Bosch C1 Error Code - Causes & Fix

Bosch C1 Error Code — What It Means

The C1 fault on Bosch gas tankless water heaters means the control has detected excessive flue blockage during startup due to insufficient combustion air. Bosch groups C1, C2, CF, and CE together as airflow and combustion-air faults. When the unit cannot get enough combustion air or cannot move flue gases properly, it shuts down for safety and displays the C1 code.

This is a combustion safety lockout. The heater will not fire until the underlying airflow restriction is identified and cleared. The condition often points to a physical obstruction in the intake or exhaust path, or to contamination of components that handle combustion air.

Jump to Fix

Common Causes

• Blocked air intake or exhaust terminal Debris, insect nests, ice, or damage at the outdoor intake or exhaust opening prevent proper airflow and trigger the fault during startup.
• Dirty or clogged air filter Accumulated dust, pollen, or contaminants on the air filter restrict combustion air supply and cause the heater to shut down with C1.
• Burner clogged with dust, pollen, or insects Contamination inside the burner area restricts combustion and airflow, especially if the outdoor air intake is exposed to these materials.
• Undersized gas supply piping Gas piping that is too small for the heater’s 175,000 BTU demand can contribute to incomplete combustion and airflow faults.
• Incorrect vent length or configuration Vent pipe that exceeds maximum length or does not match the installation manual layout creates excessive back pressure and triggers C1.
• Damaged burner tray or gas valve Physical damage to the burner assembly or gas valve can disrupt combustion air flow and cause the control to detect blockage conditions.

Step-by-Step Fix

1. Reset the heater by turning power off for thirty seconds, then back on, and confirm whether the C1 code returns on the next heating cycle.
2. Inspect the combustion air intake and exhaust terminal outdoors for blockages, debris, insect nests, ice buildup, or physical damage, and clear any obstructions you find.
3. Remove and inspect the air filter at the heater, clean or replace it if you see dust, pollen, or other contamination restricting airflow.
4. Check the vent system layout against your Bosch installation manual to verify that total pipe length, number of elbows, and termination height all match the approved configuration for your model.
5. Inspect the burner area by removing the front cover (power off first) and looking for dust, debris, or insect material on the burner and in the combustion chamber, then vacuum or brush gently if contamination is present.
6. Verify gas supply piping size by comparing your existing pipe diameter and length to the Bosch sizing chart for a 175,000 BTU heater, and upgrade to the correct size if it is undersized.
7. Call a qualified technician if the code persists after you have cleared all accessible obstructions and verified venting and gas supply, as further diagnosis may require combustion analysis, burner tray inspection, or gas valve testing.

Parts Often Needed

When to Call a Pro

Call a professional if the C1 code returns after you have cleared intake and exhaust obstructions, replaced the air filter, and verified vent configuration. Persistent codes often require combustion analysis, gas pressure measurement, or inspection of the burner tray and gas valve, all of which require specialized tools and training. Any work involving gas connections, combustion tuning, or disassembly of the burner chamber should be performed by a technician qualified to service Bosch tankless heaters. If you see visible damage to the burner tray, corrosion in the combustion chamber, or suspect an undersized gas supply that requires re-piping, a licensed professional is the safest choice.