Overview
E163 on a Baxi 600 Combi indicates a fault with the domestic hot water (DHW) NTC temperature sensor: the boiler has detected either an open-circuit (o.c.) or a short-circuit (s.c.) on the DHW NTC. The NTC (negative temperature coefficient) sensor measures the DHW temperature so the boiler can control heating and prevent overheating or scalding. An open-circuit usually means the sensor or its wiring is disconnected or broken (infinite resistance). A short-circuit means the sensor or wiring is shorted (very low resistance) or contaminated by water. This fault normally disables or locks out hot water, though central heating may still operate depending on the boiler state. Severity is medium: it is not an immediate gas safety emergency, but it does affect safe temperature control of your hot water and can lead to no hot water or incorrect temperatures. Basic checks and resets can be done by a competent homeowner, but diagnosing wiring or replacing the NTC, sealing the sensor pocket, or any work involving the boiler internals should be done by a Gas Safe registered engineer to avoid risks, preserve warranty, and ensure correct sealing and commissioning.
Possible Cause: Domestic hot water (DHW) negative temperature coefficient (NTC) thermostat sensor fault (s.c.) or DHW NTC sensor fault (o.c.).
Troubleshooting Steps
Safety precautions:
- If you are not competent with electrical work or gas appliances, stop and call a Gas Safe registered engineer. Working on boilers can be dangerous.
- Before doing any internal checks, switch the boiler off at the electrical isolator and the boiler control panel. Isolate mains power. For resistance checks, the boiler must be unpowered.
- Do not attempt to remove or replace gas-carrying components. If the boiler needs draining or components removed that affect the sealed system, a qualified engineer should do this.
Initial homeowner checks (safe, non-intrusive):
1. Note any other error codes or symptoms (no hot water, hot water too hot, boiler locked out). Try a full reset of the boiler following the user manual (reset button or selector held for required seconds). If E163 returns immediately or after a short run, further checks are needed.
2. Check for obvious signs of water leaks or corrosion near the boiler and around visible sensor wiring. Wet connectors or corrosion can cause faults.
3. If it is very cold and you have external pipework or a condensate pipe, ensure no freezing issues are present. Freezing can indirectly affect sensors/wiring but is less common for DHW NTC faults.
Diagnostic checks (requires basic tools and comfort with powered-off testing):
1. Turn off and isolate electrical supply to the boiler. Confirm power is off.
2. Locate the DHW NTC sensor and its wiring. On a combi this is usually a small probe or plug-in sensor on the hot water heat exchanger or near the DHW outlet; consult the boiler schematic or service sheet if available.
3. Visually inspect the sensor connector and wiring for damage, loose pins, or signs of water ingress. Re-seat the connector if present and dry any moisture. Restore power and reset the boiler to see if the fault clears (only if connector was dry and safe to access without removing seals). If you previously powered off, power it back on only after re-seating.
4. For a proper electrical diagnosis, with the boiler power still isolated, use a multimeter set to measure resistance (ohms). Disconnect the sensor connector from the PCB so you measure the sensor itself, not the circuit. Measure the resistance across the sensor terminals. Typical NTC sensors used on many boilers are around 10 kΩ at ~25°C (check the exact spec in the service manual). Expected behaviour:
- Open-circuit: multimeter shows OL/infinite resistance (sensor wire broken or sensor failed)
- Short-circuit: multimeter shows very low resistance (close to 0 Ω) indicating short or sensor failed
- Normal: a resistance roughly consistent with ambient temperature (for a 10 kΩ NTC: around ~10 kΩ at room temp). If you have access to the exact sensor spec, compare to the temperature-resistance curve in the manual.
5. If the sensor reads open or short at the sensor end, the sensor is faulty and should be replaced. If the sensor reads normal but the PCB terminals show open/short when measured at the boiler connector, suspect wiring or a PCB fault.
Fix steps and guidance:
- If the fault is a loose or corroded connector, re-seating and cleaning may clear the error. Ensure the connection is dry and secure. Power up and reset to confirm.
- If the sensor itself is faulty (open or short), the sensor should be replaced. On combi boilers the sensor is usually a simple replaceable probe and sometimes sits in a pocket with an O-ring. Replacement requires the correct part number and proper sealing to avoid leaks.
- If the wiring is damaged between the sensor and PCB (broken or shorted cable), the wiring needs repair or replacement. Trace the cable visually where accessible. Repair must ensure correct insulation and routing; any work on internal wiring is best done by an engineer.
- If the sensor and wiring test OK but the PCB connection shows the error, it could be a PCB fault. PCB diagnosis and replacement should be left to a Gas Safe engineer.
When to call a professional:
- Call a Gas Safe registered engineer if you are unsure at any stage, if the sensor needs replacing, if wiring repair is required, or if the boiler still reports E163 after connector and sensor checks. A professional should fit replacement sensors, change seals, test for leaks, and verify correct operation and safety interlocks. They will also check for associated faults (PCB, pump, exchanger) and recommission the boiler.
Important notes:
- Do not attempt to bypass temperature sensors or run the boiler with faulty sensors — this can cause unsafe water temperatures and damage the appliance.
- Check the boiler service book/warranty before undertaking any work; unauthorized internal work may affect warranty.
- If in doubt or if you do not have the correct part or confidence to complete electrical checks safely, stop and arrange a Gas Safe engineer visit.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Baxi 600 Combi Gas Boiler.