Buderus Furnaces Error Codes
Complete list of fault codes and troubleshooting guides. Use the search below to find your specific error.
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2F
Burner does not start, control shows 2F lockout code
Ignition Failure Lockout
3L
Furnace locks out with 3L code, burner starts briefly then shuts down
Flame Loss During Operation
6A
Display shows 6A, burner does not ignite, fan may run briefly.
No Flame Signal at Start-Up
6L
Furnace attempts to start but shows 6L error, multiple failed ignition attempts.
Repeated Ignition Failure
A7
Control shows A7, burner shuts down or will not start, temperature readings seem incorrect.
Flow Temperature Sensor Fault
A8
Display shows A8, furnace may run at reduced output or shut down intermittently.
Return Temperature Sensor Fault
A9
Furnace shows A9, may limit output or shut down on temperature error.
DHW (Domestic Hot Water) Temperature Sensor Fault
B2
Control displays B2, furnace may not fire or may shut down on safety.
Flue Gas Temperature Sensor Fault
C4
Furnace shows C4, blower or fan does not operate correctly.
Fan Speed or Fan Feedback Fault
C6
Error C6, furnace will not fire, fan may not start.
Fan Does Not Start or No Fan Signal
C7
Display shows C7, furnace shuts down shortly after fan starts.
Fan Speed Out of Range
E2
Furnace shows E2, no response to thermostat, burner does not start.
Communication Fault with Room Thermostat or Control
E9
Error E9: Furnace shuts down due to overheating, and may be very hot to the touch.
Safety High Limit or Overheat Trip
H1
Display shows H1: Furnace locked out, no burner operation.
Low Water Pressure / Water Shortage
H2
H2 error: Furnace will not fire, pressure reading on display is abnormally high or low.
Water Pressure Sensor Fault
L2
L2 code: Furnace shuts down, may restart after cooling.
Flue Gas Overtemperature / Limit Exceeded
L3
L3 error: Furnace cycles off on limit, radiators or ducts may be very hot.
Flow Temperature Limit Exceeded
O5
O5 code: Furnace does not start, safety chain open.
Safety Circuit Open (Limit, Door Switch, or External Safety)
O9
O9 error: Furnace locked out, burner will not start.
Gas Valve or Gas Supply Fault
P2
P2 code: Condensate present, furnace may shut down or leak water internally.
Condensate Drain Fault
P3
P3 error: Furnace output limited, may not reach set temperature.
Outdoor Temperature Sensor Fault (Weather Compensation)
U1
U1 code: Furnace unresponsive or randomly resets.
Supply Voltage Fault / Under- or Overvoltage
Symptom
Domestic hot water is not hot enough or runs out quickly, but heating works.
Insufficient DHW Production
Symptom
Furnace is running but making loud booming or rumbling noises at startup.
Delayed Ignition / Combustion Instability
Symptom
Furnace is short cycling, turning on and off frequently without reaching the set temperature.
Short Cycling Due to Control or Sizing Issues
Symptom
Furnace is very noisy during operation, with whistling or howling sounds from the vent.
Airflow or Vent Resonance Noise
Symptom
Furnace produces a burning smell or plastic odor when running.
Overheating Components or Foreign Material on Heat Exchanger
Symptom
Furnace runs but the house is not heating adequately; radiators or ducts are lukewarm.
Insufficient Heat Output
Symptom
Furnace starts, then shuts off after a few seconds; no error code displayed.
Short Cycling on Flame Loss
Symptom
Furnace will not start at all; no lights or display.
No Power to Furnace
Symptom
Gurgling or rushing water sounds in pipes and radiators when the furnace runs.
Air in Hydronic System
Symptom
Radiators or certain zones stay cold while others heat normally.
Zone or Circulation Imbalance
Symptom
System pressure rises excessively when heating; the relief valve discharges water.
Overpressure in Hydronic System
Symptom
The furnace fan runs continuously even when there is no call for heat.
Fan Stuck in Continuous Mode or Stuck Relay
Symptom
The furnace frequently locks out and must be manually reset, but no specific code is shown.
Intermittent Safety Lockout
Symptom
The furnace vibrates excessively or rattles during operation.
Mechanical Vibration / Loose Components
Symptom
Visible soot around the burner or in the flue, black staining near the vent termination.
Sooting / Incomplete Combustion
Symptom
Water leaking from the bottom of the Buderus furnace or nearby piping.
System or Condensate Leak
| Error Code | Symptom | Fault Type | Guide |
|---|---|---|---|
| 2F |
Burner does not start, control shows 2F lockout code
|
Ignition Failure Lockout | View Details |
| 3L |
Furnace locks out with 3L code, burner starts briefly then shuts down
|
Flame Loss During Operation | View Details |
| 6A |
Display shows 6A, burner does not ignite, fan may run briefly.
|
No Flame Signal at Start-Up | View Details |
| 6L |
Furnace attempts to start but shows 6L error, multiple failed ignition attempts.
|
Repeated Ignition Failure | View Details |
| A7 |
Control shows A7, burner shuts down or will not start, temperature readings seem incorrect.
|
Flow Temperature Sensor Fault | View Details |
| A8 |
Display shows A8, furnace may run at reduced output or shut down intermittently.
|
Return Temperature Sensor Fault | View Details |
| A9 |
Furnace shows A9, may limit output or shut down on temperature error.
|
DHW (Domestic Hot Water) Temperature Sensor Fault | View Details |
| B2 |
Control displays B2, furnace may not fire or may shut down on safety.
|
Flue Gas Temperature Sensor Fault | View Details |
| C4 |
Furnace shows C4, blower or fan does not operate correctly.
|
Fan Speed or Fan Feedback Fault | View Details |
| C6 |
Error C6, furnace will not fire, fan may not start.
|
Fan Does Not Start or No Fan Signal | View Details |
| C7 |
Display shows C7, furnace shuts down shortly after fan starts.
|
Fan Speed Out of Range | View Details |
| E2 |
Furnace shows E2, no response to thermostat, burner does not start.
|
Communication Fault with Room Thermostat or Control | View Details |
| E9 |
Error E9: Furnace shuts down due to overheating, and may be very hot to the touch.
|
Safety High Limit or Overheat Trip | View Details |
| H1 |
Display shows H1: Furnace locked out, no burner operation.
|
Low Water Pressure / Water Shortage | View Details |
| H2 |
H2 error: Furnace will not fire, pressure reading on display is abnormally high or low.
|
Water Pressure Sensor Fault | View Details |
| L2 |
L2 code: Furnace shuts down, may restart after cooling.
|
Flue Gas Overtemperature / Limit Exceeded | View Details |
| L3 |
L3 error: Furnace cycles off on limit, radiators or ducts may be very hot.
|
Flow Temperature Limit Exceeded | View Details |
| O5 |
O5 code: Furnace does not start, safety chain open.
|
Safety Circuit Open (Limit, Door Switch, or External Safety) | View Details |
| O9 |
O9 error: Furnace locked out, burner will not start.
|
Gas Valve or Gas Supply Fault | View Details |
| P2 |
P2 code: Condensate present, furnace may shut down or leak water internally.
|
Condensate Drain Fault | View Details |
| P3 |
P3 error: Furnace output limited, may not reach set temperature.
|
Outdoor Temperature Sensor Fault (Weather Compensation) | View Details |
| U1 |
U1 code: Furnace unresponsive or randomly resets.
|
Supply Voltage Fault / Under- or Overvoltage | View Details |
| – |
Domestic hot water is not hot enough or runs out quickly, but heating works.
|
Insufficient DHW Production | View Details |
| – |
Furnace is running but making loud booming or rumbling noises at startup.
|
Delayed Ignition / Combustion Instability | View Details |
| – |
Furnace is short cycling, turning on and off frequently without reaching the set temperature.
|
Short Cycling Due to Control or Sizing Issues | View Details |
| – |
Furnace is very noisy during operation, with whistling or howling sounds from the vent.
|
Airflow or Vent Resonance Noise | View Details |
| – |
Furnace produces a burning smell or plastic odor when running.
|
Overheating Components or Foreign Material on Heat Exchanger | View Details |
| – |
Furnace runs but the house is not heating adequately; radiators or ducts are lukewarm.
|
Insufficient Heat Output | View Details |
| – |
Furnace starts, then shuts off after a few seconds; no error code displayed.
|
Short Cycling on Flame Loss | View Details |
| – |
Furnace will not start at all; no lights or display.
|
No Power to Furnace | View Details |
| – |
Gurgling or rushing water sounds in pipes and radiators when the furnace runs.
|
Air in Hydronic System | View Details |
| – |
Radiators or certain zones stay cold while others heat normally.
|
Zone or Circulation Imbalance | View Details |
| – |
System pressure rises excessively when heating; the relief valve discharges water.
|
Overpressure in Hydronic System | View Details |
| – |
The furnace fan runs continuously even when there is no call for heat.
|
Fan Stuck in Continuous Mode or Stuck Relay | View Details |
| – |
The furnace frequently locks out and must be manually reset, but no specific code is shown.
|
Intermittent Safety Lockout | View Details |
| – |
The furnace vibrates excessively or rattles during operation.
|
Mechanical Vibration / Loose Components | View Details |
| – |
Visible soot around the burner or in the flue, black staining near the vent termination.
|
Sooting / Incomplete Combustion | View Details |
| – |
Water leaking from the bottom of the Buderus furnace or nearby piping.
|
System or Condensate Leak | View Details |
No matching codes found
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