Bosch Greentherm Error Codes
The Bosch Greentherm 9000 and 9800 series are condensing tankless water heaters that have grown significantly in the North American market. They offer high efficiency, modulating output, and a compact footprint — but like any sophisticated appliance, they display error codes when something goes wrong.
This guide covers the four most common Greentherm fault codes and what to do when your unit locks out.
Common Greentherm Error Codes
| Code | Description |
|---|---|
| A0 | Ignition Failure |
| C7 | Fan Motor Fault |
| E9 | Hot Water Outlet Sensor Fault |
| BB | Burner Lockout |
A0 — Ignition Failure {#a0-ignition-failure}
What it means: The Greentherm attempted to light the burner, but the flame sensor did not detect a stable flame within the trial-for-ignition period. After a set number of retries, the unit locks out and displays A0.
Common causes:
- Low or interrupted gas supply
- Dirty or failed flame sensor/electrode
- Failed igniter
- Incorrect gas type selection (NG vs. LP)
- Blocked inlet air or exhaust vent
- Moisture in the combustion chamber (new installation or after long shutdown)
Diagnosis and fix:
- Verify all other gas appliances in the home are working. If not, check the gas meter and main shutoff.
- Open the hot water tap fully and confirm you have adequate water flow — the unit will not attempt ignition below minimum flow threshold (~0.5 GPM).
- Turn off the unit, wait 30 seconds, and restart. The first firing after a cold start can fail due to air in the gas line.
- Remove the burner assembly cover (after shutting off gas and power) and inspect the flame sensor electrode. It should be clean and uncoated. Clean gently with fine steel wool.
- Check the gas type selector switch on the control board — it must match your gas supply (NG or LP).
- Inspect both vent pipes at the termination point for blockages (nests, ice, leaves).
C7 — Fan Motor Fault {#c7-fan-motor-fault}
What it means: The combustion fan motor has failed to reach the target RPM within the expected time, or the fan speed feedback signal has been lost. The fan is critical for maintaining the correct air/fuel mixture and venting combustion gases safely.
Common causes:
- Fan motor bearing failure (worn, seized)
- Fan blade obstructed by debris
- Wiring harness to fan motor loose or damaged
- Fan speed sensor (hall effect) failure
- Control board output relay for fan motor failed
Diagnosis and fix:
- With the unit powered off, try to spin the fan blade by hand through the vent opening (or after removing the access cover). It should spin freely with minimal resistance. Grinding or stiffness indicates bearing failure.
- Inspect the fan blade for debris — insects, insulation fragments, and lint can accumulate over years of operation.
- Check the wiring harness connector at the fan motor for corrosion or loose pins. A secure connection is critical for both motor power and the speed feedback signal.
- If the fan motor spins freely but the error persists, the hall effect speed sensor integrated into the motor may have failed — the entire motor assembly typically needs replacement.
- Fan motor replacement on the Greentherm 9000/9800 is a moderate DIY repair. The motor is accessible after removing the front cover and combustion air tube.
Parts:
E9 — Hot Water Outlet Sensor Fault {#e9-outlet-sensor-fault}
What it means: The NTC thermistor monitoring the hot water outlet temperature has failed, is reading out of expected range, or has an open/short circuit. The control board cannot safely regulate output temperature without a valid sensor reading.
Common causes:
- Thermistor failed (open or short circuit)
- Wiring harness damaged (pinched, corroded connector)
- Thermistor has come loose from its immersion well in the heat exchanger outlet pipe
- Scale buildup on the thermistor body insulating it from the water
Diagnosis and fix:
- Turn off power to the unit and disconnect the outlet sensor connector from the control board.
- Measure resistance across the sensor terminals with a multimeter. At room temperature (~70°F/21°C), a healthy NTC thermistor typically reads 10–12 kΩ. Open circuit (OL) or near-zero resistance indicates a failed sensor.
- Check the connector pins for corrosion or push-back. Clean with electrical contact cleaner and re-seat firmly.
- If the thermistor body is accessible, check that it is fully seated in its immersion well.
- If resistance is correct and the connector is secure, suspect the wiring harness between sensor and board — check for continuity along each wire.
Parts:
BB — Burner Lockout {#bb-burner-lockout}
What it means: BB is a hard lockout code indicating the unit has exhausted all automatic retry attempts following repeated ignition failures (typically A0 faults). The unit will not restart automatically — you must manually clear the lockout by pressing and holding the power button for 3–5 seconds.
Common causes:
- All causes listed under A0 above, having occurred multiple times
- Gas supply interruption followed by air in the line
- Severe flame sensor contamination
- Persistent venting issue
Diagnosis and fix:
- Do not simply clear the BB code and walk away. The lockout exists because the unit tried to light multiple times and failed each time — find the root cause first.
- Work through the A0 diagnosis steps above systematically.
- Most BB lockouts in the field resolve to one of three causes: (a) gas supply issue, (b) dirty flame sensor, or (c) blocked vent termination.
- After identifying and correcting the root cause, press and hold the power/reset button for 3–5 seconds to clear the BB lockout. The unit will go through a pre-purge cycle before attempting ignition.
- If the unit immediately returns to A0 or BB after clearing, a component has failed — call a Bosch-authorized service technician.
Parts That May Need Replacement {#parts-that-may-need-replacement}
| Part | Typical Cost | Where to Buy |
|---|---|---|
| Flame Sensor / Igniter Electrode | $30–$60 | Amazon |
| Fan Motor Assembly | $150–$280 | Amazon |
| Outlet Temperature Sensor (NTC) | $20–$45 | Amazon |
| Gas Valve | $180–$300 | Amazon |
When to Call a Technician
Any fault that persists after cleaning the flame sensor and verifying gas supply should be evaluated by a licensed plumber or HVAC technician. Gas valve replacement, heat exchanger inspection, and combustion analysis require specialized tools and training. Bosch maintains a network of authorized service agents — use the Bosch Home Comfort service locator at bosch-home.com to find one near you.
Pro tip: Greentherm units installed in garages or near exterior walls in cold climates frequently develop A0 or BB faults in winter due to vent freezing. If your error codes appear only in cold weather, inspect the vent termination for ice formation and consider adding a vent termination cap rated for low-temperature operation.