HeatingCodes

Rheem tankless water heater error codes

By Jason Dearmin · Last reviewed November 14, 2026

Rheem's troubleshooting manual separates warning codes (the heater keeps running) from error codes (the heater stops). The manual sends you to the gas supply, vent and flame rod first for ignition problems.

What this table coversTitles from the table of contents of the Rheem High and Super High Efficiency Tankless Water Heater Troubleshooting Manual (AP21822 Rev 02).

How to read the display

Codes show on the built-in controller or the remote controller. The manual lists the last eight faults in the controller's error history, which helps when a code comes and goes.

Several codes in the manual are marked "WiFi model only". Check that your model is in the list at the front of the manual.

Rheem code table

CodeMeaningWhat the document says
P1 Error: not enough water flow for operation
1L Warning: lime build-up detected in heat exchanger
03 Error: manifold setting failure
05 Warning: low heat exchanger temperature detected
28 Error: low heat exchanger temperature detected
10 Warning: incorrect inlet air volume
11 Error: flame not detected by flame rod Full explanation
12 Error: detected presence of flame, then lost it
14 Error: over-heat limiter is active
15 Error: excessively hot water detected
16 Error: outlet thermistor failure
17 Error: water leak detected (WiFi model only)
24 Error: DIP switch malfunction
29 Error: condensation not drained from collector
31 Error: inlet thermistor malfunction
32 Error: heat exchanger thermistor malfunction
33 Error: outlet thermistor malfunction
34 Error: ambient / air supply thermistor malfunction
35 Error: improper thermistor connection
51 Error: main gas solenoid valve (SV) malfunction
52 Error: proportional gas valve malfunction
61 Error: blower motor speed incorrect for combustion
66 Error: bypass motor failed
70 Error: microcomputer abnormality
71 Error: solenoid valve (SV) circuit failure
72 Error: flame rod detected flame before ignition
76 Error: built-in display (or remote controller) to water heater communication failure
79 Error: blower motor current failure
80 Error: detected flame presence after closing solenoid valves
82 Error: installed gas chip error
88 Maintenance notice
89 Error: high exhaust temperature detected
90 Error: blockage in venting detected (before the unit goes to ignition)
99 Error: blockage in venting detected (after the unit shut down)

Source: Rheem AP21822 Rev 02, section 2.2 error code list.

Detailed explanations

SafetyThese tables describe what the manufacturer documents say. Gas, combustion, refrigerant and line-voltage repairs are for licensed professionals. If you smell gas, leave the building and call your gas utility or 911. Safety disclaimer.

Sources