Intermittent Pilot Ignition Troubleshooting: Honeywell S8610U Sequence, LED Codes and Checks

Intermittent pilot (IP) ignition was the step between the standing pilot and hot surface ignition. You'll still find it on plenty of older furnaces, boilers, unit heaters and water heaters. The pilot only burns during a call for heat: a module sparks it, proves it, then opens the main valve.

The Honeywell (now Resideo) S8610U is the universal replacement module for many of these systems, so this guide uses it as the example. Other modules work the same way, but timings and codes vary. Always check the appliance's own wiring diagram and the module's instructions.

Safety: The spark lead carries high voltage. Keep the burner door and gas safety checks in place, and leak-test any gas connection you disturb.

How the sequence works

From Resideo's Y8610U retrofit kit instructions (form 68-0291):

1. A call for heat powers the module. It runs a self-check.
2. The trial for ignition starts: the spark turns on and the pilot valve opens.
3. The pilot flame is proved by flame rectification through the igniter-sensor and the burner ground.
4. Spark stops. After a 2-second flame stabilization period, the main valve opens.
5. If the pilot isn't proved during the trial, the module shuts the pilot valve and spark off (100% shutoff), waits 5 minutes and tries again. This repeats until the pilot lights or the call for heat ends.
6. If flame is lost during the run, the module closes both valves and starts a new trial. After more than 5 flame losses in one call for heat, it closes the valves.

The 5-minute wait can be cut short by cycling the thermostat or removing and restoring power.

Prepurge and trial time (DIP switches)

Resideo recommends 0 seconds prepurge and a 90-second trial unless the appliance calls for something else. Set the switches before powering the module.

Read the LED first

The S8610U's green LED tells you where it stopped.

No spark

1. Confirm 24 V at TH-W (and GND) on a call for heat. No voltage: check the thermostat, limits, transformer and wiring upstream.
2. Arc test: pull the ignition cable at the module and hold it near the SPARK terminal (insulated pliers). Resideo's guide: an arc of 1/8 inch (3 mm) or longer means the module's output is fine. No arc, or an arc shorter than 1/8 inch, means check the module's external fuse and power at its input; if they're good, replace the module.
3. Good arc at the module but none at the pilot: check the ignition cable for cracks and continuity, the ceramic insulator for cracks, and the electrode position.

Spark, but the pilot won't light

1. Manual gas valves open and inlet pressure to spec. See furnace gas pressure check.
2. On a new install or after a repair, recycle the call a few times to purge air from the pilot tube.
3. Check the pilot orifice for blockage (spiders and lint are common).
4. Measure 24 VAC across the pilot valve terminals (PV to MV/PV) during the trial. No voltage: replace the module. Voltage but no pilot gas: replace the gas valve.

Pilot lights, but the spark doesn't stop

The module isn't seeing the flame.

• Check continuity of the ignition cable and the ground wire. The igniter-sensor, module and pilot burner need a common ground through the GND (BURNER) terminal to the burner bracket.
• Clean the flame rod and check its ceramic for cracks.
• The pilot flame should be steady, blue and wrap 3/8 to 1/2 inch of the flame rod. Adjust the pilot if it's lazy or lifting.
• Measure flame current (below). If everything checks out and it still won't prove, replace the module.

Pilot lit, main burner won't come on

1. Check the connections between module and gas valve.
2. Measure 24 V at the main valve (MV to MV/PV) after the pilot is proved. Voltage present with no main flame: replace the gas valve. No voltage with a proven pilot: replace the module.
3. If the main valve stays open after the call ends, remove the MV lead at the module. If the valve closes, recheck the thermostat and wiring. If not, replace the gas valve.

Measuring flame current

Resideo's technician quick reference (69-2042) for the S8610U:

• Measure with the pilot lit and no main gas: disconnect the MV lead first.
• Use the DC microamp scale with correct polarity. The guide says not to measure in series with the wiring; follow its meter hookup.
• Minimum: a steady 1 µA DC. Recommended: 2 µA or more for reliable operation.

In run mode the LED also reports flame current: after 4 seconds on, it flashes once per microamp.

For more on flame rectification, see flame sensor microamps and cleaning.

Replacing an old module with the S8610U

The S8610U is a universal replacement for many Honeywell, Robertshaw, Penn-Johnson, Fenwal and other IP modules, but not all of them. Check the replacement table in its instructions before you swap. Don't use it to replace a module that isn't listed unless you're sure the specs match (lockout type, timings, pilot flow rate up to 1,500 Btuh). Label each wire before you pull it.

Common mistakes

• Replacing the module for a ground problem. A loose or corroded burner ground is the most common reason a pilot lights but won't prove.
• Calling 2 flashes a dead module. It's waiting out its 5-minute retry.
• Measuring flame current with main gas on. Measure on pilot only.
• Ignoring the vent damper. On boilers with a damper, a damper fault (5 flashes) stops the sequence.
• Changing DIP switches with power on.

FAQ

Sources

• Resideo Y8610U Intermittent Pilot Retrofit Kit, form 68-0291
• Honeywell S8610U Technician's Quick Reference Guide, form 69-2042
• Honeywell S8610U Universal Intermittent Pilot Module installation instructions, form 69-0729