CH237 on a LG THERMA V R290 Monobloc: Communication modem error – RS-485 modem on the indoor (IDU 485 modem) cannot communicate with Main PCB (outdoor cycle) (R290)
Short answer
On a LG heat pump, CH237 means: Communication modem error – RS-485 modem on the indoor (IDU 485 modem) cannot communicate with Main PCB (outdoor cycle) (R290). The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Communication modem error – RS-485 modem on the indoor (IDU 485 modem) cannot communicate with Main PCB (outdoor cycle) (R290).
Part involved: indoor (IDU 485 modem) 485 modem.
When it shows: No communication between Main PCB(Indoor) and Main PCB(Outdoor cycle) for 3 minutes or more.
Likely causes:
- Communication line wiring abnormal; input voltage abnormal; IDU PCB or ODU PCB abnormal.
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: no.
Manual reset: Service-engineer repair and restart required (LG: resumed only after certificated professional's check).
What you can check
- Read the code on the controller (LG shows 'CH' + number; older wired controllers alternate 'CH' and the number) and note it together with the model number. Report it to your installer or an LG-authorised service centre – LG owner's manuals say an error code generated by self-diagnosis is a reason to call the service. This is an 'Error'-class fault: LG says the system can only be resumed after a qualified professional has checked it, so do not keep trying to restart it. Check the heat pump's breaker/isolator at the consumer unit is on; if a breaker or RCD has tripped, do not keep resetting it – leave it off and call an engineer.
What the engineer will check
- Check the communication cable terminations and polarity between the named PCBs, look for intermediate twisted joints, shorts or open circuits, check for noise sources near the cable, confirm supply voltage and earthing, check software/EEPROM versions of both PCBs are compatible, then replace the defective PCB.
Who to ask for: Heat pump engineer / electrician (inverter and PCB diagnostics).
Certification: Competent person for electrical work (BS 7671); isolate and wait 3 min for DC-link discharge.
What you might notice
What owners and engineers report
1 owner and engineer report mentions this.
- No heating on the THERMA V R290 Monobloc: 1 report
Sources
- LG THERMA V R290 Monobloc Trouble Shooting Guide (FHNW16606C0 / HM..HF UB60/UB40) – ManualsLib 4570773 ↗ · © 2023-2024 (revision not printed on viewed pages) · page 64, 97 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026