E121 on a Samsung heat pump: Error of ROOM Temperature Sensor in Indoor unit short/open
Short answer
On a Samsung heat pump, E121 means: Error of ROOM Temperature Sensor in Indoor unit short/open. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause. The wording differs by model range — each is shown below.
Reviewed 23 September 2026 · 3 sources
What it means
On the EHS Mono R290 (AE***CXYD*K and pump-integrated AE***CXYB*K)
The manual says: Error of ROOM Temperature Sensor in Indoor unit short/open.
Part involved: Room temperature sensor / thermostat input.
When it shows: Unit self-diagnosis: Error of ROOM Temperature Sensor in Indoor unit short/open.
Likely causes:
- Room temperature sensor/thermostat input short or open circuit, wiring or configuration fault.
- Engineer checks per Samsung service manual: Check the Zone 1 room temperature sensor/thermostat and hydro unit PCB connector (CNS043 per SM).
On the EHS Mono R32 (AE***RXYD*G)
The manual says: Short- or open-circuit error of the room temperature sensor of the Zone 1 indoor unit (detected only when the room thermostat is used).
Part involved: Room temperature sensor / thermostat input.
When it shows: Unit self-diagnosis: Short- or open-circuit error of the room temperature sensor of the Zone 1 indoor unit (detected only when the room thermostat is used).
Likely causes:
- Room temperature sensor/thermostat input short or open circuit, wiring or configuration fault.
- Engineer checks per Samsung service manual: Check the Zone 1 room temperature sensor/thermostat and hydro unit PCB connector (CNS043 per SM).
On the EHS TDM Plus (R410A)
The manual says: Error on indoor temperature sensor of indoor unit (Short or Open).
Part involved: Room temperature sensor / thermostat input.
When it shows: Unit self-diagnosis: Error on indoor temperature sensor of indoor unit (Short or Open).
Likely causes:
- Room temperature sensor/thermostat input short or open circuit, wiring or configuration fault.
- Engineer checks per Samsung service manual: Check the Zone 1 room temperature sensor/thermostat and hydro unit PCB connector (CNS043 per SM).
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: unknown.
Manual reset: unknown.
What you can check
- Note the full error code shown on the controller; check the heat pump breaker/isolator in the consumer unit is ON; check thermostat/timer and controller operating-mode settings; then contact your installer or Samsung service; Samsung manuals do not describe a homeowner reset for this code.
What the engineer will check
- Check the Zone 1 room temperature sensor/thermostat and hydro unit PCB connector (CNS043 per SM). Any work inside casings, on PCBs, wiring, sensors, pumps, valves or refrigerant is engineer-only.
Who to ask for: Heat pump engineer (controls).
Certification: None specific (low-voltage controls).
What you might notice
What owners and engineers report
15 owner and engineer reports mention this.
- Controller / thermostat problem on the EHS Mono R290 (AE***CXYD*K and pump-integrated AE***CXYB*K): 2 reports
- Controller / thermostat problem on the EHS Mono R32 (AE***RXYD*G): 7 reports
- No heating on the EHS Mono R32 (AE***RXYD*G): 6 reports
Sources
- EHS R290 Mono Outdoor Unit Installation Manual AE***CXYD** (EU) DB68-12496A-03 ↗ · DB68-12496A-03 · page p.24 · Reviewed 23 September 2026
- EHS R-32 Mono Outdoor Unit Installation Manual (EU) DB68-08408A-07 ↗ · DB68-08408A-07 · page p.43 · Reviewed 23 September 2026
- Samsung EHS TDM Plus Service Manual Code AC-07002A_0 (Feb 2017) ↗ · AC-07002A_0 · page p.66 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026