I did have an issue with mine a year or so back, but don't recall the light sequence.
The car was up to temp and had switched over to LPG by the time I got onto the open road. When I got up to speed, it gave me the warning buzzer for no LPG and switched back to petrol. I tried switching back to LPG, but it reverted back less than 30 seconds later. It was due for the LPG service soon, so took it up early. Explained the issue to them, and they diagnosed it as a faulty coil pack. They said that the LPG is less tolerant to external issues, and so switches back to petrol.
Having just had work done on the engine, is there a possibility that one of the leads to a coil pack has been knocked. Either that or you now have a faulty coil pack.
Unfortunately I have no idea if you can use VCDS to test the coil packs, and the people who worked on mine said they spotted the fault with whatever software they use on the LPG system.
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2001 A8 D2 FL 3.7 Q - LPG Conversion, Engine: AKC, Gearbox:FBB, Colour:LY5X Aqua Blue pearl effect, Interior:WJ light beige/dark beige valcona leather with burr walnut inserts
My DIY jobs - Replaced alarm horn, Replaced Coolant Temp sensor, Updated RNS-D firmware, Installed Reversing Camera and Digital TV, Dashcam Installation, Retrofitted Rear Blinds, Auto-Dimming Rear View Mirror, Chrome Boot Struts
Sold!  2008 S8 D3 5.2 V10 - Engine: BSM, Gearbox:KLW, Colour:LY7F Suzuka Grey Pearl Effect, Interior:Black/Black with Silver/Black valcona leather with carbon atlas inserts, Bang & Olufsen, Heated windscreen, Soft-close doors, TV, DAB, AMI, Quad Zone, Auto Boot, Auto dim Mirrors, Keyless entry, PDC with rearview camera, AFS II Headlights
My DIY jobs - Fan Jet Windscreen Washers, Retrofitted Electric Side Blinds
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