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The fuel supply to a fuel-fired auxiliary heater restricted, so it cannot draw what it needs to burn. The heater takes fuel from the vehicle's own tank through its own small-bore line and metering pump, and that line is far finer than anything on the engine.

The heater fails to light, or lights and shuts down shortly after, and the module reports a flame not detected or lost. Listen for the metering pump's tick at start-up — a pump ticking steadily while the heater fails to light means fuel is being commanded and not arriving, which points at the line rather than the pump.

The restriction is usually the pickup screen or the fine line itself. These heaters draw through a dedicated standpipe in the tank whose screen blocks with the same debris that blocks the main strainer, and the line is small enough to be closed by wax in cold weather — which is when the heater is most needed and when the fault therefore appears.