Gas boiler engineers are failing to make basic checks and recommending expensive repair jobs instead, consumer campaigner group Which? says.
Four out of 10 engineers didn't carry out basic checks in the Which? undercover investigation.
Two of these four also recommended up to £500 of unnecessary repairs and another suggested getting a brand new boiler, which costs on average £2,459.
"Registered gas engineers should provide a professional service at a fair price, so it's inexcusable when corners are cut, faults are missed and people are ripped off," says Richard Lloyd, executive director of Which?.
In the investigation, 10 boiler engineers were asked to service a boiler. The only fault with the boiler was a disconnected lead, which meant the boiler wouldn't light. This is an easy-to-fix problem that can be rectified in five minutes.
Of the four engineers that failed to spot the fault, one was a subcontractor from national chain Homeserve. The British Gas engineer did spot and fix the problem but only after a third arranged visit.
"People should ask their friends and family for recommendations when choosing an engineer and get quotes from different suppliers to get the best deal," advises Lloyd.
Five ways to check you're using a reputable boiler engineer:
1) Always use a Gas Safe registered engineer
One in three of us base our choices on friend and family recommendations without checking qualifications, according to Gas Safe Register. A registered engineer should carry their Gas Safe ID card on them at all times and be able to show this to you.
2) Check the details
Not all engineers are registered to do the same level of work so ask to read the back of an engineer's ID card, which gives details of qualifications and the work they are authorised to carry out. For example, it is illegal to fit a gas fire if the engineer is only registered to carry out boiler and pipework.
3) Double check for fakes
You can also check the engineer's licence number by calling 0800 408 5500, at gassaferegister.co.uk or by texting ‘gas' then the licence number to 85080.
4) Know what to expect
A thorough boiler service should include a visual inspection, checking with a gauge the operating pressure or heat input or both if necessary, checking the ventilation and making sure the boiler is safe.
5) Get a free safety inspection
If you've had work carried out in the last six months you can apply for a free gas safety inspection by filling in an online form with Gas Safe.