Domestic hot water (DHW) sensor damaged
The 201 error code on Atag boilers indicates domestic hot water (dhw) sensor damaged.
Atag 201 means: domestic hot water (dhw) sensor damaged. On this component the documented failure modes are the sensor has failed — short circuit or open circuit or wiring, connector or harness of the sensor damaged or loose. You can safely reset the boiler once — if 201 returns, do not keep resetting. A qualified heating engineer should diagnose the root cause.
What Does 201 Mean
The 201 code is shown by the boiler’s control board when it detects domestic hot water (dhw) sensor damaged. The boiler then stops instead of continuing to run in that condition: the code is the appliance reporting a problem, not proof that the boiler is broken.
| What you see on the boiler |
|---|
| The display shows 201 and the boiler stops until the fault is cleared |
Common Causes
The manufacturer does not publish a cause list for this specific code. The causes below are the failure modes documented for the same component on other boilers, confirmed by the service literature of 4 different manufacturers.
| Cause |
|---|
| The sensor has failed — short circuit or open circuit |
| Wiring, connector or harness of the sensor damaged or loose |
Safety Warning
201 involves electrical or pressure-related components. Do not attempt repairs without proper qualifications. Ensure the boiler is electrically isolated before any inspection. If you notice water leaks, unusual sounds, or pressure anomalies, contact a qualified heating engineer immediately.
Parts & Prices
Prices shown are indicative. Actual prices and availability vary by region.
Step-by-Step Diagnosis
1. Note the exact error code and check the manufacturer's installation and service manual for specific guidance. 2. Check all visible wiring connections for security and condition. 3. Power cycle the boiler — switch off at the mains for 30 seconds, then switch back on. 4. If the fault recurs, check the fault history for related or recurring codes. 5. Inspect the boiler for any obvious signs of damage, leaks, or unusual sounds
How to Fix It
Measure the resistance of the sensor with a multimeter and compare it against the manufacturer’s resistance-versus-temperature table; replace it if it is open circuit, short circuit or out of range.
Unplug and re-seat the connector of the sensor, then check the harness for chafing, corroded pins and breaks in continuity.
Refer to the Atag service documentation for the detailed procedure. Only a qualified engineer should carry out these repairs.
Diagnostic Tools
TroubleAtlas Score
What Is the TroubleAtlas Score?
Every boiler model is independently evaluated and scored by TroubleAtlas across 6 dimensions: reliability, efficiency, parts availability, service network, value for money, and user satisfaction. The score helps you compare models and make informed decisions when choosing a new boiler.
See All Boiler Scores →Boiler Breakdown Cover
Boiler Breakdown Cover
Protect against unexpected repair costs. Cover plans include annual service, emergency callouts within 24 hours, and full parts replacement. Compare plans side by side and choose the level of protection that fits your home and budget.
Coverage Plans — Coming SoonConsidering a New Heating System
Considering a New Heating System?
If your boiler or heat pump is over 10 years old, replacing it with a modern condensing boiler or air-source heat pump could save you up to 30% on energy bills and significantly reduce your carbon footprint.
Compare Heating System Prices →
YouTube Channel
Watch the fixes. Follow the channel.
Step-by-step boiler repair guides, equipment reviews, and heating system comparisons from qualified engineers.
New fixes every weekFix in 5
Quick fixes for common boiler faults explained in under 5 minutes
Boiler Comparator
Side-by-side comparisons of popular boiler models
Boiler Reviews
Independent reviews with TroubleAtlas Score ratings
Find a Specialist
Don’t fix it alone. Find a trusted heating engineer near you.
TroubleAtlas connects you with vetted local heating professionals who can inspect, diagnose and repair your boiler safely — no guesswork, no unnecessary call-outs.
Local heating professionals are coming to your area
We’re onboarding Gas Safe registered and licensed HVAC engineers city by city. Check back soon, or search above to see current availability.
TroubleAtlas Pro
Get cited by AI. Get found by customers.
TroubleAtlas Pro puts your business in front of homeowners searching for exactly this fault, and structures your listing so AI assistants can cite you as a trusted source.
- ✓ Appear on error code pages viewed by thousands of homeowners
- ✓ AI assistants cite your listing as a verified local specialist
- ✓ Receive direct leads with customer name and contact details
- ✓ TroubleAtlas Certified badge for your marketing materials
Affected Models
Atag models that can display fault code 201:
| Model Family | Output Range | Frequency |
|---|---|---|
| All Atag models | — | Possible |
Frequently Asked Questions
Measure the resistance of the sensor with a multimeter and compare it against the manufacturer’s resistance-versus-temperature table; replace it if it is open circuit, short circuit or out of range. According to the technical documentation, this is the first check on the list for 201.
201 is not classified as a high-severity fault, so the boiler can be reset once. If 201 comes back, stop resetting: the underlying cause is still there and needs a qualified heating engineer.