Cause
Faulty fuel fired heater
The auxiliary heater itself at fault — its glow pin, metering pump, combustion blower or control unit. It is an independent burner rather than part of the engine's cooling system, which is why its codes sit apart and why an engine that warms up normally says nothing about it.
Its failures are mostly consumable rather than catastrophic. The glow pin is a wearing part with a service life, the metering pump ticks audibly and can lose its prime or block, and the screen in the fuel pickup fouls. Listen for the pump's tick at start-up — a heater attempting to run with no pump ticking has a fuel delivery fault, and one that ticks and never lights has an ignition or air fault.
The unit is safety-interlocked and will lock itself out after repeated failed starts, which is correct behaviour and requires a deliberate reset with the right tool rather than a battery disconnection. Check the simple inhibits first: fuel level below the reserve threshold, a flat battery, or an exhaust pipe blocked by road debris will all stop a perfectly healthy heater.
Typically specialist work. Needs equipment or training most home workshops do not have.
Codes this can cause (3)
Codes where this is a likely explanation first, rather than merely a possible one.
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