EMCOR Drake & Scull's painful and costly restructuring helped the UK outfit return to the black last year.
The West London-based firm made £3.1 million before tax on £383.2 million turnover in 2004 after losing £11.7 million on £347.4 million turnover in the previous year.
Chief executive Tony Whale said: 'We've gone through a lot of pain to get to where we're at.'
The business made £125,000 at an operating level, with £3 million pocketed from selling plant arm Delcommerce and a South African subsidiary.
The firm's US parent continues to take a hit on the restructuring, which covered other parts of the group.
Emcor made £8.9 million before tax in the first half of this year on static turnover of £1.3 billion after losing £2.4 million a year ago, but still booked a £3 million loss from restructuring.