Check engine light has come on..

My 1998 discovery 3.9 V8 LPG runs perfectly but while sitting behind a bus belching out fumes in slow moving traffic, my check engine light has come on....never seen it before, didn't even know I had one.
Oil level ok,
temp ok,
water/coolant level ok
Power steering level ok
Auto trans fluid level ok

Car still starts and idles effortlessly runs like a dream.

Any ideas on what could be causing light to come on.

How do I reset.

Thanks
Craig

Hi
Mine is on 100% of the time something to do with LPG system, doesn't affect car or performance, Same thing happens on wife LPG Jag X Type