Bosch error code C7 on a Greentherm tankless or Greenstar combi indicates a flame failure — the unit is calling for heat and attempting ignition, but no flame is detected within the safety timeout window. The unit locks out for safety until manually reset or until the underlying cause is resolved.
What triggers a C7
The Bosch flame sensor (a metal rod in the flame path that conducts microamperage when surrounded by ionized flame gases) reports no signal during the trial-for-ignition window. This can be a genuine no-flame condition (gas not arriving at burner) or a sensor problem (flame is present but unsensed).
Diagnostic steps
- Verify gas supply: check the gas valve at the unit is fully open. Check other gas appliances (range, dryer) work — if those are also dead, the problem is upstream of the water heater. Verify gas meter is on and supply pressure is in spec (typically 11" WC for natural gas, 11" WC for LP).
- Verify ignition spark: with the front cover off and the unit calling for heat, listen for spark click at the burner. If no click, the igniter or ignition control board is failed.
- Inspect flame sensor: Bosch flame sensors collect carbon and oxidation. Carefully remove and clean with fine steel wool or 400-grit paper. Reinstall with sensor tip in the flame path.
- Check burner orifices: spider nests are common in stored units. Remove burner assembly and verify orifices are clear.
- Verify air intake: blocked intake (lint, debris, snow on outdoor units) prevents combustion. Clear and verify free airflow.
Reset procedure
After addressing the underlying cause, reset by cycling the power switch on the unit OR using the controller reset sequence (typically holding the reset button for 3 seconds on Greentherm models). If C7 recurs immediately on next call for heat, the underlying problem is not resolved — do not repeatedly reset, as repeated ignition attempts can damage the flame rod and burner.
When to call a pro
If gas pressure measurements require a manometer, if the ignition module shows no spark after sensor cleaning, or if you're not comfortable removing the burner assembly. Bosch authorized service techs have the diagnostic equipment to test ignition modules and gas valves under load.
Preventive maintenance
Annual flame sensor cleaning prevents C7 in most installations. In dusty or salt-air environments, increase to semi-annual. Verify the unit's annual descaling schedule is being followed — heavy scale on the heat exchanger increases ignition difficulty by changing combustion dynamics.