Rheem code 05: improper combustion warning

Code 05 is an early warning that the burner is not burning cleanly. Treat it seriously, because poor combustion can also produce carbon monoxide.

This page uses two Rheem sources: the current Rheem and Ruud tankless troubleshooting manual, and Rheem's error code list for earlier tankless models. Codes can differ between generations, so the meaning for each is shown where they differ. Rheem technical support: (800) 432-8373.

What it means by model generation

What to check

• Venting: blocked or damaged vent terminals, wrong materials, too long a run
• Gas supply: line or meter too small, low pressure when other appliances run
• Altitude setting: DIP2 on the control board must match your elevation
• Earlier models: clean the air inlet filter, combustion fan and heat exchanger fins, and check combustion air openings

What you can do

Check the vent terminals outside for leaves, nests, snow and ice. Note whether the code appears when a furnace or dryer runs at the same time, which points to gas supply. Then call a technician.

Before you call

Have the model and serial number from the rating label ready, and note whether your unit is a condensing (high-efficiency) or non-condensing model. Tell the technician the exact code, when it appears and any recent work on the gas, water, venting or electrical supply. That often saves a second visit.

See every code on our Rheem error codes page.