Viessmann A9 fault codewhat it means and how to fix it
On a 100-W, 111-W or 050-W, A9 is a communication error with the OpenTherm room controller: the boiler carries on heating without it. On a 242-F it means the internal pump is blocked.
Before you start
Which Viessmann is showing A9?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
Which Viessmann is showing A9?
Communication error with the OpenTherm room controller. The boiler keeps heating without it. Viessmann: check the connections and lead, and replace the OpenTherm device if required.
Check the room controller has power and a display. If A9 persists, your installer needs to check the two-core cable and the controller.
Which Viessmann is showing A9?
Internal circulation pump blocked. The burner runs at reduced output or is blocked depending on the heating circuits fitted. Viessmann: check the circulation pump.
Book your heating contractor; the pump is inside the casing.
What A9 means
A9 is documented by Viessmann, in two different rows on two different families of control unit. On the two-character Vitodens 100-W WB1B, WB1C, B1HA/B1KA and B1HC/B1KC, the 111-W B1LA to B1LD and the 050-W BPJD it reads "Control mode without OpenTherm device — Communication error, OpenTherm device — Check connections and lead; replace OpenTherm device if required." The OpenTherm device is the room controller wired to the boiler's OpenTherm terminals — a Vitotrol or a third-party programmable thermostat. When the two stop talking, the boiler does not stop: the system characteristic says it drops back to control mode without the device, which means it heats to its own set flow temperature and ignores the room controller until the link returns. No reset is listed and none is needed. A9 that appears when a new thermostat is fitted is a wiring or compatibility matter; A9 that appears on a working installation is usually the two-core cable or a controller that has lost power.
On the Vitodens 242-F B2UA, controlled by a Vitotronic 200, A9 is unrelated: "Internal circulation pump blocked — Check the circulation pump", with the burner running at its lower output if a mixer circuit is connected and blocked altogether if only one circuit without a mixer is connected. The 200-W B2HA and B2HB tables have no A9 row at all. This page previously said Viessmann published nothing on A9 and repeated a third-party reading of it as an internal communication fault on a "Vitodens 200"; both rows above are Viessmann's own and replace that. On the 2020-on numeric-table units the OpenTherm sense is F.95 (OpenTherm remote control not connected) and the pump sense is F.89 (internal circulation pump blocked).
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| OpenTherm cable or terminals (100-W, 111-W, 050-W) | The two-core cable between controller and boiler, or the plug at the boiler. Viessmann's first measure. | Common |
| Room controller without power or faulty | A battery-powered OpenTherm controller with flat batteries, or a failed controller. "Replace OpenTherm device if required." | Common |
| Controller not OpenTherm-compatible | A thermostat wired to the OpenTherm terminals that does not speak OpenTherm, or that needs the boiler coded for it. A commissioning finding. | Occasional |
| Internal circulation pump seized (242-F B2UA) | On the 242-F the code reports the pump, not a controller. Sludge or a seized rotor after a summer idle. | Occasional |
What to try yourself
On a 100-W, 111-W or 050-W: look at the room controller. If its display is blank, replace its batteries or check its power supply; a controller that has gone dark is the most common reason the boiler loses contact with it.
If the controller is alive and A9 stays on the boiler display, note the code and have your installer check the cable and the controller. The boiler will keep heating at its own set temperature in the meantime — adjust it at the boiler if the house is too warm or too cool.
On a 242-F, A9 is the internal pump and none of this applies; call your heating contractor.
Get a Gas Safe registered engineer to look at your Viessmann A9
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What it costs to fix
Typical repair cost
£0–£300
Time on site
30 minutes
Batteries or a re-made connection cost nothing. A replacement OpenTherm controller is the middle of the range; a circulation pump on a 242-F the top.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Viessmann internal circulation pump
£150–£300Authorised Viessmann parts distributor
Price checked 28 September 2026
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Viessmann fault codes
- F.95Get it checkedThe OpenTherm room controller a 2020-on Vitodens expects is not connected (F.95) or connected but not configured (F.738). On a 100-W or 111-W the burner will not run; the 050-W carries on.Related: controls and communication
- F.102Usually simple to fixThe WiFi communication module of a 2020-on Vitodens has lost contact: F.102 on the fault list, E10 or E12 during connection. Heating is unaffected; only the ViCare app link is down.Related: controls and communication
- F4Stop — call an engineerNo flame signal was detected during burner start on any two-character Vitodens, so the burner has locked out. Reset once; if it returns, call a Gas Safe engineer.Related: same brand
- F.74Get it checkedSystem pressure too low on a 2020-on Vitodens with the numeric display. The burner and pump stop until the pressure is back. Viessmann tell the householder to top up below 1.0 bar.Related: same brand
- F.65Stop — call an engineerNo flame signal at burner start on a 2020-on Vitodens — the numeric table's equivalent of F4. Burner locked out. Check the gas is on, reset once; if it returns, call a Gas Safe engineer.Related: same brand
- EAStop — call an engineerA Lambda Pro combustion-controller code on Vitotronic-controlled 200-W, 222-F and 242-F boilers: the ionisation current during calibration was outside the permissible range or implausibly different from last time. Lockout; engineer.Related: same brand
Own a Viessmann boiler? Save it and its codes come first next time.
Related guides
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.