Citroen C4 VTi LPG Converted - depollution system fault

Hello! I found there many forum threads regarding this problem, but I couldn't find any similar to mine.
I have 1.6 16V VTi, 2008, LPG converted. Few days ago, while driving, at the moment when from petrol switches to gas, the depollution system fault appear. The engine switched off. After moment I switched it on, engine was working normally, but the depollution system fault and check engine still appeared. After driving, when I turned off the engine, engine cooling fan was still working and it switched off only after some 10 minutes. The next day, when I turned the engine on, just to check, isn't the fault disappeared, the fault appeared again (even not driving) and at the same time cooling fan started to work again (it wasn't hot day). After I switched engine off, fan was still working and it switched off only after some time.

Does anyone experienced the same problem?

What can be wrong?

Thanks