North West Ambulance Service is replacing engines in 103 ambulances after five caught fire.
The trust has so far installed 70 new engines in the Mercedes 515 vehicles at a cost of £5,000 each.
The total bill is expected to reach about £515,000.
The most serious was an incident involving Kellie Lamb, 23, of Maryport, Cumbria, who was in labour and travelling by ambulance last October when it set alight.
The ambulance service says mother and baby were unharmed and ‘completely fine’.
The vehicle was one of three involved in fires in Cumbria in 2012. In each case, the cause was attributed to engine failure and the ambulances had to be written off.
In addition, there were two less serious incidents involving vehicles from Cheshire and Merseyside area damaged by fire due to an issue with the ‘remote boost start control’.
An ambulance spokeswoman said: “The trust is totally committed to ensuring the health and safety of all its staff and patients and therefore, as a precautionary measure, and in line with recommendations made by the manufacturer, the trust has and is continuing to undertake an engine replacement programme on all the Mercedes 515 vehicles as and when the vehicle mileage reached 100,000 miles.”
A Mercedes Benz spokesman said the company was working closely with the ambulance service to ensure the reliability of vehicles.
He added: “Mercedes-Benz sells 24,000 light commercial vehicles a year in the UK, and whilst three issues experienced by the NWAS is a small number of technical failures, to our standards it’s not acceptable. And that’s why we're not going to stop working hard to ensure our vehicles are correctly maintained and supported to help avoid any future instances.”