Error 203

Safety precautions:

1) Do not attempt gas or burner work yourself. If you suspect a gas issue, turn off the gas supply at the meter and call a qualified engineer.

2) Before touching electrical components, isolate the boiler from mains power at the boiler isolator or consumer unit. Wait a few minutes for electronics to discharge.

3) If you need to access the tank probe in a hot water cylinder, be aware of hot water and pressure; do not remove a probe or immersion sleeve without isolating and partially draining the cylinder unless you are experienced. When in doubt, stop and call an engineer.

4) Never bypass or short a sensor to force the boiler to run.

Initial checks a homeowner can do:

1) Note the exact code (203) and whether a spanner/service symbol is also displayed. Try a single reset of the boiler following the manufacturer's reset procedure (switch off at the mains for 30 seconds or use the reset button per user manual) and see if the error returns. Temporary faults can clear on a reset but persistent return indicates a real fault.

2) Visually inspect accessible wiring and connectors between the boiler and the hot water tank (probe cable). Look for loose connectors, corroded terminals, crushed or frayed cable, and signs of moisture or leaks around the cylinder or probe pocket.

3) Check for any obvious water leaks or damage at the immersion sleeve/where the probe enters the cylinder. If there is leakage, isolate water and call an engineer.

4) Record working conditions: any recent power outages, servicing, freezing weather, or physical work near the boiler or cylinder that might have disturbed wiring.

Specific diagnostic steps (for a competent DIYer with basic tools only):

1) After isolating mains power, remove the boiler cover only if you are comfortable and competent; otherwise leave this to an engineer. Inspect the connector block where the tank probe cable terminates for tightness and corrosion. Re-seat any plug-in connector (take a clear photo first so you can reconnect exactly as found).

2) If you can access the probe at the cylinder, check that the probe is properly located in its pocket or well and that the pocket is not filled with insulating material or damaged. Do not force removal of a probe from a sealed sleeve.

3) If you have a multimeter and basic knowledge: with power isolated you can check continuity and resistance of the probe cable. Measure resistance across the probe terminals at the boiler connector and compare with the reading at the probe end (if you can access both ends). You are mainly looking for an open circuit (infinite resistance) or a short (very low ohms) which indicates a failed sensor or broken cable. Note: exact resistance vs temperature values depend on probe type—do not attempt to 'calibrate' without the manufacturer's values. If you find an open or short you have isolated a cable or sensor fault.

4) If the cable resistance looks OK and connections are good but the boiler still flags 203, the issue may be the probe element itself. Replacing the probe usually requires draining part of the cylinder or removing an immersion sleeve and should be done by an engineer.

Recommended fixes and next steps:

1) If you found a loose connector or visible corrosion and you are competent to clean and re-seat terminals, do so, restore power and test. If the error clears and remains cleared, monitor the system.

2) If a cable is damaged (chafed, cut) and you are skilled in appliance wiring, you can replace the internal probe cable; otherwise isolate power and call an engineer.

3) If the probe itself is faulty (open/short or inconsistent resistance), the correct action is to replace the probe with the manufacturer-specified part and refit correctly into the cylinder or immersion pocket.

4) If after replacing or repairing the probe the boiler still shows errors, the problem may be in the control PCB or in the wiring harness—stop and call a qualified engineer for further electronic diagnostics.

When to call a professional:

1) If the error persists after basic checks and a reset, or if you are not comfortable isolating mains power or removing covers, contact a Gas Safe registered heating engineer. Replacement of probes, working on cylinder sleeves, and PCB-level troubleshooting should be done by a professional.

2) Call an engineer immediately if you find water leaks, a damaged cylinder, or any sign of electrical damage or scorching.

3) Provide the engineer the boiler model, serial number, and that the fault code is 203 (tank probe damaged) and describe the checks you have already done—this speeds diagnosis.

Final notes:

Do not attempt to bypass the sensor or force the boiler to run; this will defeat safety controls and could damage the boiler or tank. Document photos of connectors and the probe position before any disassembly to help the engineer. A qualified engineer will be able to test the probe against manufacturer specifications, replace the sensor and/or cable properly, and verify correct system operation and safety.