Overview
On Baxi 816/825/830 Heat boilers a single red flash indicates a sensor/flow fault. The boiler has detected a problem with the flow or return temperature sensing (thermistor/NTC) or a condition that makes the sensor readings invalid — common causes are a bad or corroded connector, a failed flow or return temperature sensor, poor or no circulation (pump not running, trapped air, closed valves), or reverse flow through the heat exchanger. The control board uses these sensors to verify safe flow and temperature; if it cannot trust those readings the boiler will fault to protect itself. Severity is moderate — the boiler will usually lock out or refuse to fire until the fault is cleared, preventing potential dry-fire or overheat. Some basic checks are safe for a competent homeowner (reset, pressure, bleeding radiators, visual wiring check), but diagnosing and especially repairing sensors, wiring inside the casing, pump replacements or any work that involves gas, sealed parts or electrical components should be carried out by a Gas Safe registered engineer. If the fault persists after simple checks, call a professional rather than repeatedly resetting the boiler.
Possible Cause: Bad connection to flow or return temperature sensor. Fault with flow or return temperature sensor(s). Low or no flow through boiler. Reverse flow through boiler.
Troubleshooting Steps
Safety first
1) If you smell gas: do not operate electrical switches or the boiler. Turn off the gas supply at the meter, evacuate the property, and call the gas emergency number immediately. 2) If you are unsure or uncomfortable doing any check that involves opening the boiler casing, stop and call a Gas Safe engineer. Isolate power at the mains before opening the casing.
Initial homeowner checks (safe, simple checks)
1) Record the fault: note model, serial if visible and that the boiler shows 1 red flash. Try a single reset: press and hold the boiler reset button or use the selector to R and hold for 5–10 seconds. Wait for restart. Do not keep repeatedly resetting if it returns immediately. 2) Check system pressure: look at the boiler pressure gauge. Aim for about 1.0–1.5 bar when cold. If pressure is low, top up using the filling loop (follow your boiler manual). After topping up, reset the boiler and see if the fault clears. 3) Check heating circulation: ensure all radiator valves and TRVs are open; check central heating is selected on the boiler/controls. Feel radiator temperatures (top vs bottom) to detect airlocks; bleed radiators if the top is cold and bottom hot. 4) Listen for the pump: when the heating calls, you should hear/feel the pump running (mild vibration through the pipes). If the pump appears dead or noisy, that points to a circulation issue.
Visual and basic electrical checks (only if competent and power isolated)
1) Turn off mains power to the boiler and isolate the gas if you will open the cover. Remove the outer casing only if you are confident and it does not void warranty. 2) Visually inspect temperature sensor connectors, wiring loom and PCB connectors for loose plugs, bent pins, corrosion or signs of water ingress. Re-seat push-fit connectors. Do not rewire or work on gas components. 3) If you own and know how to use a multimeter and you are competent, you can measure the temperature sensor leads for open-circuit (infinite resistance) which indicates a broken sensor or wiring. Do not guess component values — if you see open/infinite or a completely shorted sensor, that indicates a fault. If you are unsure what readings should be, stop and call an engineer.
Other checks and considerations
1) Reverse flow or plumbing changes: if the boiler has recently been serviced or pipework altered, tell the engineer — incorrect pump direction, an installed bypass or an incorrectly fitted valve can cause unexpected sensor readings. 2) If you have a system with zone valves, ensure they are operating correctly and not stuck closed. 3) Keep a log: note when the fault occurs (on CH only, on DHW, after reset, after topping up pressure) and any noises or visible leaks. Take photos of wiring/labels and the error light for the engineer.
When to call a professional
1) Call a Gas Safe registered engineer if the fault persists after the safe checks above, if visual wiring reseating did not help, if the pump is dead or suspected faulty, if sensor replacement or PCB work is needed, or if you are not comfortable opening the boiler. 2) Provide the engineer with the boiler model, the exact symptom (1 red flash), what checks you’ve already completed, and any readings (pressure, radiator behaviour, pump/noise). 3) Do not attempt to replace temperature sensors, pumps, the PCB or to alter gas components unless you are Gas Safe qualified. These are controlled, safety-critical components.
Final notes
1) Repeated resets without remedy can cause further lockouts and may hide an underlying safety issue. 2) The likely fixes an engineer will perform are: confirm wiring/connectors, measure sensors and replace a failed flow/return thermistor, correct circulation issues (pump repair/replacement, bleed or reconfigure), and check for reverse flow or valve faults. 3) For your safety and to preserve warranty and compliance, any internal electrical or gas work must be done by a Gas Safe registered engineer.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Baxi 816 Heat, 825 Heat, 830 Heat.