Run capacitor
Common no-start and weak-compressor failure point. Match the microfarad and voltage rating.
LG Mini-Split CH21 Error Code — Outdoor Unit High Pressure
Error Code CH21 on LG mini-split systems means the high-pressure protection device has tripped on the outdoor unit. The system has detected refrigerant pressure above the safe operating limit and shut down the compressor to prevent damage.
Jump to Fix
What Triggers CH21
The CH21 code is triggered by one of two conditions:
- The high-pressure switch (electromechanical) opens due to excessive discharge pressure
- The high-pressure sensor (transducer) reads a pressure above the board’s software limit
For R-410A systems, the high-pressure switch typically trips at 590–620 psig.
Common Causes {#most-likely-cause}
| Cause | Likelihood |
|---|---|
| Dirty or blocked condenser coil | Very High |
| Outdoor fan motor or capacitor failed | Very High |
| Refrigerant overcharge | Medium |
| Non-condensables in refrigerant system | Medium |
| Restricted liquid line (TXV issue) | Medium |
| Failed high-pressure switch | Low |
| High ambient temperature with undersized system | Low |
Step-by-Step Diagnosis {#diagnosis}
Step 1 — Inspect the condenser coil
- Look for dirt, cottonwood, leaves, or grease on the condenser coil fins
- Clean with alkaline coil cleaner from inside out (fins will bend easily — use low pressure)
- Even 30% coil blockage can cause CH21 in summer
Step 2 — Check the outdoor fan
- Fan not running = blown capacitor or failed motor
- Fan running slowly = weak capacitor (check µF)
- Typical LG outdoor fan capacitor: 3–5 µF, 370–440V AC
Step 3 — Connect refrigerant gauges
- High-side pressure (R-410A) at 95°F ambient: 280–320 psig is normal
- High-side above 400 psig with fans running and coil clean = refrigerant or non-condensable issue
- Calculate subcooling: temperature difference between liquid line temp and saturation temp
- Subcooling above 20°F suggests overcharge
Step 4 — Check for non-condensables
- Non-condensables (air, nitrogen) cause high head pressure without high subcooling
- Recover refrigerant, evacuate to 500 microns, recharge to nameplate specification
Step 5 — Test the high-pressure switch
- Check switch continuity at ambient temperature — should be closed
- If switch is open at ambient pressure, replace
CH21 Reset Procedure
LG mini-splits allow 3 high-pressure trips before hard lockout:
- Correct the root cause
- Turn off the unit from the remote
- Wait 3 minutes for the minimum off-timer
- Restart — CH21 should clear
- For hard lockout: cycle the breaker (off for 30 seconds)
Replacement Parts {#parts}
| Part | Notes |
|---|---|
| Outdoor fan run capacitor | LG-specific — match µF and voltage |
| Outdoor fan motor | Match HP, RPM, and shaft diameter |
| High-pressure switch | LG part — match trip pressure (590 psig for R-410A) |
| Condenser coil | Extensive fin damage — clean before condemning |
Need Replacement Parts?
Before ordering, match the HVAC part to the model number, electrical rating, and failure symptom. Check current prices on Amazon - prices and availability change frequently.
Pro tip: In areas with heavy cottonwood or pollen, schedule annual coil cleaning in spring before cooling season. A clean coil prevents most CH21 faults without any refrigerant work.