Why won't my lawn mower start?

Seven things to check before you bring it in. Most of them take five minutes and cost nothing.

Every spring we get the same machine through the door about forty times: it ran fine in March, it sat in the shed all winter, and now it will not fire. Nine times out of ten it is one of three things, and two of those you can sort out yourself in the time it takes the kettle to boil.

Here is the order we check things in on the bench, which is also the order that finds the fault fastest.

Check these first

1. Old fuel. Petrol goes off in about thirty days, and modern fuel with ethanol in it goes off faster. If it has been sitting since autumn, drain it and put fresh in before you do anything else.
2. The spark plug. Pull it out. If the tip is black and wet, it is fouled. If it is white and chalky, it has been running lean. Either way a new plug costs less than a coffee.
3. The air filter. Hold it up to the light. If you cannot see through it, the engine cannot breathe through it either.
4. The fuel tap and primer. Obvious, and skipped constantly. Make sure the tap is open and prime it the number of times the manual says — not more.
5. The kill switch and bail arm. The cable stretches over time. If the bail is not pulling the switch fully closed, you get no spark at all.
6. The blade and the underside of the deck. A deck packed with old clippings will stall an engine that is otherwise fine. Disconnect the plug lead before you tip it.
7. The oil level. Many modern mowers have a low-oil cutout that will refuse to start rather than let you destroy the engine. It is doing you a favour.

If it has fuel, spark and air, it will run. Everything else is just working out which of the three is missing. — The bench, most Mondays

When to bring it in

If you have done all seven and it still will not start, stop. The next steps are the carburettor and the ignition coil, and both of those are jobs where the wrong guess costs you a part you did not need.

One thing that genuinely helps us: do not drain the fuel before you bring it in. What is in the tank tells us a great deal about what has happened, and a machine that arrives dry is one we have to diagnose from scratch.