What will the engineer check during a gas boiler service?

Check Gas pressure, to ensure the boiler is operating at the optimal pressure for safety and efficiency.
Inspect the boiler to check for corrosion, leakage or other damage
Test the gas boiler flue, to ensure there are no unsafe emissions from the boiler
Inspect your boiler and controls to make sure they are operating safely
Certify your boiler has been checked and running safely and efficiently

If you are having problems, try these checks first. They could save you money.