a mate of mine had a problem like this before!!!!
happened when he put diesel into his petrol astra
light was on full time, then would turn off and back on. turned out that after replacing the cat converter it was #
still on. so took the manifold apart and could see that the seal around the manifold had melted away, (due to the diesel)
filled her up with silicone, and is perfect!!!!!
maybe im well off but, worth a try