No spark: diagnosing the ignition chain

No spark: ignition chain

If a plug that looks fine (not fouled, not wet, gapped correctly) throws no spark when grounded against the head while the engine is cranked, the fault is in the ignition chain upstream of the plug. A weak or intermittent spark points the same way. The chain has a genuine order. Kill switch and safety interlocks come first, because they cost nothing to rule out and fail more often than people expect.

Then the stator or pickup coil, the signal source. Then the ignition module, either CDI or ECU. Then the ignition coil, stepping up to spark voltage. Last the plug cap, plug wire and plug itself, which the won't-start check order already covered.

2. Stator or pickup coil; No signal being generated at all, the most common true ignition failure on an older or high-hour machine.

5. Plug cap, plug wire, plug; A physical connection or the plug itself, closest to the symptom but not the most common cause.

2. Stator or pickup coil: is a signal being generated at all.

The stator (on most small-engine ignition systems) or pickup coil generates the raw signal the rest of the chain depends on. A failed stator produces no signal at all. Age, heat cycling and vibration make it one of the more common true failures on a machine with real hours on it.

Test it with a multimeter: phase-to-phase resistance should be low and equal across all three pairs, typically 0.1 to 1.0 ohm, with no continuity to the case, and AC output at about 5,000 rpm should be equal across the pairs at roughly 60 volts or more.

3. Ignition module: CDI or ECU.

The module takes the raw signal, times it to the engine's position, and fires the coil at the right moment. A failed module is a less common cause than a failed stator but not rare. It is usually diagnosed by elimination, confirmed signal in from the stator, confirmed no output to the coil, instead of by a simple pass/fail test most owners can run without a manual's specific procedure.

Where this stops being a home diagnosis

Testing the stator, ignition module and coil takes a multimeter and the resistance and output figures for your machine. A typical three-phase stator reads 0.1 to 1.0 ohm between phase wires and shows no continuity to the engine case; coil and module figures vary too much between machines to generalize, so get them from the manual.

If you have ruled out the kill switch and interlocks and cannot get the test procedure for the remaining three components, hand the diagnosis to a shop. Do not swap parts on a guess.

Troubleshooting questions

What causes a weak or intermittent spark?

A weak or intermittent spark usually means a component that is failing, not one that has failed: a plug near the end of its life, a coil that breaks down when hot, a marginal stator connection or a corroded plug cap. Work the same order (kill switch, stator, module, coil, plug cap), looking for a degraded reading at each step, and test again when the engine is hot if the fault only appears then.

Can a dead battery cause no spark on a machine that otherwise cranks normally?

On most kick-start and many electric-start machines, the ignition system runs on its own separate low-voltage circuit or the stator's own output, not solely on battery voltage. A machine that cranks with real effort on the starter but still has no spark is not usually a simple dead-battery problem.

A battery voltage low enough to affect the ignition module on some fuel-injected machines is possible, though; it is worth ruling out early if the battery is also suspect.

Is it worth just replacing the stator, module and coil together to save time?

Not usually. These are not inexpensive parts on most machines. Replacing all three without testing means paying for two working parts alongside the one that was bad; it gives no better odds of fixing it than testing each in order. Working the chain in sequence, even without special tools for every step, at minimum narrows which one or two parts are worth paying for.

Can moisture or a wet ride cause no spark?

Yes, and it is worth ruling out before assuming a component has failed. Water intrusion into a plug cap, a connector, or a poorly sealed coil can cause spark to fail temporarily while wet. It returns once everything dries out. A no-spark condition that appeared right after riding through deep water or a heavy rain points at moisture instead of a genuine component failure. So does one that clears once dry.

How do I test a stator if I do not have the exact spec?

Compare the phases with each other. With the stator unplugged, each pair of phase wires should read the same low resistance, typically 0.1 to 1.0 ohm, and no wire should read continuity to the engine case. A phase that differs from the other two, or any ground reading, means a failed stator even without the manual.

Then run the engine at about 5,000 rpm and check AC volts across each pair: all three should match and typically land at 60 volts or more.

My kill switch tests fine with a multimeter but I still suspect it, what am I missing?

A switch that tests fine sitting still on a bench can still fail intermittently under vibration or heat once installed. A hairline crack in a switch contact, or a connector that is marginal but not fully open, can pass a static continuity test; it can still cause an intermittent no-spark symptom while riding. Wiggle the switch and its connector while testing, not testing it motionless; that catches a class of fault a static test misses.

Does no spark always mean an electrical component has failed?

No. The chain starts with the kill switch deliberately. A mechanical or positional issue (a switch not in run, an interlock not satisfied) produces an identical symptom to a genuine electrical failure; it is far more common as a cause. Reaching this page assumes those simplest causes were already ruled out on the won't-start page. If they were not, that is worth rechecking before testing any electrical component.

Can a blown fuse cause no spark?

On a machine where the ignition system shares a fuse with other circuits, yes. A blown fuse can present as a no-spark condition that looks identical to a component failure further down the chain. Check the fuse box, including any inline fuses near the ignition components themselves. It costs almost nothing. Do it before testing the stator, module or coil.

Why does my machine have spark at idle speed cranking but lose it under load?

This pattern points more at a marginal or overheating component than a fully failed one. A stator or module right at the edge of working can produce enough signal at low cranking speed; it can fail once the demand or heat increases under running load. A harder fault to catch with a static test; it often needs testing while the engine runs. That is home diagnosis's practical limit.

Is a CDI box the same thing as an ignition module?

Yes, in the sense most powersports manuals use. CDI (capacitor discharge ignition) is one module type. On a machine that uses it, "CDI box" and "ignition module" refer to the same component. Fuel-injected machines more often integrate this function into the main ECU instead of a separate CDI box; that is why this page uses "ignition module (CDI or ECU)" instead of assuming one term fits every machine.

Can aftermarket electrical accessories cause an intermittent no-spark fault?

Yes. An improperly grounded or wired accessory (lights, a heated grip kit, an alarm) can introduce electrical noise or a voltage drop that interferes with the ignition system. This is most likely on a machine with a marginal stator or module already close to its limit. An intermittent fault that started after adding an accessory is worth investigating as a wiring interaction before assuming a component failed on its own.

Should I check for spark before or after checking fuel?

Fuel first, per the won't-start check order. Fuel problems are more common than ignition failure. Ruling out fuel before spending time on the deeper ignition chain on this page saves effort in the majority of cases; this page assumes you have already confirmed fuel is present. It also assumes spark is genuinely, confirmedly absent, not just that spark seemed like a reasonable guess.

Won't start

The fuel/spark/air/compression order this page picks up from at step 2.

Spark plugs: fouling, gap and replacement; For a plug that does fire but fouls or wears, instead of one confirmed dead.

Battery and charging troubleshooting

A failing stator affects charging too, not just spark: worth checking both if either is suspected.