault is cleared by re starting the engine?

Normally a glow plug fault would not need you to switch off and restart in order to get full power back, so I suspect there is more to this problem.

Was it a generic code reader that you used ?

If yes, then they often can not properly diagnose Ford specific DTC's which need something like FORScan.

It probably is the glowplug fault triggering limp mode.  Fairly common on the 16v 1.6 TDCI.  The plugs are only used for the first few minutes during warm up but the limp mode remains until it's cleared with a restart.

If you leave the faulty plugs long enough, eventually the limp mode can't be cleared at all, you realise you don't have a bendy glowplug socket for access, and then have to drive across town, over the 70mph ring road, stuck in limp mode, which is not fun.  Or maybe that was just me...