Fault codes / Aqua-Hot / ENGINE PREHEAT RED

Aqua-Hot: Engine preheat red

What it means: Short in the preheat pump circuit.

What to check

Light is only active with preheat switch ON plus burner and/or element ON.

Learn the system

• Furnace sequence
• LP system basics

Practice it on a service call

• Furnace blows cold
• Furnace sparks but never lights
• Furnace keeps shutting off, fan keeps blowing

Guided diagnosis

• Furnace
• Thermostat

Other Aqua-Hot codes

• Aqua-Hot: Low tank-level cutoff (red) Boiler fluid low
• Aqua-Hot: Low battery voltage (red) 12V too low
• Aqua-Hot: Overload (red) 12V short or controller overheated
• Aqua-Hot: Zone light red Short in that zone
• Aqua-Hot: Pump #1/#2/#3 red Short in that pump circuit
• Aqua-Hot: Electric element red Short in the element's 12V control circuit

Full field guide: Aqua-Hot 450 hydronic heater

Codes come from the maker manuals and public service sheets. Your model's own chart always wins, and LP and 120 volt work belongs to a qualified technician.