Stihl TS 420 No Spark (Ignition Troubleshooting)
A TS 420 with no spark may have a fouled plug, a damaged lead, a shorted stop-switch wire, or a failed ignition module. Work outdoors with the machine cool and dry, and test with an approved spark tester. Never hold a plug against the cylinder to check spark. Repair the stop circuit before using the machine.
A pull cord that turns the engine without starting can point to an ignition fault, but it does not prove one. A wet plug may be flooded, while a damaged stop wire can cut spark in and out. I start with simple checks, then isolate parts in order, so a small fault does not lead to an unnecessary module replacement.
The TS 420 is a cut-off machine, not a chainsaw. Its 2-stroke engine uses a 50:1 gasoline-to-oil mix, and it takes a compatible cut-off wheel up to 350 mm (14 in). Use the serial-number-specific manual for replacement parts, setup details, and service limits.
Prepare the TS 420 for a safe spark test
A safe test starts with a cool, dry machine on stable ground, outdoors and away from fuel vapors. Remove the wheel for ignition diagnosis if practical, and follow the manual when handling or securing the machine. Wear suitable eye, face, hearing, hand, foot, and respiratory protection. Keep people at least 15 m (50 ft) away.
Before touching the ignition, move the stop switch to RUN. Inspect the plug cap, high-tension lead, and visible wires. Look for loose connections, cracks, fuel contamination, pinched wires, or dark carbon tracks that may let electricity leak. Do not run the engine with its wheel guard removed.
A spark tester is a tool that checks whether the ignition system sends a spark through the plug lead. Use an approved inline or adjustable tester according to its maker’s instructions. Do not hold a removed plug against the cylinder, or make an exposed spark near fuel, vapors, or spilled fuel. Ignition sparks can ignite fuel.
| Check before testing | What to confirm |
|---|---|
| Machine position | Stable, outdoors, engine cool |
| Stop switch | Set to RUN for the test |
| Ignition area | Dry, with no spilled fuel nearby |
| Test equipment | Approved spark tester, used as directed |
| People and protection | Bystanders at least 15 m (50 ft) away; suitable protective gear |
Check the plug, cap, lead, and stop switch
These parts carry or control the ignition signal. A loose cap, worn plug, damaged lead, or stop wire touching ground can all prevent spark. Inspect them before suspecting the ignition module, and use a correct replacement plug confirmed by the machine’s manual.
Inspect the spark plug and test with a known-good plug
A spark plug is the part that makes the ignition spark inside the cylinder. With the engine cool, remove it and look for wet fuel, heavy carbon, cracks, or damage. A wet or carbon-fouled plug can give a misleading result, so do not assume the module has failed based on that plug alone.
Check the electrode gap against the specified 0.5 mm (0.020 in). If the plug is damaged or fouled, test with a known-good plug of the correct type. Confirm that type in the serial-number-specific manual; do not choose a plug based only on appearance or a general parts listing.
Connect the approved tester as its instructions describe, then pull the starter as directed. Keep your hands clear of moving parts, and keep the test area free of fuel. If the tester shows spark, the ignition is producing a signal under that test. Refit the plug and continue to fuel, air, and compression checks if the engine still will not start.
Inspect the cap, lead, and stop-switch wiring
The high-tension lead carries high voltage from the ignition module to the plug. Check that the cap fits firmly and the lead has no cuts, crushed sections, loose joints, or signs of heat damage. Replace damaged parts with the correct parts, then route wires as specified so they cannot touch the flywheel or hot components.
The stop switch shuts down the engine by grounding the ignition circuit. Its wire can suppress spark if it is pinched, damaged, or touching a grounded metal part. Inspect the visible wiring and switch for damage. If the fault comes and goes when the machine or wiring moves, intermittent contact is possible; do not keep using the machine until the cause is found.
Isolate the stop circuit and ignition module
If a known-good plug and careful inspection do not restore spark, the next checks involve the stop circuit, ignition module, and flywheel. These parts need safe, controlled testing. A qualified technician should handle isolation and internal checks if you lack the right tools or service information.
Use stop-wire disconnection only as a controlled test
A shorted stop wire can prevent ignition even when the module is working. A technician may disconnect the stop wire briefly to see whether spark returns, following the service manual and proper test procedure. This is a diagnostic step only, not a repair or a way to operate the machine.
Never run or return the TS 420 to service with the stop wire disconnected. The switch must remain able to stop the engine. After the test, reconnect the wire and repair any damaged insulation, switch, connector, or routing. Confirm that the stop switch shuts the engine off before cutting.
Have the module and flywheel checked to manual limits
An ignition module creates and sends the spark signal. If the stop circuit is sound but the tester still shows no spark, a technician can inspect the module, wiring, and flywheel magnets. The module-to-flywheel clearance must match the service-manual specification for that machine. Do not bend or pry parts to change the gap.
A failed module is only one possible cause. Loose connections, damaged wiring, a faulty stop switch, or a flywheel problem can produce similar symptoms. Use the correct part for the machine’s serial number and market, and follow the manual for clearance, fastener torque, and assembly details.
| Test result | Likely next step |
|---|---|
| Spark appears with a known-good plug | Check fuel, air, and compression if it still will not start |
| No spark; visible wiring damage | Repair or replace the damaged lead, cap, switch, or wire |
| No spark; stop circuit may be grounding | Have a qualified technician isolate and test the circuit |
| Stop circuit checks out; still no spark | Inspect module, flywheel, and clearance to manual specification |
| Spark loss comes and goes | Find and repair loose or intermittent wiring before use |
Repair, reassemble, and verify operation
A repair is complete only when the machine sparks reliably and the stop switch works. Use correct parts and restore all wiring, covers, and guards before starting. If ignition checks pass but the engine does not start, move to separate fuel, air, and compression checks rather than replacing ignition parts at random.
Use correct parts and service information
Replace a defective plug, lead, cap, stop switch, or ignition module with a part confirmed for the TS 420’s serial number. Route and secure the wiring as the manual shows. If module clearance or fastener torque is not listed in your manual, have a qualified technician complete that work rather than guessing.
The TS 420 has a 66.7 cm³ 2-stroke engine. Use fresh 50:1 gasoline-to-2-stroke-oil premix made with suitable oil. Stale or incorrect fuel can cause a no-start even when spark is present, but it does not by itself prove an ignition fault. Follow the STIHL manual and local rules for fuel handling and storage.
Confirm spark, starting, and shutdown
After testing, reconnect every wire, refit covers and guards, and check that the wheel guard is in place. Start the machine only when it is secured as directed in the manual. Confirm stable idle and test that the stop switch shuts the engine off. Do not cut until both checks pass.
The TS 420 is designed for a compatible 350 mm (14 in) maximum wheel, with maximum spindle speed of 5,350 rpm. Check the machine manual and wheel markings before use. These wheel limits do not change the ignition test, but they matter when returning the machine to service.
Illustrative diagnostic scenario: A TS 420 has no spark on its old plug. A correct known-good plug still shows no spark on an approved tester. Inspection then finds a pinched stop-switch wire. A technician tests the circuit under controlled conditions, repairs the wire, reconnects it, and verifies both spark and shutdown. This example shows the diagnostic sequence; it is not evidence that every no-spark machine has a damaged wire.
FAQ: TS 420 ignition troubleshooting
These short answers cover the most common checks when a TS 420 will not show spark. They do not replace the machine’s manual or a qualified technician’s inspection. Stop testing if fuel has spilled, wiring is damaged, or you cannot keep the test area safe.
Why does my TS 420 have no spark?
Common causes include a fouled plug, loose or damaged plug lead, stop-switch wire shorted to ground, faulty switch, or ignition-module or flywheel problem. Test with a correct known-good plug and approved spark tester, then inspect wiring.
Can I test spark by holding the plug against the cylinder?
No. Do not create an exposed spark near fuel or fuel vapors. Use an approved inline or adjustable spark tester and follow the tester maker’s instructions.
What should the spark plug gap be?
The specified electrode gap is 0.5 mm (0.020 in). Confirm the correct plug type and service details in the manual for your machine’s serial number.
Can a wet plug cause a false no-spark diagnosis?
Yes. A wet or carbon-fouled plug can mislead a test. Try a known-good, correct-specification plug with an approved tester before concluding that the ignition module has failed.
Should I disconnect the stop-switch wire to get spark?
No. A qualified technician may disconnect it briefly as a controlled diagnostic test, but it must be reconnected before operation. Running with the stop circuit disabled removes reliable engine shutdown control.
What if the tester shows spark but the engine still will not start?
Check fuel, air, and compression as separate systems. Confirm fresh 50:1 premix and follow the manual’s starting procedure. Spark alone does not confirm that the engine has enough fuel or compression to run.
When should I replace the ignition module?
Replace it only after the plug, lead, stop circuit, wiring, and related checks point to module failure. Confirm the part and setup for the machine’s serial number, and follow the service manual or use a qualified technician.
Can I operate the machine if spark cuts in and out?
No. Intermittent spark may come from a damaged wire or loose connection, including a stop wire that touches ground. Repair the fault and verify reliable shutdown before returning the machine to work.
What should I do after a spark test?
Reconnect all wiring, refit covers and guards, and check the stop switch. Start only with the machine secured as directed in the manual; confirm stable idle and proper shutdown before cutting.
(This article was written by one of our staff writers, Harrison Croft. Visit our Meet the Team page.)
