ATAG 203 Fault Code: Causes, Fixes & Repair Costs
What does the ATAG 203 fault code mean?
Fault code 203 on an Atag boiler indicates that the boiler's control system has detected a problem with the tank probe — the sensor responsible for monitoring the temperature of the hot water storage tank or heat store in your system. When this sensor returns a reading that is outside the expected range, or stops responding altogether, the boiler logs code 203 and may lock out to prevent unsafe operation. This code is closely related to code 201, which covers the domestic hot water (DHW) probe directly on the boiler; code 203 specifically relates to the external tank or cylinder sensor used in system or heat-store configurations. If you see either code, the diagnostic approach is broadly the same.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Failed or worn tank temperature sensor Common
Over time, the thermistor inside the tank probe degrades, particularly in systems that run at high temperatures or have not been regularly serviced. A failed sensor will send a signal that falls outside the boiler's acceptable range, triggering code 203. This is the most frequent root cause engineers find when attending this fault.
- Loose, corroded, or broken wiring to the probe Common
The wiring connecting the tank probe to the boiler's PCB can suffer from corrosion, loose terminals, or physical damage — especially if the probe is mounted on an older cylinder or in a damp plant room. A poor connection can cause intermittent or out-of-range readings that the boiler interprets as a damaged sensor.
- Water ingress or moisture around the probe Sometimes
If there is a slow leak from the cylinder, pipework, or fittings close to where the probe sits, moisture can track along the wiring or get into the sensor body itself. This corrupts the electrical signal and often causes the fault to appear suddenly after a period of normal operation.
- Faulty PCB misreading the probe signal Rare
In less common cases, the boiler's printed circuit board (PCB) develops a fault on the input channel that reads the tank probe, causing it to report an error even when the probe itself is healthy. This is typically only confirmed after the probe and wiring have been checked and found to be in good order.
How to fix it
- Reset the boiler once or twice DIY safe
Press and hold the reset button on the boiler's control panel (refer to your user manual for the exact location on your model) until the boiler attempts to restart. Wait a few minutes to see if code 203 reappears. A one-off sensor glitch can occasionally be cleared this way. If the fault returns after one or two resets, do not keep resetting — further attempts will not resolve an underlying component fault and may complicate diagnosis.
- Check that your hot water cylinder or tank is correctly connected and that there are no obvious leaks around the probe location DIY safe
Without removing any panels or touching any internal components, visually inspect the area around your hot water cylinder or heat store. Look for signs of damp, dripping water, or corrosion near the wiring that runs to the tank sensor. If you spot an active leak, turn off the water supply and contact a plumber or heating engineer promptly. Do not attempt to move or disconnect any wiring yourself.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
A qualified engineer will use a multimeter to test the resistance of the tank probe against the manufacturer's expected values, inspect the wiring loom for continuity and corrosion, and check the PCB input channel if the probe tests healthy. In most cases, the fix is straightforward — replacing the faulty probe and, if needed, any damaged wiring. Only a Gas Safe registered engineer should carry out this work on your boiler's components.
Parts you may need
- Tank temperature probe / sensor (NTC thermistor) · from £25
- Sensor wiring harness / loom · from £35
- Atag boiler PCB (control board) · from £280
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £100–£220, depending on the underlying cause.
Frequently asked questions
Can I fix Atag fault code 203 myself?
The only safe DIY action is a boiler reset (one or two attempts) and a visual check for obvious leaks around the tank sensor area. The probe itself sits inside your heating system's wiring and components, so replacing it — or checking the wiring in any detail — must be done by a Gas Safe registered engineer. Attempting it yourself risks both personal safety and invalidating your boiler's warranty.
What is the difference between Atag fault codes 201 and 203?
Both codes relate to temperature sensor faults, but they cover different sensors. Code 201 points to the domestic hot water (DHW) probe mounted within the boiler itself, while code 203 refers to the external tank probe used to monitor a separate hot water cylinder or heat store. If your system includes a storage cylinder, code 203 is the relevant one. The diagnosis and repair process is very similar for both.
How much does it cost to fix an Atag 203 fault code in the UK?
For the most common scenario — a straightforward tank probe replacement — most homeowners pay somewhere between £100 and £220 all-in, covering the engineer's call-out, labour, and the part itself. The sensor component typically costs £15–£40, with the rest being labour. If the underlying cause turns out to be a faulty PCB, costs rise significantly — a replacement PCB can cost £200–£350 for the part alone, plus labour. Your engineer should be able to give you a clearer estimate once they have diagnosed the root cause.
Is it safe to keep using my boiler when fault code 203 is showing?
In most cases, code 203 will cause the boiler to lock out, meaning it stops operating until the fault is resolved. You should not attempt repeated resets to force the boiler to keep running — this can mask a genuine fault and, depending on the underlying cause (such as a water leak near electrical components), could create a safety risk. Contact a Gas Safe registered engineer promptly to get the fault properly diagnosed and repaired.