Boiler not working after filling loop replaced

Usually this happen when engineer drain down the boilers to fit the filling loop. Solid red light mean, boiler is 1.overheating 2. Air trap in heat exchange This maybe fix the issue 1. Make sure that an automatic air vent on the boilers(top of the pump) is opened 2. Make sure the pump is fully spinning, take the silver knot off the front of the pump and try to stop it With screw driver (you should not be able to stop it) 3. Make sure the flow and the return isolator is fully open. As if this is a plastic type isolation valve, sometime it may look that fully open but it not. (take the plastic off and turned it with a spanner)—p.s ignore this if it is a brass type 4. If try all these and not working then 4.1 replace overheat sensor 4.2 Replace PCB Make sure you are competent to be able to do this

Hard to say if its connected with the replacement filling loop or something hes done or not done when fitting it. I'm thinking he would've isolated boiler valves to fit the loop so has he reopened them fully because if he didn't open them fully the boiler would run for a short time through the boiler bypass then water would get to hot then lock out and would only start again once the heating water cooled down enough. If hes a respectable engineer I'm sure he wouldn't mind or charge to come back and have a quick look but obviously youd have to ask him that question. I wouldn't recommend trying to fix anything yourself as has been suggested as if the engineer has caused a fault he might blame you for it by trying to fix it yourself...

Original answer is sort of correct but you must use a Gas Safe engineer to carry out rectification work. Once the cover is removed by non registered operative or householder it becomes illegal gas work. At least that's what every assessment center I have been to has told me.