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Lawn mower won't start spark plug

Updated: October 2026

The mower will not fire, and a spark plug is the first part you are about to order. Sometimes that is the right part. Often the same silence comes from stale fuel, a loose lead, oil in the cylinder or a coil that has failed. This page is the decision, not the fitting guide. It follows the Briggs & Stratton UK fault notes. Prices and the twelve listings stay on the [spark plug lawnmower comparison](/hub/lawn-and-garden-equipment/spark-plug-lawnmower). Peter F. did not diagnose your mower in a workshop. A qualifying purchase can earn a commission at no extra cost to you.

The mower will not fire, and a spark plug is the first part you are about to order. Sometimes that is the right part.

Our top picks

Briggs & Stratton Briggs & Stratton RC12YC pair

Best overall

Briggs & Stratton Briggs & Stratton RC12YC pair
9.6/10about £7.99
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NGK NGK BPMR7A

Best value

NGK NGK BPMR7A
9.4/10about £3.91
View offer
UYUYong UYUYong RJ19LM, 3

Rank 3

UYUYong UYUYong RJ19LM, 3
8.6/10about £4.89
View offer
On this page
  1. 1.A dead mower is not automatically a dead plug
  2. 2.What the plug can tell you
  3. 3.A new plug and it still will not run
  4. 4.When to stop

A dead mower is not automatically a dead plug

Briggs UK lists a disconnected, dirty or fouled plug as a common no-start, and the same page lists the HT lead, a shorted kill switch and a damaged flywheel key. Stale petrol and a blocked air filter feel the same at the cord. Buying a plug first is reasonable when the plug is old, cracked or fouled, and a delay when the tank smells sour or the lead is off. Work with the engine cool. The Health and Safety Executive says refill petrol outdoors and only when the mower is cool. Do not tip a four-stroke so oil runs onto the plug. If you already know the code, the steps sit on how to change a lawn mower spark plug. A seasonal plug does not replace fresh fuel.

What the plug can tell you

Briggs UK tells you to replace a plug with stubborn deposits, cracked porcelain or burnt electrodes. A wet plug that smells of petrol has been flooded. Dry it, and do not keep cranking in the same stale fuel. A black, sooty nose points to a rich mixture, which is what Briggs links to black smoke. White or blue smoke means the engine is burning oil. Oil that returns after a new plug is a dealer job. A plug that looks ordinary is a weak reason to buy another. Check the lead, the kill switch and this season’s fuel. Many Briggs gaps are 0.030 in (0.76 mm). Some Quantum rows are 0.020 in (0.51 mm).
Briggs & Stratton Briggs & Stratton RC12YC pair

Testsieger

Briggs & Stratton Briggs & Stratton RC12YC pair
9.6/10about £7.99
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A new plug and it still will not run

A correct new plug can still leave the engine dead. Briggs then points to the lead, a shorted kill switch, and a flywheel key sheared after the blade hit something. Another plug will not recut that key. Do not hold the lead against the block to look for a spark. Petrol vapour makes that a fire risk, and the lead can shock you. One RC12YC review found the terminal too large for a new coil boot, so the code was right and the connection was not. A hot stall can be fuel or a coil, not only the plug. Settle the code before you condemn the coil. A listing only helps after that, which is the limit of the best lawn mower spark plug page.

When to stop

Change the plug when it is old, cracked, burnt or heavily fouled, and the code matches. Stop blaming it when a correct new plug still gives no spark, when oil returns, or when the thread is damaged. White or blue smoke is burning oil. Black smoke points at mixture. Call a dealer for a workplace machine when PUWER 1998 means the service has to be recorded, and call one at home when the flywheel key, the coil or returning oil is beyond a plug and fresh petrol. If the nose is clean and the lead is secure, check fuel and air before you open another listing.

Common questions

Why will my lawn mower not start, and is it the spark plug?
It might be. Briggs UK names a disconnected, dirty or fouled plug as a common no-start, and also names the lead, a shorted kill switch and a damaged flywheel key. Stale fuel and a blocked filter feel the same at the starter cord. Look at the plug if it is due, every 25 hours or once a season on the Briggs interval. Do not assume it is the only part.
What does a black plug mean?
A black, sooty nose usually means a rich mixture or a plug that has fouled, which matches Briggs linking black smoke with mixture rather than with a failed coil. Replace it if the deposits will not clean off or the porcelain is cracked. A new plug in an engine that is still running rich will blacken again. The colour is a clue, not a parts number.
What does an oily plug mean?
Oil on the nose means oil is reaching the cylinder, or you tipped the mower and drained oil onto the plug. White or blue smoke, on the Briggs fault page, means the engine is burning oil. Dry a one-time wet plug after a careful tip, with the lead off. If oil returns on the next plug, stop buying plugs and have the engine checked. A different brand will wet in the same way.
The mower starts then dies. Is the new plug faulty?
Not by default. A mower that fires and then stops can be starving of fuel, venting badly, or losing spark as a coil warms up. One UK review described RJ19LM-marked plugs that overheated and stopped a Macallister, which is a wrong-plug example, not every hot stop. If your new plug’s code matches the old one and the gap is the chart figure, look at fuel and the lead before you return the plug.
I fitted a new plug and it still will not start. What next?
Confirm the code and the gap, and confirm the lead actually grips the terminal. Briggs then points to the lead, a shorted kill switch and a sheared flywheel key. Another plug of the same code will not repair those. Do not spark-test by holding the lead near the block in petrol vapour. If you have no spark with a correct plug and a sound lead, that is a dealer job.

Related pages

Which lawn mower spark plug

Spark plug lawnmower

Best lawn mower spark plug for a code you already know

How to change a lawn mower spark plug and set the gap

RJ19LM vs RC12YC spark plug codes for Briggs engines

Sources

Buying notes from the sources below, checked for this page on 22 October 2026. Prices, stars and review counts are from the amazon.co.uk captures of that date. Two listings had no customer-reviews line.

  1. Briggs & Stratton UK: identifying the correct spark plug and gap (retrieved 22 October 2026)
  2. Briggs & Stratton UK: servicing the spark plug (retrieved 22 October 2026)
  3. Briggs & Stratton UK: engine problem solving (retrieved 22 October 2026)
  4. NGK: BPMR7A nickel spark plug (retrieved 22 October 2026)
  5. Husqvarna UK: spark plug spare (retrieved 22 October 2026)
  6. Husqvarna UK: how to check a spark plug gap (retrieved 22 October 2026)
  7. HSE: staying safe with petrol (retrieved 22 October 2026)
  8. Consumer Rights Act 2015, section 9 (retrieved 22 October 2026)