Glow plug issue

On a octavia 110 TDI AFH engine, the glow plug light go stright out on trying to start, with no voltage goint to the plugs untill the engine has started than gives voltage for several minutes without the glow plug light showing, then have tryed unplugging the coolant senser, this keepes the glow plug light on on the dash and voltage to the plugs for a time, the engine starts easly with the plug light going out but still suplying power to the plugs for warm up. Have fitted a new coolant senser but still the same folt. The temperture gauge has been working fine all along.