check it runs well on petrol, ignore the LPG initially and check it as normal.. once you are happy see how well it switches to LPG, whether it runs smooth or badly when first switched over..

check the airbox for signs of backfire
check that it drives as well as on petrol when on gas
check the filler for the gas is secure and not loose where someone has had a driveaway
floor it hard at a low speed and listen for backfires
check the heating works - gas conversions can have a detrimental impact on heating, and the rest of the engine coolant system especially when they are not used properly - switching to gas too early can cause the coolant pipes to the heater matrix to freeze as they are shared with the gas thingy (I'll remember what it's called in a minute)

sure others will suggest other checks as well..
 
just check the colour is ok!! well thats what my wife does!!!! lol

andy drips, clunks, wirrs bangs, anything taht can sound look wrong question it.

now look for the tidyness of the lpg installation as rough looking jobs can be rough running!!!

check operation of door locks, windows, boot lock lights, sunroof,

also check for water in the oil or oil in the water, check aircon works and every other button. check heat goes from floor to window when buttons pressed.

a couple of things i have had to do: oil pressure switch, leaking. door locks, not working correctly. windows didn't go up correctly. couple of pipes under bonnet not fastened properly. all round brake pads and caliper refurbish

basically check everything, a good couple of hours checking, note the bad bits and barter!
 
Yeah all LPG conversions should be sequential LPG conversions which are teh latest technology and accepted by insurance companies. Plus they must be accompanied by a certificate otherwise its worthless.

Other things to check.

Has the car had issues with the remotes? battery drain etc?
Has the RF Receiver been replaced with the latest 3rd revision that actually works and wont get you stranded in car parks with Wifi signals that wake up the BECM and drain your battery? (worth £140 plus labour)
Does the air suspension work? Press the button and raise the car up and down a few times and check the ride height of all tyres. Are they all the right height?
Has the compressor been replaced? (£400)
When was the last service? What was carried out?
Has the exhaust system been replaced? £600 for new system)
Has it had a new battery?
Has it had an upgraded alternator? 120 amp or above? This is required as its electrics are quite hungry and put a strain on the electric system and an upgraded alternator is a must.
 

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