2000 Vauxhall Corsa Diagnostic Light

Electrical problem
2000 Vauxhall Corsa 4 cyl Two Wheel Drive Manual 50.000 miles

Hi there i have a problem with my corsa when the Engines cold it runs great as soon as the Engine reaches its running temp Approx 90 i take my foot off the power to slow down the Diagnostic light comes on as soon as i stop the light goes out it only does this when the Engine is hot i can also turn on the Heater to get rid of some hot Air and that stops the light coming on can you Help


Thanks ian