lpg conversion

[subo+]

firstly i have no mechanical experience. i had my 2001 s80 140 bhp with approx 70,000 miles converted to lpg about 10 months ago. last month my engine management light came on and the car was running very lumpy. is it called the lamba sensor at the back of the cat anyway that was diagnosed as the problem by the chap who by the way is fully registered to carryout conversions and i have good faith in, changed the sensor, within switching the ignition off twice the light came back on. by this time i was home which is 50mile round trip from where i had the car converted, so i went to my local garage who know the car for mot,s etc, checked and diagnosed the same fault. i told them that the part had been changed ( i saw it being changed) they said that (1) this can be gas conversion problem and (2) it may be a stored fault code, therefore i asked them to put out the light, by the way when the sensor was changed the car did and still is running very well, but after driving home and switching off the ignition i started it again and the same light came on. my question is? is there anyone who has experienced a similiar problem or maybe there is modification that can be done as i have been informed that the conversion to gas create,s alot more heat. thanks