LG AC / Heat Pump Error CH21 – DC Peak / IPM Fault (Compressor Drive)
CH21: the outdoor inverter's power module saw a dangerous current peak and cut power to the compressor.
At a glance
Severity: High
- Meaning
- DC Peak / IPM Fault (Compressor Drive)
- Typical repair cost
- $0–$3,800
- DIY difficulty
- Hard
LG defines CH21 as a DC peak error or IPM fault. The intelligent power module (IPM) on the outdoor inverter board switches the high-voltage DC that drives the compressor, and it monitors its own current. If a peak goes beyond the safe limit – even for a fraction of a second – the module shuts down and the indoor unit shows CH21. The power module itself is the most frequent failure, especially after surges or years of heat stress, followed by a compressor that is shorting internally or struggling to turn. An overloaded system can also push the current over the edge: a blocked outdoor coil, an outdoor fan that isn't running, or too much refrigerant. Wiring between board and compressor and poor supply power round out the causes.
How it looks on the display
Model families & display variants
| Model family | Shows | Note |
|---|---|---|
| Wall units with 2-digit display (Art Cool, Mega, Standard) | CH ↔ 21 | The display alternates between CH and the number. |
| Models without a numeric display | LEDs 2 + 1 | The heating LED blinks twice and the cooling LED once, then the pattern repeats. |
| Outdoor board (service) | LEDs 2 + 1 | The green and red LEDs on the outdoor PCB repeat the code for the technician. |
Safety first · Consult a professional
The outdoor inverter holds a high DC voltage in its capacitors after the power is cut. Stay with the breaker, the disconnect and the outside of the unit. Refrigerant work requires an EPA 608-certified technician.
- Unplug the appliance or switch off the breaker before opening any panel. Mains voltage can kill.
Symptoms
- CH and 21 alternating on the indoor display
- Outdoor fan may start, but the compressor never runs or stops within seconds
- Indoor unit blows room-temperature air
- Error returns after every restart, sometimes after a thunderstorm or power flicker
- On models without a code display, the indoor LEDs blink two and one times
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Intelligent power module (IPM) or outdoor inverter board failed
- Compressor winding short, insulation breakdown or mechanical binding
- System overload – dirty outdoor coil, outdoor fan not running, high outdoor temperature
- Loose, burnt or swapped compressor leads (U-V-W)
- Supply problems – voltage sags, surges, undersized wiring
- Overcharge or service valves not fully opened after installation
Fix it step by step
Observe the outdoor unit
Start cooling and watch outside. Note whether the outdoor fan runs, whether you hear the compressor try to start and how many seconds pass before it quits. This tells the technician whether to start with the fan, the compressor or the board.
Think about the power supply
Did CH21 start after a storm, a utility outage or while running on a generator? Surges are a classic killer of power modules, and unstable generator power can trip the peak protection repeatedly.
Clean and clear around the unit
With the disconnect off, brush debris off the coil and trim plants back so there is at least 1 ft of free space around and 4–5 ft above the unit. A choked coil raises compressor current on every start.
Tools: Soft brush, Gloves
One full power reset
Switch off the breaker for 10 minutes so the inverter fully discharges and resets. Restart once. If CH21 is back within seconds, leave the system off – repeated attempts on a shorted compressor or module can do further damage.
Technician measurements
The technician checks the IPM terminals for shorts, measures compressor windings for equal low resistance and insulation to ground, and may read live data with LG's LGMV diagnostic software. New installs also get a check that both service valves are fully back-seated open.
Tools: Multimeter
How to reset CH21
- Turn the unit off with the remote
- Switch off the breaker for 10 minutes
- Restore power and start once; if CH21 returns immediately, keep the system off
Where the parts are
- 1Fan
- 2Temperature sensor
- 3CAN bus
- 4Outdoor unit
- 5Compressor
- 6Engine computer (ECM)
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Clean outdoor coil | Yes | $0–$150 |
| Technician diagnosis incl. IPM and compressor checks | No | $120–$250 |
| Outdoor inverter board / IPM replacement | No | $500–$1,300 |
| Compressor lead repair | No | $150–$350 |
| Compressor replacement incl. refrigerant work | No | $1,800–$3,800 |
Rough US estimates; technician call-out fees vary by region. Not a quote.
When to call a technician
Call an HVAC technician right away if CH21 returns after one long power reset. Unlike airflow codes, there is nothing further the owner can safely check, and every extra start on a shorted part risks the other: a failed IPM can take out a good compressor, and a shorted compressor can destroy a new board.
Frequently asked questions
Is CH21 the board or the compressor?
It can be either – the protection sits on the board but reacts to whatever draws too much current. A technician must test the compressor before fitting a new board, otherwise a shorted compressor can kill the replacement board immediately.
Could a power surge cause CH21?
Yes. Surges and lightning often damage the power module. A whole-home surge protector is worth considering once the unit is repaired.
Will my LG warranty cover CH21?
LG typically offers longer coverage on the compressor than on other parts, provided the unit was registered and installed by a licensed contractor. Check your registration and paperwork before paying for parts.
Sources
- Official LG user documentation and support pages (facts only; wording is our own)
- Methodology: How We Build Code Pages · Data Sources We Use · Editorial Policy