Caterpillar GPE35N Ser # T13F58011 forklift with a K25 engine Ser # 036622X. This machine has been repaired in the field before. At the time it had low power as if it was going to derate mode. It also had a fault code E-21 ( air flow meter fault ), the sensor was replaced by a contractor. I believe this did not fix it. Other fitters went to the machine at different times to troubleshoot the engine but in doing so introduced may more fault codes. The electrical harness was also tested by electricians. At this stage most of the sensors and controllers had been replaced and fitted with the incorrect ECM. The machine was then brought to the workshop were I started to work on it. I started to test and clear all the fault codes until I ended up with only one E-36 ( LPG pressure sensor fault. At this time I found that the ECM is off an older model engine with a different LPG sensor. Therefore the ECM was unable to read the LPG pressure and the engine ran very poorly. For some reason the original ECM was lost. The factory people gave us the correct part # ECM and I then fitted it. The ECM now reads LPG pressure but will not let the engine start on LPG and has no fault codes. The engine runs very well on gasoline. I then refitted the wrong ECM and the engine would then run on LPG, again with code E-36. I refitted the new ECM and rechecked all the learning calibrations with a laptop. Still it did not want to start on LPG. I also checked the LPG pressure at the rail at 4 psi ( in spec ). With a laptop I can't see anything that would stop it from running. I also tested the signal to the LPG injector but the ECM is not sending anything. Can someone please help us.