Limit thermostat fault
The A02 error code on Vokera boilers indicates limit thermostat fault.
Vokera A02 means: limit thermostat fault. The usual causes are circulation pump failure or air lock in the system, blocked or restricted heat exchanger (sludge or limescale) or system valves closed restricting water flow. You can safely reset the boiler once — if A02 returns, do not keep resetting. A qualified heating engineer should diagnose the root cause.
What Does A02 Mean
The A02 code is shown by the boiler’s control board when it detects limit thermostat fault. 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 A02 and the boiler stops until the fault is cleared |
Common Causes
Causes documented in 1 technical sources.
| Cause |
|---|
| Circulation pump failure or air lock in the system |
| blocked or restricted heat exchanger (sludge or limescale) |
| system valves closed restricting water flow |
| faulty thermistor giving incorrect temperature reading |
| thermostat wiring fault |
| insufficient water flow through the boiler |
Safety Warning
A02 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. Check the circulation pump — verify it is running and water is flowing (feel the pipes either side of the pump for temperature difference). 2. Check system valves — ensure all isolation valves and zone valves are fully open. 3. Check the system for air locks — bleed radiators and check for gurgling noises. 4. Inspect the heat exchanger — check for limescale build-up or sludge. 5. Test the flow and return thermistors — compare the temperature reading with an external thermometer. 6. Check pump speed setting — increase if necessary. 7. If the system has a filter (magnetic or inline), check if it needs cleaning
How to Fix It
Bleed the system of air
flush the heating system if sludge is present (consider a power flush)
replace the pump if it is not circulating
descale or replace the heat exchanger if blocked with limescale
open any closed valves
replace faulty thermistors
Refer to the Vokera 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
Vokera models that can display fault code A02:
| Model Family | Output Range | Frequency |
|---|---|---|
| All Vokera models | — | Possible |
Frequently Asked Questions
Bleed the system of air. According to the technical documentation, this is the first check on the list for A02.
A02 is not classified as a high-severity fault, so the boiler can be reset once. If A02 comes back, stop resetting: the underlying cause is still there and needs a qualified heating engineer.