It “might” be oil in the line. This was a gas problem a while ago, I remember. Oil is included in the LPG we buy and can find it’s way into the lines. One way to stop it is by having vertical pipe runs from the tank and “low points” that collect the oil and stop it getting to inaccessible points in the pipes. The other option is to fit a filter.
Any plumber/gas fitter can help you. You need to switch off gas AT THE BOTTLE . Then attach a pump to the appropriate gas line. Pressurise it to clear any build up of oil. The gas fitter will know if all is normal from his pressure gauge.
Another potential problem is dirt, dust and rust that collects on and round the gas jet and Piezoelectric igniter, situated under the hot tank. It is extremely difficult to get at – the best route to start with is via the exhaust outlet on the rear nearside corner of the Exsis. Pages 117 and 118 of the Little Exsis Book show how to remove the ‘chimney’ covers to give you access, albeit limited, to the boiler burner jet and igniter. You will need a long brush (one of those long, ‘bendy’ brushes are ideal) and a vacuum cleaner nozzle. Brush clean as much as you can and vaccuum as much debris as you can, from the gas jet and igniter area. This might solve the problem.
The other problems (if the igniter seems to work and there’s no obstruction in the gas line) could be the automatic open/shut gas valve OR the circuit board alongside the boiler, are defective. I hope it’s not these because they’re difficult or expensive to sort. This is all discussed in the boiler entry starting on page 105 of the Little Exsis Book.
Let us know how you resolve this?
Good luck.
Barry