Bosch Furnaces Error Codes
Complete list of fault codes and troubleshooting guides. Use the search below to find your specific error.
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A2
Furnace will not start; control shows A2 lockout code after failed ignition attempts
Ignition Failure Lockout
A21
Furnace shows A21; burner does not start or shuts down quickly
Air Pressure / Fan Speed Fault
A3
Furnace attempts to start but shuts down quickly; control shows A3 flame supervision error
Flame Detection Failure
A41
Furnace locks out with A41; may indicate flue gas temperature too high
Flue Gas Temperature Overheat
A61
Furnace displays A61; unit may not respond to thermostat or internal controls
Communication Error with Control Interface
A7
Furnace shuts down; display shows A7 temperature sensor fault
Flow Temperature Sensor Failure
A8
Furnace locks out with A8; unit does not fire and may show overheat or sensor error
Return Temperature Sensor Failure
A9
Furnace stops and displays A9; may run briefly then shut down on temperature error
DHW (Domestic Hot Water) Temperature Sensor Fault
C1
Furnace shows C1; burner does not start, fan may run briefly
Air Pressure Switch Not Closing
C2
Furnace shows C2; burner shuts down during operation
Air Pressure Switch Opening During Run
E2
Furnace displays E2 and will not fire; may indicate ignition lockout
Ignition Lockout (No Flame Detected)
E9
Furnace shows E9; unit shuts down on overheat
Safety High-Limit Overtemperature Trip
EA
Furnace shows EA; burner shuts down and locks out
Flame Loss During Operation
F0
Furnace display shows F0; unit may not respond to commands
Internal Control Board Fault
F1
Furnace shows F1; may indicate sensor short circuit
Temperature Sensor Short Circuit
F2
Furnace shows F2; may indicate sensor open circuit
Temperature Sensor Open Circuit
H1
Furnace shows H1; may continue to run at reduced output
Low System Water Pressure
H2
Furnace shows H2; may shut down on high pressure
High System Water Pressure
P1
Furnace shows P1; burner output limited or unit stops
Circulation Pump Fault
P2
Furnace shows P2; unit may overheat or shut down
Insufficient Water Flow
U9
Furnace shows U9; may indicate incorrect configuration or parameter error
Configuration / Parameter Fault
Symptom
Furnace blower or circulation pump runs continuously even when there is no call for heat
Stuck Fan/Pump Relay or Overheat Recovery Mode
Symptom
Furnace causes noticeable vibration or rattling noises through the ductwork or piping
Mechanical Vibration / Loose Mounting
Symptom
Furnace display is blank or unreadable but unit still runs
User Interface / Display Failure
Symptom
Furnace emits a burning smell or plastic odor when running
Overheating Components or Dust Burn-Off
Symptom
Furnace emits a high-pitched whine or loud humming during operation
Combustion Fan or Inducer Noise
Symptom
Furnace frequently loses pressure and needs to be topped up with water
Chronic System Pressure Loss
Symptom
Furnace ignites but produces yellow, lazy flames or soot at the flue terminal
Poor Combustion / Sooting
Symptom
Furnace leaks water from the bottom or inside the cabinet
Condensate or System Water Leak
Symptom
Furnace locks out intermittently during windy conditions
Wind-Induced Flue or Air Pressure Issues
Symptom
Furnace makes loud banging or booming noise at startup
Delayed Ignition / Burner Light-Off Noise
Symptom
Furnace produces heat but house never reaches set temperature; runs for long periods
Insufficient Heat Output
Symptom
Furnace runs but does not produce heat; blower or pump operates but burner never ignites
No Heat / Burner Not Firing
Symptom
Furnace runs but some rooms are cold while others are hot
Uneven Heat Distribution
Symptom
Furnace starts then shuts off after a few seconds; repeated short cycling without heating the home
Short Cycling on Flame Loss or Overheat
Symptom
Furnace will not start at all; no lights or display, completely dead
No Power to Furnace
Symptom
Thermostat calls for heat but furnace starts and stops frequently, with very short run times
Thermostat or Control Cycling Issue
Symptom
Visible rust or water stains around the flue connection or top of furnace
Flue Condensate Leakage
| Error Code | Symptom | Fault Type | Guide |
|---|---|---|---|
| A2 |
Furnace will not start; control shows A2 lockout code after failed ignition attempts
|
Ignition Failure Lockout | View Details |
| A21 |
Furnace shows A21; burner does not start or shuts down quickly
|
Air Pressure / Fan Speed Fault | View Details |
| A3 |
Furnace attempts to start but shuts down quickly; control shows A3 flame supervision error
|
Flame Detection Failure | View Details |
| A41 |
Furnace locks out with A41; may indicate flue gas temperature too high
|
Flue Gas Temperature Overheat | View Details |
| A61 |
Furnace displays A61; unit may not respond to thermostat or internal controls
|
Communication Error with Control Interface | View Details |
| A7 |
Furnace shuts down; display shows A7 temperature sensor fault
|
Flow Temperature Sensor Failure | View Details |
| A8 |
Furnace locks out with A8; unit does not fire and may show overheat or sensor error
|
Return Temperature Sensor Failure | View Details |
| A9 |
Furnace stops and displays A9; may run briefly then shut down on temperature error
|
DHW (Domestic Hot Water) Temperature Sensor Fault | View Details |
| C1 |
Furnace shows C1; burner does not start, fan may run briefly
|
Air Pressure Switch Not Closing | View Details |
| C2 |
Furnace shows C2; burner shuts down during operation
|
Air Pressure Switch Opening During Run | View Details |
| E2 |
Furnace displays E2 and will not fire; may indicate ignition lockout
|
Ignition Lockout (No Flame Detected) | View Details |
| E9 |
Furnace shows E9; unit shuts down on overheat
|
Safety High-Limit Overtemperature Trip | View Details |
| EA |
Furnace shows EA; burner shuts down and locks out
|
Flame Loss During Operation | View Details |
| F0 |
Furnace display shows F0; unit may not respond to commands
|
Internal Control Board Fault | View Details |
| F1 |
Furnace shows F1; may indicate sensor short circuit
|
Temperature Sensor Short Circuit | View Details |
| F2 |
Furnace shows F2; may indicate sensor open circuit
|
Temperature Sensor Open Circuit | View Details |
| H1 |
Furnace shows H1; may continue to run at reduced output
|
Low System Water Pressure | View Details |
| H2 |
Furnace shows H2; may shut down on high pressure
|
High System Water Pressure | View Details |
| P1 |
Furnace shows P1; burner output limited or unit stops
|
Circulation Pump Fault | View Details |
| P2 |
Furnace shows P2; unit may overheat or shut down
|
Insufficient Water Flow | View Details |
| U9 |
Furnace shows U9; may indicate incorrect configuration or parameter error
|
Configuration / Parameter Fault | View Details |
| – |
Furnace blower or circulation pump runs continuously even when there is no call for heat
|
Stuck Fan/Pump Relay or Overheat Recovery Mode | View Details |
| – |
Furnace causes noticeable vibration or rattling noises through the ductwork or piping
|
Mechanical Vibration / Loose Mounting | View Details |
| – |
Furnace display is blank or unreadable but unit still runs
|
User Interface / Display Failure | View Details |
| – |
Furnace emits a burning smell or plastic odor when running
|
Overheating Components or Dust Burn-Off | View Details |
| – |
Furnace emits a high-pitched whine or loud humming during operation
|
Combustion Fan or Inducer Noise | View Details |
| – |
Furnace frequently loses pressure and needs to be topped up with water
|
Chronic System Pressure Loss | View Details |
| – |
Furnace ignites but produces yellow, lazy flames or soot at the flue terminal
|
Poor Combustion / Sooting | View Details |
| – |
Furnace leaks water from the bottom or inside the cabinet
|
Condensate or System Water Leak | View Details |
| – |
Furnace locks out intermittently during windy conditions
|
Wind-Induced Flue or Air Pressure Issues | View Details |
| – |
Furnace makes loud banging or booming noise at startup
|
Delayed Ignition / Burner Light-Off Noise | View Details |
| – |
Furnace produces heat but house never reaches set temperature; runs for long periods
|
Insufficient Heat Output | View Details |
| – |
Furnace runs but does not produce heat; blower or pump operates but burner never ignites
|
No Heat / Burner Not Firing | View Details |
| – |
Furnace runs but some rooms are cold while others are hot
|
Uneven Heat Distribution | View Details |
| – |
Furnace starts then shuts off after a few seconds; repeated short cycling without heating the home
|
Short Cycling on Flame Loss or Overheat | View Details |
| – |
Furnace will not start at all; no lights or display, completely dead
|
No Power to Furnace | View Details |
| – |
Thermostat calls for heat but furnace starts and stops frequently, with very short run times
|
Thermostat or Control Cycling Issue | View Details |
| – |
Visible rust or water stains around the flue connection or top of furnace
|
Flue Condensate Leakage | View Details |
No matching codes found
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