Youre supposed to have the boiler running for a few minutes before sticking in the pokey bit in the pokey bit recepticle. To let the boiler clear up pooled up condensate, but if the dummy installer has put in the flue horizontally, that wouldnt help. I swear they need to put in flue level sensors that beep like a reversing lorry if the flue is not at the right angle or slopes outwards.
Open the device to the smallest bits and dry with a hairdryer. I would even source the cell if its really bad. Then send it for calibration (they dont open them for this). If you send it in as it is, they will most likely tell you its got baby crocodiles inside and make you buy a new one. I was an electronics engineer before getting my gas card, and I swear manufacturers make their owners think that the insides is rainbow butterflies and unicorns, and you need to feed it magic dust to fix it. Its a rather simple device with the only proprietary thing inside being the main IC (the chip). Everything else is readily available sensor cells, resistors transistors and diodes.