Engine Warning Light comes on itermittently

[DealgaO]

My V40 T4 1.9 has been converted to run on LPG and is now officially a bi-fuel car. It has 123,000 miles and some change on the clock. It runs very well, but every now and again the engine warning light comes on. This means, according to the handbook a fault in the ignition or fuel or exhaust systems. I know that both the petrol and gas systems are OK, the exhaust seems to be fine and the plugs only have 10,000 or so miles on them. The warning light stays on for a little while but the car runs very well indeed. After about 3 or four trips the light goes off and stays off for a good while. Anyone got any ideas as to what could be causing the problem?