Suburban SF-42Q Furnace Shuts Off After 5-10 Minutes: Troubleshooting

Bottom Line Up Front

Your furnace's symptoms point to a thermal limit switch that's opening due to overheating and not resetting properly, or a failing control board that's losing signal to the gas valve after the initial startup cycle. Since you've confirmed the sail switch works and cleaned the unit, the fact that it runs perfectly for 5-10 minutes then shuts down the burner while the fan continues strongly suggests the thermal limit switch is cycling open and either not closing again or the board isn't recognizing it's closed. The igniter attempting multiple times before giving up indicates the board is trying to restart but getting blocked by a safety circuit.

Root Causes

Common causes include a thermal limit switch that opens at manufacturer-specified temperatures (verify specifications with dealer) to prevent overheating, and your symptoms are typical for this safety device cycling. Even though you confirmed it was closed when cold, it may be opening during operation due to restricted airflow, a partially blocked heat exchanger, or the switch itself becoming heat-sensitive with age. When the switch opens, it cuts power to the gas valve, stopping combustion while allowing the fan to continue purging.

Another common cause includes a failing control board that's losing the ability to maintain gas valve control after the initial startup sequence. The board manages multiple safety circuits and timing sequences, and internal component degradation can cause intermittent failures. The fact that cycling power restarts the system perfectly suggests the board's startup logic works fine, but something in the run cycle is failing.

Less likely but possible causes include a gas valve that's overheating and shutting its internal thermal protection, or flame rollout issues causing the limit switch to trip. Since you've cleaned the unit and confirmed the sail switch operation, these are secondary considerations.

Before You Start

SAFETY FIRST - Complete Shutdown Procedure:

1. Turn off the furnace at the thermostat

2. Switch off the main LP gas supply at the tank or regulator

3. Turn off electrical power to the furnace at the main panel or disconnect switch

4. Allow the unit to cool completely (minimum 30 minutes) before beginning work

5. Verify gas is off by checking that the gas valve is in the closed position and waiting for any residual gas in lines to dissipate

You'll need a multimeter capable of reading continuity and DC voltage, along with basic hand tools to access the limit switch and control board connections. Have your furnace service manual handy for the wiring diagram and diagnostic procedures. Most importantly, ensure you have adequate lighting and workspace since you'll need to monitor the unit while it's running to catch the fault in action.

The thermal limit switch is typically mounted on the heat exchanger and will require removing the outer cover to access for testing, but only after the unit has been shut down and cooled completely. Plan to work with a helper if possible, as you'll need someone to monitor the thermostat while you watch the unit operation.

Gather some electrical tape and wire nuts in case you need to extend wires for testing access. The control board on these units is usually mounted behind the gas valve assembly and can be tricky to access for voltage measurements during operation.

The Repair Process

Start by running the furnace through a complete cycle to observe the symptoms, then shut down completely and allow to cool before testing. After the unit has cooled for at least 30 minutes, test the thermal limit switch for continuity in the cold state - it should read closed (near zero resistance). If the switch tests good when cold but you suspect it's opening during operation, the switch likely needs replacement as thermal limit switches can become heat-sensitive with age and open prematurely.

If the thermal limit switch is opening, determine why it's overheating. Check for obstructions in the combustion air intake and exhaust, verify the blower wheel is clean and spinning freely, and inspect the heat exchanger for debris or corrosion that could restrict airflow. The switch should reset (close) within a few minutes of the burner shutting down if it's working properly.

If the thermal limit switch remains closed throughout the cycle, focus on the control board. With the unit running, check for 24V AC at the gas valve terminals when the burner stops. If voltage drops to zero, the board is cutting power to the valve. This could indicate a failing board or another safety input the board is responding to.

Test the flame sensor circuit if your furnace has one - a dirty or failing flame sensor can cause delayed shutdowns. Clean the sensor rod with fine steel wool and verify proper positioning in the flame path. Also check all wire connections at the control board for corrosion or looseness that could cause intermittent signals.

Professional Help Needed

Call a qualified RV technician if you find the thermal limit switch is functioning properly and maintaining continuity throughout the run cycle, as this points to control board issues that require specialized diagnostic equipment and knowledge of the board's internal circuits. Control boards typically run $150-250, and proper diagnosis prevents unnecessary part replacement.

Also seek professional help if you discover gas pressure irregularities or suspect issues with the gas valve's internal thermal protection. These components require specialized tools and safety procedures that go beyond typical DIY capabilities. A mobile RV tech can often diagnose these issues on-site with proper gas pressure testing equipment.

If multiple limit switches or safety devices appear to be involved, or if you're uncomfortable working with the gas system components, professional diagnosis is worth the service call cost. Furnace repairs involving gas components should meet NFPA codes and manufacturer specifications for safety and warranty compliance.