Outdoor Inverter DC Peak
LG CH21 Error Code
The LG CH21 error code is raised when the outdoor inverter sees a sudden spike of current in its DC power circuit. The board cuts power to the compressor in a fraction of a second to protect itself. CH21 can be a one off after a power disturbance or a sign of a deeper compressor or inverter fault, so it pays to understand it.
What It Means
What Is a DC Peak Error on an LG Air Conditioner?
An inverter air conditioner does not run its compressor directly from mains power. Instead, the outdoor board converts the incoming AC supply into DC, then uses a power module to build a variable output that drives the compressor at whatever speed is needed. That power module is often called the IPM, short for intelligent power module.
The IPM watches the current flowing through it extremely closely. If the current jumps above a safe peak, even for a moment, the module shuts the drive off to stop itself from being destroyed. The board then logs the event and the indoor unit shows CH21.
LG service information describes CH21 as a DC peak or IPM fault. In plain words, the compressor drive circuit tripped on a current spike. It is a protective action, not a diagnosis of exactly which part is to blame.
Some third party lists word CH21 as overcurrent in the inverter DC circuit. That is consistent with the same idea. On current LG residential split systems, CH21 sits within the family of outdoor compressor and inverter faults.
Outdoor Unit
Which Parts Are Linked to an LG CH21 Fault?
CH21 is an outdoor unit fault. Nothing inside your indoor unit causes it directly, so the technician’s attention goes straight to the condenser unit outside.
The Power Module
The IPM switches high currents rapidly to drive the compressor. It can fail with age, heat or after a surge, and a failing module can trip on its own.
The Compressor
If the compressor motor windings are breaking down, or the compressor is struggling mechanically, it draws more current than normal and triggers the peak protection.
Compressor Wiring
Three wires connect the board to the compressor. A loose, burnt or shorting terminal on any of them can cause a current spike.
Refrigerant Conditions
Very high pressures, from a blocked condenser or an overcharged system, make the compressor work against a heavy load and can push current up.
Reading It
How Will I See CH21 on My LG Unit?
On LG wall splits with a numeric display, you will see CH and 21. On units without one, an indicator light blinks a two part pattern. Because the code comes from the outdoor unit, it is passed to the indoor display over the communication link.
CH21 often shows at a particular moment. Many owners notice it just after the compressor starts, when current naturally rises. Others see it during a hot afternoon when the system is working at full speed. Note when it appeared, as this helps the technician narrow down the cause.
You may also notice the outdoor unit start, hum for a few seconds, then go quiet while the fan keeps turning. That pattern is typical of a compressor drive trip.
Likely Causes
What Triggers the LG CH21 Code?
The causes run from a temporary disturbance on the power supply to serious internal faults. Here they are, roughly from simple to serious.
Temporary or External
- A voltage spike or dip on the grid during a storm
- Restarting the unit too quickly after a power cut
- Several heavy appliances starting on the same circuit
- A very hot day pushing the system to its limit
Internal and Serious
- A blocked or dirty outdoor coil raising pressures
- Loose or burnt compressor terminals
- A failing power module on the outdoor board
- Compressor winding breakdown or mechanical wear
A useful distinction is whether CH21 happens once or keeps returning. A single trip after a storm that never repeats is usually a supply event. A code that returns every time the compressor starts points firmly at the hardware.
Owner Checks
What Safe Checks Can I Do for CH21?
Inverter boards store a high voltage charge even after the power is turned off. Keep well away from the outdoor unit’s electrical covers. These checks are all safe and external.
- Note the code, the time and the weather. Was there a storm or outage recently?
- Switch the system off with the remote, then off at the breaker. Leave it off for ten minutes so the board fully discharges and resets.
- Look at the outdoor unit. Remove leaves, rubbish and plant growth from around the coil and fan. Keep at least an arm’s length of clear space around it.
- Check that the outdoor unit is not boxed in by stored items, a new fence or a cover.
- Restore power and try once more. Avoid running other heavy appliances at the same moment.
If CH21 returns after this single reset, switch off at the breaker and book a technician. Repeated attempts put extra stress on the compressor and power module.
Using the Unit
Should I Keep Running My LG After a CH21 Code?
Not if it repeats. CH21 is the board stepping in to prevent damage. Each time the compressor tries to start into a fault, it subjects the windings and the power module to a heavy current surge.
If the cause is a failing component, those repeated surges can finish it off. A power module that might have been the only failed part could take the compressor drive circuit with it, or a weak compressor could fail completely. That turns a moderate repair into a major one.
One reset after a storm is reasonable. After that, the safest course is to leave the system off and wait for a technician.
Technician Tests
How Does a Technician Diagnose LG CH21?
Diagnosing CH21 calls for proper electrical test equipment and knowledge of inverter safety. The technician typically follows these steps.
Review History
They ask when the code occurs, whether it follows storms, and whether it happens at start up or under load.Check the Supply
Mains voltage at the outdoor unit is measured, including under load, to rule out a weak or unstable supply.Discharge and Inspect
After isolating power and allowing the board to discharge, they inspect for burnt marks, swollen parts and loose terminals on the board and compressor.Test the Compressor
Winding resistance is measured between each pair of compressor terminals and to earth. Unequal readings or leakage to earth indicate a compressor fault.Test the Power Module
The IPM outputs are checked for shorts. Some technicians also run the board with the compressor disconnected to see whether the trip persists.Check Operating Pressures
With gauges fitted by an ARCtick licensed technician, operating pressures are checked to rule out a condenser or charge problem pushing the load up.
Open more detail
Inverter compressors and power modules are closely matched. Replacing one without understanding why it failed can lead to a repeat failure. A compressor with a winding fault can destroy a new board, and a faulty board can stress a healthy compressor. That is why a thorough technician tests both before recommending a repair.
If a major part needs replacing, the technician will explain the options and the age of the system will be part of the conversation. On older systems, replacement can sometimes be the more sensible choice. That decision is always yours.
Weather and Grid
How Do Australian Storms and Heatwaves Affect CH21?
CH21 has a strong link to power quality. Summer thunderstorms bring lightning and surges, and they also cause brief outages that restart thousands of air conditioners at once. That restart can create a current spike as compressors try to start against pressure that has not yet equalised.
Heatwaves add load. When outdoor temperatures soar, the compressor works against higher pressures, current rises, and any weak point in the drive circuit is more likely to trip. Grid voltage can also sag on very hot evenings when demand peaks, which makes inverter drives work harder.
Coastal salt air corrodes compressor terminals and board connections over years, raising resistance and heat at those points. In dusty inland suburbs, a coil packed with dust raises head pressure and adds to the load. Homes with outdoor units in full western sun see higher condenser and board temperatures on summer afternoons, which also stresses the inverter.
Avoiding Repeats
How Can I Lower the Risk of CH21 Happening Again?
While some CH21 trips come from the grid and cannot be prevented, a few steps reduce the chance and protect the inverter.
Wait After an Outage
When power returns after a blackout, wait several minutes before switching the air conditioner back on. This lets system pressures settle.
Keep the Coil Clean
A clean outdoor coil keeps pressures and current down. Have it cleaned during routine servicing.
Consider Surge Protection
A licensed electrician can advise on surge protection for your switchboard. It may reduce the impact of grid disturbances.
Shade over the outdoor unit can help too, provided it does not restrict airflow. A shade sail or awning well above the unit is better than anything placed close around it.
Inside the Inverter
Why Do Inverter Systems Need Peak Current Protection?
Older fixed speed air conditioners switched their compressors fully on and fully off. Inverter systems, which include almost every LG split sold in recent years, work very differently. They vary compressor speed smoothly, which saves energy and keeps rooms at a steadier temperature.
To do that, the outdoor board switches large currents on and off thousands of times every second. The parts that do this switching are fast and efficient, but they have little tolerance for overload. A current spike that an old fixed speed relay would shrug off can destroy a power transistor almost instantly.
That is why LG builds peak current detection directly into the power module. It reacts far faster than any fuse or breaker could. When CH21 appears, it means that protection worked. The part may still be healthy, but something made it see a dangerous current, and that something needs to be understood.
The protection also explains why CH21 can appear without any obvious damage. The module may have tripped a split second before harm occurred. If the trigger was a passing grid disturbance, the system may run perfectly after a reset, and it may never show the code again. If it was a developing fault, the trips will continue.
Helpful Details
What Should I Mention When Booking a CH21 Repair?
Because CH21 has both supply and hardware causes, the background story matters a great deal. Share as much of the following as you can.
- Whether there was a storm, an outage or electrical work shortly before the code appeared.
- Whether the code appears at start up, after running a while, or only on hot days.
- Any change in cooling or heating performance in recent weeks.
- Any unusual noises from the outdoor unit, such as clunks, buzzing or rattling.
- The approximate age of the system and the model number if you can see it on the outdoor unit label.
Access to the outdoor unit matters for this code, since almost all of the work happens there. If the unit sits on a roof, a high wall bracket or behind a locked side gate, tell us when you book. The technician can then bring the right ladder, safety gear and test equipment, and plan the visit safely. Clearing a path to the unit and keeping pets inside on the day also helps the job run smoothly.
One Off or Ongoing
How Do I Know if My CH21 Is Serious?
Owners often ask whether a single CH21 means trouble. The answer depends on the pattern, and these two scenarios cover most cases.
A Single Event
The code appeared once, during or after a storm. After a full reset, the unit runs normally for days and weeks with no repeat. This is most likely a supply disturbance. Keep an eye on it and note any recurrence.
A Repeating Pattern
The code returns each time the unit starts, or regularly on hot afternoons. Cooling may also have weakened. This points to a hardware fault, and the unit should stay off until a technician has tested it.
Related Codes
Which LG Codes Sit Near CH21?
CH21 belongs to a cluster of outdoor inverter and compressor codes. These pages explain the closest relatives.
LG CH26
Compressor start or rotation failure.
LG CH62
Inverter heat sink overheating on the outdoor board.
LG CH61
Outdoor condenser coil running too hot.
Other related codes include CH23 for low DC voltage and CH29 for excessive compressor current. Visit our LG error code hub for the full set we cover, or the error code library for other brands.
Getting a Technician Out
Where Can I Get an LG CH21 Fault Diagnosed?
We arrange licensed technicians for LG inverter faults in Sydney, Melbourne, Brisbane, Perth and Adelaide. Storm season brings a spike in calls across all five, so ring early with your code.
Read more on our LG air conditioning page. Rapid Air Cooling is a booking service, not affiliated with LG, and the technicians we send hold the licences their work requires.
Questions
Frequently Asked Questions
Does CH21 mean my compressor is dead?
Not necessarily. CH21 can come from a supply disturbance, a loose terminal, high pressures or a failing power module. Only testing can show whether the compressor is at fault.
Why did CH21 appear after a blackout?
Restarting immediately after an outage can make the compressor start against unequal pressures, causing a current spike. Wait a few minutes before switching back on.
Is it safe to open the outdoor unit to look?
No. Inverter boards hold a dangerous charge after power is off. Leave all covers in place and let a licensed technician do the inspection.
Can a dirty outdoor coil cause CH21?
It can contribute. A blocked coil raises pressures, which raises compressor current. Clearing debris from around the unit is a safe first step.
Will a power module replacement fix CH21?
Only if the module is the cause. The technician should test the compressor and wiring first, because a faulty compressor can destroy a new module.
How is CH21 different from CH26?
CH21 is a current spike trip. CH26 is a failure to detect proper compressor rotation. They overlap in causes but are tested slightly differently.
Where can I learn more?
See our troubleshooting guide for everyday faults, or our page on air conditioning service Australia to learn how bookings work.
Related Guides
You May Also Find These Helpful
LG Tripping With CH21?
Leave the system off and give us a call with the code. We will arrange a licensed technician to test the supply, compressor and inverter safely.
Reviewed by AC repair and service specialist on 22 October 2026. How we write our articles