ServiceNomad RV · Knowledge
Suburban Furnace Won't Ignite
Suburban furnaces are the most popular brand in RVs, known for their reliability—but when they won't ignite, the cause is almost always related to the ignition system, gas supply, or safety lockouts. Understanding the ignition sequence and what can interrupt it helps you communicate effectively with a technician and avoid unnecessary parts replacement. Suburban furnaces follow a precise startup routine, and if any step fails, the system shuts down for safety.
The Suburban Ignition Sequence
When you call for heat, the Suburban control board performs a pre-purge by running the blower motor for 30-60 seconds to clear any residual gas from the combustion chamber. After pre-purge, the board opens the gas valve and energizes the igniter (either a spark electrode or hot surface igniter depending on model). If flame is detected by the flame sensor within a few seconds, the furnace continues to run. If no flame is detected, the board closes the gas valve, waits, and tries again—usually up to three attempts before entering lockout. This sequence is safety-critical. Any interruption—whether from low battery voltage, weak spark, restricted airflow, or gas supply issues—will cause the furnace to abort and either retry or lock out entirely.
Spark Igniter and Electrode Issues
Older Suburban furnaces use a spark igniter system with a ceramic electrode that produces a visible spark to ignite the gas. Over time, these electrodes can become carbon-fouled, cracked, or misaligned from vibration. If the gap between the electrode and ground is too wide or too narrow, the spark won't reliably ignite the gas. You'll hear clicking during the ignition sequence but see no flame. A technician can inspect the electrode gap (typically 1/8 inch) and clean or replace the electrode if needed. They'll also check the ignition module that generates the high-voltage spark—these modules can fail internally and produce weak or intermittent spark.
Hot Surface Igniter Failures
Newer Suburban furnaces use a hot surface igniter (HSI)—a silicon nitride element that glows orange-hot to ignite the gas. HSIs are more reliable than spark igniters but are fragile and can crack from thermal shock or rough handling. If the HSI is cracked, it may still glow but won't produce enough heat to ignite the gas, or it may draw excessive current and cause the control board to shut down. Technicians test HSI resistance with a multimeter (typically 40-90 ohms depending on model). If the resistance is out of spec or the element is visibly cracked, replacement is necessary. Never touch an HSI element with bare hands—skin oils cause hot spots and premature failure.
Gas Supply and Pressure
Even with perfect ignition, the furnace won't light if gas isn't reaching the burner. Common gas-related issues include an empty or low propane tank, a tank valve that isn't fully open, a regulator in lockout (after running out of gas mid-cycle), or a kinked or blocked gas line. Some RVs have shutoff valves at the furnace or in the bay that may have been accidentally closed. A technician will verify gas supply by testing pressure at the furnace inlet (typically 11 inches water column) and checking that the gas valve solenoid clicks when energized. If no click is heard, the valve may be electrically failed or the control board isn't sending the signal.
Flame Sensor and Lockout Behavior
After ignition, the control board monitors the flame sensor (a metal rod positioned in the flame). If the flame sensor is dirty, corroded, or improperly grounded, it won't detect the flame even if the furnace is burning properly. The board will shut off the gas valve within seconds and enter a retry or lockout mode. Cleaning the flame sensor with fine steel wool or emery cloth often resolves this issue. The sensor must be properly positioned so the flame engulfs it—if the burner is dirty and the flame is lazy or yellow, the sensor may not get hot enough to signal flame presence.
⚠️ Safety Notes
- Never bypass safety switches or attempt to manually hold the gas valve open—this can cause dangerous gas buildup and explosion risk.
- If you smell propane during ignition attempts, immediately turn off the furnace and propane supply, ventilate the RV, and call a technician.
💡 Prevention Tips
- Test your furnace before winter camping season to catch issues while you still have time for repairs.
- Keep the combustion chamber and flame sensor clean by having the furnace serviced annually.
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