Hitachi Error Code 20, When the Compressor Sensor Fails

The word compressor tends to make owners nervous. It is the heart of the system, and most people know it is a costly part. So when the controller flashes ALARM and Hitachi error code 20 appears, it is easy to fear the worst.

Here is the reassuring part. This code is about a sensor that watches the compressor, not the compressor itself. In many cases, the fault is a small part, a plug or a lead. This guide explains the difference and what to do next.

What does Hitachi error code 20 mean in plain words?

Hitachi error code 20 points to a fault with the compressor discharge thermistor. That sensor clips onto the discharge pipe or the top of the compressor in the outdoor unit. It reads the temperature of the hot gas that the compressor pushes out.

When the outdoor board cannot get a believable reading from that sensor, it raises code 20. Usually that means the circuit reads as open or shorted. Because the board can no longer watch the hottest point in the system, it stops the compressor to stay safe.

The code shows on the wired controller along with the unit number. The outdoor PCB also shows it on its seven segment display, which helps a technician confirm the fault on site. Many Hitachi ducted, cassette, Utopia and Set Free systems report it this way.

Why does the system watch discharge temperature so closely?

The compressor squeezes refrigerant gas, and squeezing gas makes it hot. The gas that leaves the compressor is the hottest part of the whole system. If that temperature climbs too far, the oil inside the compressor can break down and parts can wear quickly.

So the outdoor board keeps a close eye on discharge temperature. It uses the reading to adjust the compressor speed, the expansion valve and sometimes the outdoor fan. In other words, the discharge sensor helps the system protect its most expensive part.

What happens when the reading disappears

Without that reading, the board is running blind at the hottest spot. It cannot tell whether the compressor is cool or close to its limit. Rather than take that risk, it shuts down and asks for help with code 20.

Does code 20 mean the compressor is broken?

No, not on its own. Code 20 is a sensor fault code. It tells you the board cannot read the discharge thermistor, which is a very different message from a failed compressor.

Hitachi has a separate code for real overheating at the compressor. Code 08 means the discharge gas temperature at the top of the compressor is excessively high. The table below shows how the two codes differ.

If you want to read about real high discharge temperature, see the Hitachi error code 08 page. For now, the key point is simple. Code 20 is usually a small part problem, while code 08 asks a deeper question about how the system runs.

What usually causes the discharge sensor fault?

The discharge sensor lives a hard life. It sits on a very hot pipe, inside an outdoor unit that faces sun, rain, wind and wildlife. That mix of heat and weather explains most of the causes below.

Heat stress on the sensor

The sensor heats up and cools down every time the compressor runs. Over the years, those cycles can crack the element or the coating around it. Once that happens, the sensor may give no reading or a reading that jumps wildly.

A lead that has rubbed through

The compressor vibrates while it runs. If the sensor lead touches a sharp edge or another pipe, that vibration can slowly wear through the sheath. Then the bare wire can short against metal, and the board sees a fault.

Corrosion in the plug

Outdoor units face rain and humid air. In coastal areas, salty air speeds up corrosion on metal pins. A plug that looks fine from the outside can have a poor contact inside, and that can be enough to raise code 20.

Insects and small animals

Ants, spiders and other insects like the warmth inside an outdoor unit. Ants in particular sometimes crowd into plugs and onto boards, where they can cause shorts. Small animals may also chew on leads.

A loose sensor holder

The sensor sits in a clip or pocket on the pipe. If that holder loosens, the sensor can fall away from the pipe. That alone gives a false reading, and on some units it may lead to a fault code.

A fault on the outdoor board

Sometimes the sensor and lead test fine, but the board input has failed. Power surges, storms and age can all damage board parts. This cause is less common, so a technician checks the simpler parts first.

What can an owner do about code 20?

Most of the real work sits inside the outdoor unit, behind panels that carry high voltage. Still, you can take a few safe steps before you book a visit. These steps help rule out a short glitch.

1. Switch off at the controllerStop the system with the wall controller first, so it is not trying to restart.
2. Turn off the breakerSwitch off the air conditioner circuit at the switchboard, and wait several minutes.
3. Look at the outdoor unitFrom a safe distance, check for obvious damage, nests, ants or debris around the unit.
4. Restore powerTurn the breaker back on and start the system again as usual.
5. Record what happensNote whether code 20 returns at once, after a short run or not at all.

If the code clears and stays away, a brief glitch was the likely cause. However, if it returns, the system needs a proper test. Our page on what to do when a reset will not clear the code explains why repeated resets rarely help.

How does a technician track down the fault?

A technician starts with the code, then works along the sensor circuit until they find the weak link. They also check the seven segment display on the outdoor PCB, which can confirm the code and sometimes show more detail. The steps below give a general idea of the process.

Isolate and inspect

First, the technician isolates the power and waits for the inverter board to discharge. Next, they open the outdoor unit and look at the sensor, its holder and its lead. They check for rub marks, melted sheathing, insects and corrosion.

Measure the sensor

After that, the technician unplugs the sensor and measures its resistance with a meter. They compare the result with the resistance table for that model at the pipe temperature. If the figure is far outside the table, or there is no reading, the sensor has failed.

Check the board

When the sensor reads well, the technician tests the board input. They can check the voltage on the sensor plug and look for damage on the board. A board fault may then call for a repair or a new board.

Run and confirm

Once the fault is fixed, the technician restarts the system and watches it run. They check that the discharge reading climbs and settles in a sensible way as the compressor works. That confirms the board can now see the hottest point in the system again.

What does the repair usually involve?

In many cases, the fix is a new discharge thermistor. The technician removes the old sensor from its holder and unplugs it from the board. Then they fit the new one, secure the lead away from sharp edges and make sure the holder grips the pipe firmly.

The sensor sits on the outside of the pipe, so this work does not usually involve the refrigerant circuit. If the visit reveals a refrigerant problem, though, that is a separate job. In Australia, refrigerant work needs a technician licensed by the Australian Refrigeration Council.

Sometimes the repair is even simpler. A cleaned and reseated plug, or a repaired lead, can be enough. For more about sensor work, see our sensor replacement page.

Should you worry about the compressor after code 20?

Usually not. The code itself means the compressor stopped because the board lost its reading. That shutdown protects the compressor rather than harming it.

That said, a careful technician will still look at the bigger picture. If the system has been running hot for a long time, the sensor may have aged faster. For example, a dirty outdoor coil or a low refrigerant charge can push discharge temperature up. Checking these things on the same visit can prevent a future code 08.

When the compressor does need attention

In rare cases, the technician may find other signs of compressor trouble during the visit. These might include noise, high current or poor pressures. If that happens, they will explain what they found. Our compressor repair page covers what that work can involve.

How do weather and location affect code 20?

Australia has a wide mix of climates, and each one is hard on outdoor units in its own way. Knowing what your unit faces can help you protect it.

• Coastal homes see salt air, which speeds up corrosion on plugs and pins.
• Inland areas often have dust, which builds up on coils and adds heat.
• Hot regions put long hours on the compressor, which means more heat cycles for the sensor.
• Humid areas encourage insects to move into the warm outdoor unit.

None of this means your sensor will fail early. However, it does explain why regular servicing matters more in some places. A technician can clean the outdoor unit, check plugs and seal gaps where insects get in.

How can you help the discharge sensor last?

You cannot stop normal wear, but you can make life easier for the outdoor unit. These simple habits help the whole system, not just the sensor.

• Keep at least a little clear space around the outdoor unit, free of plants and stored items.
• Hose loose leaves and dust off the outer coil gently, with the power off, if the manual allows it.
• Watch for ant trails or nests around the unit, and deal with them early.
• Book regular servicing, so a technician can check the sensor holder, lead and plug.

Above all, act early when the system behaves oddly. A short run of strange stops or a code that clears by itself is worth reporting. Fixing a loose plug today is far easier than dealing with a full shutdown on a hot day.

What does the outdoor unit do while the code is active?

Once the board raises code 20, the compressor stays off. On many systems, the outdoor fan also stops soon after. You may hear a few clicks as relays drop out, and then the unit goes quiet.

Inside, the indoor fan may run for a short while before it settles. That is normal, because the system is winding down in a controlled way. The controller keeps showing the alarm until someone clears the fault or cycles the power.

Why the system may try again

Some systems will make another attempt after a pause. If the sensor reads well for a moment, the compressor may start and then stop again. This stop and start pattern is a common sign of a loose plug or a lead that only shorts when it moves.

What questions are worth asking your technician?

A good technician is happy to explain what they found and why. Asking a few simple questions helps you understand the repair and plan for the future. Here are some that owners often find useful.

• Did the sensor test outside its table, or was the fault in the lead or plug?
• Were there signs of insects, corrosion or rubbing that could cause a repeat?
• Did the discharge temperature look healthy once the system was running again?
• Is the outdoor coil clean enough, or does it need a deeper clean?
• Is there anything about the location of the unit that makes it run hot?

The answers tell you whether the fault was a one off or part of a wider pattern. For instance, a sensor that failed from age is a simple story. A lead that rubbed through because of loose mounting may need a further fix to stop the same thing happening again.

Key points about code 20

Hitachi error code 20 means the outdoor board cannot read the compressor discharge thermistor properly. It is a sensor circuit fault, not a sign that the compressor has failed. The sensor, its lead, its plug or the board input can all be the cause.

A single power cycle is a fair first step. If the code returns, a technician will test the circuit, fix the weak point and check that the system runs at a healthy temperature. With good care of the outdoor unit, this sensor can keep guarding your compressor for years.

More Hitachi guides

• Hitachi Split System Short Cycling, Why It Happens and What to Do
• Hitachi Ducted Short Cycling, What Causes It and How to Stop It
• Hitachi Operation Light Flashing, Normal Warm Up or a Fault?

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