Truma Furnace: “W18H”
warning RV furnace fault codes
What it means
Warm-air outlet or circulated-air (return air) intake is blocked, so the unit cannot move air and shuts down to protect itself. On heating, blocked airflow causes the heat exchanger to run too hot.
Most likely causes
- A warm-air outlet vent or ducting hose is covered, kinked, or blocked most likely
- The circulated-air (return) intake grille is obstructed by furniture, bedding, or stored items common
- Crushed or disconnected warm-air ducting reducing airflow less common
Safe checks you can do yourself
- With the unit off and cool, check every warm-air outlet in the vehicle and make sure none are closed or covered
- Locate the return-air intake grille and clear anything stacked in front of it
- Visually inspect accessible warm-air ducting for crushes or kinks
- After clearing blockages, restart the heater and see if the warning clears on its own
How to reset / clear W18H
- Remove the cause (clear the blocked outlet/intake) — the warning usually clears by itself once airflow is restored
- If the code stays on the CP plus panel, press and hold the rotary/selector or double-click the displayed code until the menu/'back' option appears, then back out to acknowledge it
- Switch the unit off and on at the control panel if the code persists after the blockage is cleared
When to call a technician
If outlets and intake are confirmed clear and the warning keeps returning, have a Truma service partner check the fan and ducting.
Applies to
Combi 4 · Combi 6 · Combi 4 E · Combi 6 E · Combi Eco Plus · Combi Comfort Plus
FAQ
What does Truma Furnace code "W18H" mean?
Warm-air outlet or circulated-air (return air) intake is blocked, so the unit cannot move air and shuts down to protect itself. On heating, blocked airflow causes the heat exchanger to run too hot.
How do I reset Truma Furnace W18H?
Remove the cause (clear the blocked outlet/intake) — the warning usually clears by itself once airflow is restored If the code stays on the CP plus panel, press and hold the rotary/selector or double-click the displayed code until the menu/'back' option appears, then back out to acknowledge it Switch the unit off and on at the control panel if the code persists after the blockage is cleared
What is the most common cause of W18H on a Truma Furnace?
The most likely cause is a warm-air outlet vent or ducting hose is covered, kinked, or blocked.
Related · Truma furnace
VarioHeat ignition/flame fault: the furnace released gas and… E43H
Over-voltage: the supply voltage to the heater is too high (… E44H
Under-voltage: the supply voltage to the heater is too low (… E112H / E212H
Gas/flame fault group: the burner did not ignite or the flam… W25H–W29H
VarioHeat voltage and airflow warnings: W25H = overvoltage a…
Sources
- Tech Tip: Truma Combi Error Codes — Truma Combi error code reference, W18H (Truma (official US site))
- Truma CP plus Error Codes (Combi) — CP plus error code table, code 18 (Ember RV (OEM-hosted Truma table))
Note: Truma displays the same fault as W18H on CP plus and as flash-code/fault number 18 on older panels; wording paraphrased.
● Last verified 2026-06-29 · compiled from manufacturer documentation. Codes can vary by model year — always cross-check your unit’s manual.