Stihl Ignition Coil No Spark (Testing & Fixes)
A STIHL saw with no spark needs a step-by-step test, not a guessed coil replacement. Check the correct plug and spark tester first, then isolate the stop-switch wire, inspect the lead and connections, and verify the flywheel and module gap. Replace the ignition module only after those checks point to it.
Thirty pulls with no start is frustrating, but a no-spark test can narrow the fault before you spend money on parts. Identify your saw’s exact model and serial range first. STIHL plug types, module gaps, torque values, and test steps can vary by model, so use its manual or service information for final specifications.
Equipment Setup and Ignition-Test Safety
This section covers the safe setup for checking a STIHL chainsaw’s ignition. A magneto uses the turning flywheel to create ignition voltage. Since a spark test can ignite fuel vapor, work outdoors in a ventilated area, with the saw cool and away from spilled fuel or other ignition sources.
Engage the chain brake and, if practical for bench work, remove the bar and chain. Keep the saw stable. Disconnect the spark-plug cap before inspecting or removing the plug, and keep the ignition lead clear of moving parts.
Use an approved inline or adjustable ignition spark tester. Follow the tester maker’s instructions for connection and grounding. Do not hold an exposed plug against the cylinder to look for spark. Do not create sparks near the plug hole, fuel system, or fuel vapor.
Confirm the model before testing
The model and serial range help you find the correct plug, wiring diagram, starting steps, and service limits. A specification from another STIHL saw may not apply. Check the model label and match it to the correct manual before setting a gap or tightening parts.
Many STIHL saws specify a spark-plug gap of 0.5 mm (0.020 in). Treat that as a common value, not a universal one. Use the plug type and gap listed for your saw. The ignition-module air gap is commonly 0.2–0.3 mm (0.008–0.012 in) on STIHL magneto systems, but confirm the model’s service specification.
Field Diagnosis: Isolate the No-Spark Fault
This section follows the ignition path from the plug toward the module. Test one part at a time, with the stop switch in RUN during spark checks. A controlled sequence helps separate a bad plug, a grounded kill circuit, a damaged lead, and a possible module or flywheel fault.
1. Check the plug, cap, and lead
Remove the plug and inspect it for a cracked insulator, heavy carbon, fuel wetness, or a damaged electrode. A wet or fouled plug can fail to spark well even if the ignition module works. Check the gap against the model manual.
If you have a known-good plug of the correct type, use it for the test. Connect the spark tester as directed and crank the saw with the switch in RUN. A visible spark at a plug in open air does not prove it will spark under compression; the tester gives a more useful check.
Inspect the plug cap and high-voltage lead for cracks, loose fit, chafing, or corrosion at a connection. Do not assume a new plug fixes a damaged cap or lead.
2. Test the stop-switch circuit
The kill wire stops the engine by grounding the ignition module. In RUN, it must not ground the module. In STOP, it must ground it. Check the model’s wiring diagram before probing wires, because connector layout and wire routing vary.
If the tester shows no spark, switch off the saw and disconnect the kill-wire lead at the module. Keep the disconnected lead secured away from fuel and moving parts. Retest with the switch in RUN.
- Spark returns: The module can produce spark, so look for a shorted stop switch, pinched insulation, damaged wiring, or a connector fault.
- No spark: Continue with the plug, cap and lead, module gap, flywheel, and module checks.
Reconnect the kill wire after the test. Do not start or operate the saw with the kill wire disconnected. The stop switch cannot shut the engine off in that state.
3. Inspect the flywheel and module
With the saw off, inspect the flywheel magnets and the ignition-module faces. Look for debris, corrosion, damage, or marks that suggest the parts have touched. Check that the flywheel is secure. If the saw took an impact or parts may have shifted, a damaged keyway or displaced flywheel may need inspection under the model’s service procedure.
Check the module air gap with the specified gauge or method. Align the flywheel magnet with the module as directed by the manual, then set the gap to the model’s value. The common 0.2–0.3 mm range is a guide only; do not use it in place of the correct specification.
Quick test guide
| Test result | What it suggests | Next step |
|---|---|---|
| Spark with a known-good plug | Original plug may be fouled or damaged | Fit the correct plug and gap |
| Spark only with kill wire disconnected | Switch circuit or wiring may be grounding the module | Inspect switch, wire routing, and connectors |
| No spark; plug cap or lead is cracked | High-voltage path may be open or leaking | Repair or replace the affected part as specified |
| No spark; gap is outside model spec | Module-to-flywheel distance may be wrong | Set the specified gap and retest |
| No spark after all checks | Module or another ignition fault remains possible | Use model-specific service tests before replacing parts |
Ignition-System Repair, Maintenance, and Verification
This section explains when to repair a connection, adjust the module, or consider module replacement. A coil or ignition module is the part that sends high voltage to the plug, but a no-spark symptom alone does not prove the module is bad. Confirm the simpler causes first.
Avoid the resistance-test trap
There is no single resistance value that passes or fails every STIHL ignition module. Electronic modules can also give misleading readings on a basic multimeter. Do not condemn a module based only on a generic online resistance figure.
Use the service test for the exact model, or compare with a known-good compatible module if the correct procedure calls for it. Replace the module only after verifying the plug, cap and lead, kill circuit, connectors, flywheel condition, and air gap.
Correct wiring and gap faults
Repair damaged insulation and loose or corroded connectors using the model’s parts and service instructions. Route wires as designed, away from hot or moving parts. If the stop switch or wiring is the cause, retest spark with the circuit correctly connected.
If you adjust or remove the ignition module, follow the manual for fastener torque and the setting method. Do not guess at torque values. After the work, rotate the flywheel by hand as allowed by the service procedure and confirm that the module does not contact it.
Confirm the repair safely
Reconnect the kill wire and all ignition connections. Refit any covers or parts removed for access, using model-specific torque values. Before starting, verify that STOP grounds the module and shuts the engine off. Start outdoors using the manual’s cold- or warm-start procedure, then check for stable idle and throttle response.
Do not run the saw with guards or covers omitted. If the engine starts unexpectedly while the kill wire is isolated, keep clear, do not operate the saw, and restore the kill circuit before further use.
Diagnostic Examples: What the Test Results Can Tell You
These are illustrative fault patterns, not claims about a specific STIHL model. They show how test results guide the next check. The key point is to change one condition at a time and avoid replacing an ignition module until the test evidence supports it.
Pattern A: Spark returns when the kill wire is isolated
A saw has no spark with the switch in RUN. With the engine off, the kill wire is disconnected at the module, then the spark test is repeated. Spark returns.
That result points toward the stop-switch circuit or its wiring, not automatic proof of a failed switch. Inspect for pinched or chafed insulation and damaged connectors, then test the switch against the wiring diagram. Reconnect the kill wire and verify STOP shuts the engine off.
Pattern B: No spark remains after the kill-wire test
A known-good, correct-type plug is installed, the tester still shows no spark, and isolating the kill wire changes nothing. The plug cap and lead look intact, but the module gap is outside the model’s specification.
Set the gap using the manual’s method, then retest with the kill circuit restored. If there is still no spark, inspect the flywheel and connections and use the model-specific module test before ordering a replacement. This avoids treating one symptom as proof of one failed part.
Storage and Seasonal Care for Reliable Starting
Storage care can reduce avoidable ignition faults, though it cannot prevent every electrical failure. Before putting the saw away, follow its manual for fuel and storage steps. Keep the plug cap, ignition lead, and stop-switch wiring free of damage, and store the saw where it will not be crushed or exposed to conditions outside the manual’s limits.
Before the next use, inspect the plug and visible wiring. If a saw develops no spark after storage, repeat the same sequence: correct plug and tester, kill-circuit isolation, wiring inspection, then flywheel and module checks. Do not skip to a coil purchase because the saw sat unused.
Conclusion: Replace the Module Only After Testing
A no-spark fault is best diagnosed by tracing the ignition path in order. Check the correct plug and tester, test the kill circuit, inspect the cap and wiring, and verify the flywheel and module gap. Use model-specific service information for limits and fasteners. Once repaired, reconnect the kill wire and confirm the stop switch works before running the saw.
Frequently asked questions
Why does my STIHL chainsaw have no spark?
Possible causes include a fouled or wrong plug, damaged cap or lead, a kill wire grounded in RUN, a poor connection, an incorrect module gap, flywheel damage, or a failed ignition module. Test in that order before replacing parts.
Can a bad stop switch cause no spark?
Yes. A faulty switch or damaged wire can ground the module even in RUN. Disconnect the kill lead only for the stationary test described above, then reconnect it and verify shutdown before operating the saw.
What spark-plug gap should I use?
Many STIHL saws specify 0.5 mm (0.020 in), but the correct plug and gap depend on the model. Check the manual for your exact model and serial range.
What is the STIHL ignition-module air gap?
A 0.2–0.3 mm (0.008–0.012 in) gap is common on STIHL magneto systems, but it is not universal. Use the service specification and setting method for your saw.
Can I test spark by grounding the plug against the cylinder?
Do not use that method. Use an approved ignition tester, and test outdoors away from fuel, vapors, and spilled fuel. Never make a spark near the plug hole.
Does spark in open air prove the ignition module is good?
No. A visible open-air spark does not reliably prove the spark will remain adequate under compression. Use an approved tester and the model’s service procedure.
Should I replace the ignition module if a multimeter shows a bad reading?
Not from a generic resistance value. STIHL modules do not share one universal pass/fail reading, and electronic parts can mislead basic meters. Use the model-specific test or an approved comparison method.
Can I run the saw with the kill wire disconnected?
No. The stop switch cannot shut the engine off while the wire is isolated. Keep the lead secured during testing, reconnect it afterward, and confirm the STOP function before use.
What should I do if there is still no spark after the kill-wire test?
Recheck the plug, cap, lead, connectors, module gap, and flywheel condition. If those pass, follow the exact model’s service test before replacing the module or seeking repair.
How do I know the repair is complete?
Reconnect the kill circuit, refit covers, and verify that STOP shuts the engine off. Then start the saw outdoors using the manual’s procedure and check for stable idle and throttle response.
(This article was written by one of our staff writers, Robert Callahan. Visit our Meet the Team page.)
