Van fleet operators are being urged to either buy vehicles converted by manufacturers or go to a company which is a member of the LP Gas Association for aftermarket conversion.
The investigation was carried out by Birmingham Trading Standards, which found a variety of faults, some of which could potentially have led to an explosion.
At present, about 72,000 vehicles run on LPG in the UK, but only 35-40% of these have been converted by LPGA members.
Paul Oxford, sales development manager at LPG provider Calor Autogas, said: 'LPG has an enviable safety record. If fleets are going down the LPG route, they will have no problems if they used an LPGA installer.'