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Most "won’t light" problems trace back to fuel supply, a connection, or air in the line. It’s rarely the heater itself. Work through the checks below in order. Most owners find the cause in the first three steps, well before reaching anything that needs a replacement part.
Step 1: Confirm the Basics
Check that the tank or cylinder actually has fuel. Don’t assume based on when it was last used. Confirm the tank valve is turned fully open, not just cracked. Confirm the hose or cylinder connection to the heater is seated properly and hand-tight. These three checks take under a minute. They rule out the single most common cause: a heater that’s actually fine but isn’t getting fuel yet.
Step 2: Purge Air From the Line
Air can occupy the line where fuel should be. This happens any time you connect a new tank or cylinder, or after a heater has sat unused for a while. Most heaters need a brief purge: repeat the ignition sequence two or three times before fuel reaches the burner consistently. A heater that seems dead on the first attempt but catches on the second or third try isn’t malfunctioning. It just needed the air worked out of the line.
Step 3: Check for a Regulator Lockout
Did the heater stop lighting specifically after you connected a fresh tank? An excess-flow safety feature in the regulator may have triggered from opening the valve too quickly. This shuts fuel flow as a precaution, not a fault. Our regulator troubleshooting guide covers the reset procedure, which fixes this in under a minute once you know it’s the cause.
Step 4: Listen and Watch for a Spark
Press the igniter and pay attention to what happens. A click or spark with no flame points toward a fuel-delivery issue, not the ignition system. Go back to steps 1 through 3. No click or spark at all points to a dead or failing igniter. Our igniter troubleshooting guide covers this, including when a replacement part is warranted.
Step 5: Check the Pilot or Thermocouple Timing
Releasing the control knob too early during lighting is a very common mistake on heaters with a pilot and thermocouple. The flame seems to catch, then immediately goes out. That looks like "won’t light," but it’s really a timing issue. Hold the knob for the full time your model’s manual specifies, typically 30 to 60 seconds, before releasing. Our thermocouple guide covers how to tell this apart from an actually failing part.
Step 6: Inspect the Burner for Debris
Dust, spider webs and insect nests inside burner ports cause a lot of ignition problems. This is especially common on a heater that’s sat unused in storage for months. See our burner cleaning guide for the safe way to clear this without damaging the burner.
Step 7: Check Cold-Weather Fuel Pressure
Propane’s vapor pressure drops in cold temperatures. A 1 lb cylinder in genuinely cold conditions can struggle to deliver enough pressure to light reliably, even with fuel still inside. This shows up more with small disposable cylinders than with larger 20 lb tanks. Do lighting problems only happen in cold weather? Warming the cylinder slightly — never with direct heat, just bringing it to room temperature before use — can fix this.
What "Won’t Light" Actually Covers
This phrase gets used for several different symptoms. Narrowing down which one you actually have speeds up diagnosis considerably. "No spark at all" is an ignition problem. "Spark but no flame" is a fuel-delivery problem. "Flame catches then dies immediately" is usually a thermocouple-timing issue. Figure out which of these three you’re seeing before you troubleshoot further. It saves time compared to working through every possible cause in order.
Forced-Air Heaters Add a Power Check
Forced-air construction heaters use electronic ignition and a fan motor. That adds a step ahead of the fuel checks above: confirm the power source first. A bad extension cord, a tripped breaker, or a generator not delivering clean power can all stop electronic ignition from firing, even with plenty of fuel available. If power checks out, the same fuel and connection checks above apply.
A Heater That Lit Fine Last Season but Not This One
Did a heater work perfectly at the end of last season but won’t light now, with nothing else changing about how it’s used? That points strongly to a storage-related issue. A hose or O-ring can degrade from age and temperature exposure while sitting unused. Debris can build up in a burner tube over months in storage too. Run through steps 1, 2 and 6 above before you assume anything more serious.
Testing a Suspected Fix Safely
After making any change — reseating a connection, purging the line, cleaning a burner — test in a well-ventilated space. Stand back slightly on the first attempt. A line that was genuinely air-locked can produce a brief larger flame as trapped air burns off before settling to normal. This is expected. It isn’t a sign something is wrong, but it’s worth knowing to expect rather than being startled by it.
Isolate the Cause With a Known-Good Tank
Stuck between a fuel problem and an ignition problem? Borrow or connect a cylinder you know is full and working, ideally one used successfully on another heater recently. Does the heater light with that tank? The original tank, hose or regulator was the cause, not the heater itself. Does it still fail to light? The problem is in the heater’s own fuel path or ignition system. This one swap rules out an entire category of causes in under a minute, and it’s especially useful when you can’t tell if a "spark but no flame" symptom is a fuel issue or a placement issue.
Keeping a Written Log Speeds Up Repeat Troubleshooting
If a heater develops the same intermittent problem more than once across a season, jotting down what fixed it the first time saves real effort the second time. Note the exact symptom, what step resolved it, and roughly how long the heater had been in storage or how cold it was outside. Owners who skip this step often re-diagnose the same regulator lockout or air-in-the-line issue from scratch every single time it recurs.
When to Stop Troubleshooting and Call It a Repair
Have you worked through fuel, connections, purging, a possible regulator lockout, ignition, timing and debris, and the heater still won’t light? A genuinely failed internal component — a valve, a control module, or wiring on an electronic-ignition model — is the remaining likely cause. Our DIY vs. replace guide covers how to decide what’s worth repairing at that point.