Truma Water Heater: “W21H”
warning RV water heater fault codes
What it means
Room-temperature sensor warning on the Combi: the CP plus has detected that the room temperature sensor or its cable is faulty (open, shorted, or out-of-range resistance), so the heater cannot regulate to the set room temperature properly.
Most likely causes
- Damaged, pinched or disconnected room-sensor cable most likely
- Failed room temperature sensor (out-of-range resistance) common
- Corroded or loose sensor connector less common
Safe checks you can do yourself
- Inspect the room temperature sensor cable for visible damage or a loose plug.
- Reseat the sensor connector if accessible.
- If you have a multimeter, the sensor should read roughly 16.2 kohm at 15 C, 12.6 kohm at 20 C, 10.0 kohm at 25 C.
How to reset / clear W21H
- Repair or replace the sensor/cable as needed.
- Power-cycle the CP plus panel; the warning clears once a valid sensor reading is restored.
When to call a technician
The cable checks good and a known-good sensor still triggers W21H, pointing to a control-board fault.
Applies to
Truma Combi 4 / Combi 6 · Truma Combi 4 E / Combi 6 E · CP plus control panel
FAQ
What does Truma Water Heater code "W21H" mean?
Room-temperature sensor warning on the Combi: the CP plus has detected that the room temperature sensor or its cable is faulty (open, shorted, or out-of-range resistance), so the heater cannot regulate to the set room temperature properly.
How do I reset Truma Water Heater W21H?
Repair or replace the sensor/cable as needed. Power-cycle the CP plus panel; the warning clears once a valid sensor reading is restored.
What is the most common cause of W21H on a Truma Water Heater?
The most likely cause is damaged, pinched or disconnected room-sensor cable.
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Sources
- Truma CP plus Operating Instructions — Troubleshooting guide for Combi gas heater (H) — CP plus operating instructions p.17, fault code W21H (incl. resistance table) (Truma (OEM-hosted, Imperial Outdoors))
Note: Resistance reference values are taken directly from the Truma chart; the diesel-Combi equivalent code is E111H/E621H with the same meaning.
● Last verified 2026-06-29 · compiled from manufacturer documentation. Codes can vary by model year — always cross-check your unit’s manual.