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.
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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?
Finding the right replacement parts for your equipment is critical to getting it running again. 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.