Overview
H.03 -.00 on a Baxi 200/400 Combi means the boiler has detected a temporary communication fault with the flue NTC (negative temperature coefficient) sensor or its wiring/connection. The flue sensor monitors exhaust gas temperature; if the boiler cannot read that sensor correctly it will flag a fault and usually lock out or stop to protect against unsafe conditions. Causes are commonly a loose or corroded connector, a disconnected lead, a failed sensor, or less commonly a PCB/communication fault. Severity is moderate. Often the fault is temporary and will clear after a reset, but repeated or persistent faults must be treated seriously because they affect the boiler's ability to confirm safe flue operation. Homeowners can perform basic visual checks and a reset, but diagnosing or replacing sensors, wiring or internal parts should be done by a Gas Safe registered engineer. Do not attempt gas or sealed-panel repairs yourself.
Possible Cause: Temporary error with communication, negative temperature coefficient thermostat (NTC) or flue sensor. Sensor could also be unconnected or have a bad connection.
Troubleshooting Steps
Safety first:
- If you smell gas or suspect a carbon monoxide issue, leave the property immediately and call the gas emergency service. Do not stay to investigate.
- If you are not confident working near a gas appliance, stop and call a Gas Safe engineer. Do not attempt internal electrical or gas repairs.
- Always isolate electrical supply at the mains before removing any covers and wait for capacitors to discharge.
Initial checks a homeowner can do:
1. Record the full fault code and time it occurred. Note if it is intermittent or persistent. This information helps the engineer.
2. Reset the boiler: use the reset button or switch off power to the boiler for 30 seconds then switch back on. If the code clears and does not return, monitor the boiler for a few hours/days.
3. Check obvious external issues: ensure the flue terminal outside is not blocked by birds' nests, debris, snow or vegetation and that condensate drain is not frozen/blocked. Ensure electricity supply to the boiler is stable and other appliances are working.
If the fault returns and you are reasonably competent with basic electrical checks (only proceed if you are confident and comfortable doing this):
4. Turn off the boiler power at the mains and isolate gas if you will be working inside the unit. Remove the outer cover only if you are comfortable doing so and only after isolation.
5. Visually inspect the flue sensor wiring and connector (usually a two-wire NTC fitted near the flue path). Look for loose connectors, damaged or chafed wires, signs of corrosion or moisture, and poor connector seating. Re-seat any loose connectors and ensure a firm connection.
6. If you have a multimeter and know how to use it safely: measure the resistance across the sensor leads. A healthy NTC will show a finite resistance (not open-circuit). If you get an open-circuit (infinite) or a short (near zero), the sensor or wiring is faulty. Do not touch live circuits; the boiler must be isolated before probing. Note that exact resistance values vary by sensor and temperature, so use only the open/short check unless you have the correct sensor spec.
7. Refit the cover, restore power and test the boiler. If re-seating a connector or clearing an obvious external blockage cleared the fault and it stays cleared, continue to monitor.
When to call a professional:
- If the H.03 error persists after a reset and basic external checks, or if you found a damaged wire, corroded connector, or open/short on the sensor.
- If you are not comfortable opening the boiler or using a multimeter, or if the fault returns intermittently.
- If there are any signs of gas smell, poor combustion, soot, or unusual noises.
What the engineer will do / what you should tell them:
- Explain the exact code (H.03 -.00), when it appeared, what resets or checks you performed, and any external observations (blocked flue, water ingress, visible wiring damage).
- The Gas Safe engineer will check wiring, measure the sensor properly, replace the flue NTC if faulty, and investigate PCB communication if required. They will also verify safe flue operation and combustion after repair.
Do not use the boiler if the fault recurs repeatedly without an engineer's inspection. Repeated flue-sensor faults affect safety and must be resolved by a qualified technician.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Baxi 200 / 400 Combi Gas Boiler.