Hitachi Error Code 24, The Outdoor Coil Sensor and Defrost

It is a cold evening, the heating has been running well and then the room starts to cool. The wall controller shows ALARM, and the code reads 24. Hitachi error code 24 often turns up in the heating season, and there is a good reason for that.

The sensor behind this code helps the outdoor unit judge how cold its coil has become. That judgement matters most when the system heats your home and the coil is at risk of frost. This page explains the sensor, the faults behind the code and the steps that get the warmth back.

What does Hitachi error code 24 mean?

Hitachi error code 24 means there is a fault with the evaporating temperature thermistor. On Hitachi systems that show this code, the sensor sits on the outdoor unit, on or near the outdoor coil. The outdoor board reads it to know the coil temperature.

When the reading drops out, shorts or jumps to a value that cannot be real, the board raises code 24. Then it stops the compressor, because it can no longer judge the coil safely. The wired controller shows the code with a unit number, and the outdoor PCB shows it on the seven segment display.

You will most often see this code on systems with a wired controller. That includes many ducted and cassette systems, the Utopia commercial range and the Set Free VRF range. Wall splits tend to report sensor faults through blinking lamps instead.

How does the outdoor coil change roles?

To see why this sensor matters, it helps to know a little about reverse cycle systems. A reverse cycle air conditioner can move heat in either direction. It uses a reversing valve to swap the job of each coil.

In cooling mode, the indoor coil soaks up heat from your room, and the outdoor coil throws it outside. In heating mode, the roles flip. The outdoor coil now pulls heat out of the outside air, even cold air, and the indoor coil releases it into your home.

The outdoor coil as the evaporator

When the outdoor coil gathers heat, the refrigerant inside it boils and evaporates. That is why it is called the evaporator in heating mode. During that process, the coil runs colder than the air around it, sometimes well below freezing.

Why does this sensor matter so much in winter?

A coil that runs below freezing will collect frost from the moisture in the air. A thin layer is normal. However, if the frost keeps building, it blocks the airflow, and the system can no longer gather heat well.

So the outdoor unit runs a defrost cycle from time to time. It briefly reverses the flow, warms the outdoor coil and melts the frost. The board decides when to start and stop defrost by using sensor readings, and the evaporating temperature reading is a key part of that decision.

Without a trustworthy coil reading, the board cannot manage defrost safely. It might defrost too often, which wastes heat, or not often enough, which lets ice build up. Rather than guess, the board stops and shows code 24.

What a healthy defrost looks like

During a normal defrost, the indoor fan usually slows or stops so it does not blow cold air. The outdoor fan may also stop, and you may see steam rising from the outdoor unit. After a short time, the system goes back to heating. This is all normal and is not a fault.

What usually causes the evaporating thermistor fault?

The sensor sits in one of the harshest spots in the system. It deals with frost, melt water, sun, rain and the constant swing between hot and cold. Those conditions explain most of the causes below.

Repeated freeze and thaw

Every defrost cycle takes the sensor from very cold to warm and back again. Over many seasons, this can crack the sensor bead or its coating. Water then reaches the element, and the reading fails.

Water in the plug or lead

Melt water from defrost runs down the coil and through the base of the unit. If it reaches a damaged lead or a poorly sealed plug, it can cause a short. In some cases, water can even freeze inside a cracked sheath and split it further.

A sensor that has lost contact with the coil

The sensor needs firm contact with the coil pipe to read well. Clips and holders can loosen with vibration and ice. Once the sensor pulls away, it reads air instead of pipe, and the board may decide the signal is wrong.

Corrosion in coastal and humid areas

Salt air and humidity attack metal pins and plugs. A plug that looks fine can have a poor connection inside. That extra resistance can push the reading out of range.

Physical damage

Garden tools, pets, hail and even ice falling from a roof can damage outdoor units. Insects and small animals also chew or nest near leads. Any of these can break the sensor circuit.

A fault in the board input

Now and then, the sensor and lead test fine but the board input has failed. Storms, power surges and age can all play a part. A technician checks this after ruling out the more common causes.

What signs might come with code 24?

Some owners notice odd behaviour before the code appears. Others see no warning at all. Here are some signs worth noting.

• Heating that fades or turns cool for long spells.
• Defrost cycles that seem to happen far more often than usual.
• Thick ice on the outdoor coil that does not clear.
• An outdoor unit that stops and starts in an irregular pattern.
• The code appearing briefly and then clearing by itself.

Heavy ice can also come from other faults, such as low refrigerant or a weak outdoor fan. So these signs are clues, not proof. Our page on an outdoor unit that has iced up covers those other causes.

What can you do when code 24 appears?

You can take some safe steps to rule out a short glitch. Keep them simple and stay outside the unit. Here is a good order to follow.

1. Stop the systemTurn the system off at the wall controller so it stops trying to restart.
2. Cut the powerSwitch off the air conditioner breaker and leave it off for several minutes.
3. Look at the outdoor unitFrom a safe distance, check for heavy ice, fallen branches, nests or rubbish around the unit.
4. Let ice melt naturallyIf the coil is covered in ice, let it melt on its own rather than trying to clear it.
5. Restart onceTurn the power back on, start heating and note whether code 24 returns.

If the code stays away and heating returns to normal, you may have cleared a brief glitch. Still, keep an eye on the system over the next few cold nights. If the code comes back, the sensor circuit needs a proper test.

How does a technician test for the fault?

A technician aims to find the exact point of failure, then fix only what needs fixing. They also check the defrost cycle, because that is where this sensor does its main work. The process usually runs like this.

Safety and inspection first

To start, the technician isolates the power and waits for the inverter board to discharge. Then they open the outdoor unit and find the evaporating thermistor on the coil. They check the sensor, its holder, the lead and the plug for cracks, water, corrosion and chew marks.

Measuring the sensor

After that, the technician unplugs the sensor and measures its resistance with a meter. They compare the result with the table for that model at the coil temperature. An open circuit, a short or a figure far off the table shows a failed sensor.

Checking the board input

If the sensor tests well, attention turns to the board. The technician checks the supply at the sensor plug and looks for damage on the board. A fault here may need a board repair or a replacement board.

Watching a defrost cycle

Once the repair is complete, the technician runs the system in heating mode. Where the system allows it, they watch the live coil reading as the coil cools. Ideally they also see the system enter and leave defrost in a normal way.

What repair will you likely need?

Most of the time, the fix is a new evaporating thermistor that suits the model. The technician removes the old sensor and its clip, fits the new one and secures it firmly to the coil pipe. They route the lead away from sharp edges and the path of melt water.

If the holder or clip is worn, they replace or secure that too. A sensor that sits loose will only cause the same problem again. They may also clean and protect the plug while the panel is off. Our sensor replacement page explains this type of job.

When the problem goes deeper

While the unit is open, the technician may spot signs of other trouble. Oily marks on pipes, for example, can point to a refrigerant leak. Any refrigerant work, such as finding a leak or adjusting the charge, needs a technician licensed by the Australian Refrigeration Council. Most Hitachi systems use R32 or R410A, and each needs the right tools and handling.

How does code 24 compare with nearby sensor codes?

Hitachi uses a separate code for each important sensor. That helps a technician go straight to the right part. Here is how code 24 sits alongside some related codes.

Codes 22 and 24 often work as a pair in heating mode. The ambient sensor reads the outside air, while the evaporating sensor reads the coil itself. The board compares them when it plans defrost. You can learn more about the air sensor on the Hitachi error code 22 page.

Which homes see code 24 most?

Any reverse cycle system can show this code. Still, homes that rely heavily on heating put more freeze and thaw cycles on the sensor. That means cooler areas and long winters tend to bring the code to light.

• Cold inland and highland areas see frequent frost on outdoor coils.
• Southern cities with long heating seasons run many defrost cycles each winter.
• Coastal homes face salt air, which attacks plugs and pins all year.
• Humid areas encourage insects and moisture inside outdoor units.

These are general patterns, not firm rules. A well serviced unit in any climate can run for years without this code. However, knowing your local conditions helps you plan care at the right time of year.

Is it safe to wait until spring?

If you rely on the system for heating, waiting can mean cold nights. The system will not run normally while code 24 stays active. In addition, a fault that lets ice build up can strain other parts if it keeps happening.

So it is best to arrange a check when the code returns, rather than waiting for warmer weather. If you have another safe heat source, use it in the meantime. If you have poor heating without a code, see our guide to poor heating in cold weather for other likely causes.

How can you keep code 24 away?

You cannot stop frost from forming in winter, but you can help the outdoor unit manage it. Good habits reduce the strain on the sensor and its wiring.

• Keep the base and sides of the outdoor unit clear, so melt water can drain away freely.
• Make sure gutters and roof runoff do not drip onto the outdoor unit.
• Trim plants back so air can reach the coil on all sides.
• Book a service before winter, so a technician can check sensors, plugs and the defrost cycle.
• Report frequent defrost cycles or heavy ice early, before a fault stops the system.

A pre winter service is especially useful for this code. During that visit, the technician can check the sensor holder, clean the coil and confirm that defrost works well. As a result, you go into the cold months with more confidence.

What to keep in mind about code 24

Hitachi error code 24 means the outdoor board cannot read the evaporating temperature thermistor properly. The sensor tracks the outdoor coil, which makes it vital for defrost in heating mode. Common causes include freeze and thaw damage, water in the plug, a loose holder, corrosion and board faults.

As an owner, you can safely cycle the power, check the area around the unit and let ice melt on its own. If the code returns, a technician will test the circuit, replace what has failed and check that defrost runs normally. With good care before winter, you can keep the heat flowing on the coldest nights.

More Hitachi guides

• Hitachi Ducted Coil Iced Up, Why It Freezes and How to Thaw It
• Hitachi Split System Defrost Problems and How to Spot Them
• Hitachi Ducted Not Heating, Why It Happens and What to Do

Frequently asked questions