Thermo King Alarm & Fault Codes

Food and medicine are only some of the temperature-sensitive things when transported. It doesn’t matter whether through air or land.

Thermo King has been leading the way to transport temperature control since 1938. It has a temperature control system that can address all your needs, from trucks to airplanes. With this guide, you will be able to troubleshoot your unit.


Thermo King alarm & fault codes appear when something is wrong with the temperature control system, including codes 63, 10, 20, & more. Here’s a quick look at what the most common ones mean. Scroll down for more information.

  • Thermo King code 63 means there are engine or vapor motor issues.
  • Thermo King code 10 means there’s a high discharge pressure.
  • Thermo King code 20 means the vapor motor has failed.

Table of Contents

Thermo King Alarm & Fault Codes

Here’s a list of frequent alarm codes or APU faults that may get you into trouble. Do note that some of them may come up simultaneously as alarm code 84. It may also clear out automatically. 

Status colorWhat it means
GreenYou’re good to go.
YellowFollow the instructions specified
RedImmediate action is needed

This list will help you with your alarm codes for trucks, trailers, CRs, DAS, & DSR. It includes multi-temp units that are divided into different zones. You’ll learn about things affecting the vapor motor, glow plugs, & auto switch.

Here are some of the warning codes and their associated colors



A Thermo King code 63 has a red status, which needs immediate action. It indicates that the engine or vapor motor stopped working. It’s sometimes called vapor lock too.

A common cause of vapor lock is heat. When there is too much heat in the exhaust system, engine, or ambient temperature, fuel may vaporize in the lines. It causes the vapor motor to stop working.

The most effective way to address this issue is to let the engine cool down. Look for a shade and park your vehicle. Allow the engine to rest for a few minutes. If possible, pour cold water over the fuel pump and fuel lines. Be sure the engine has cooled before doing this.


Code 10 has a red status, which requires immediate action. It indicates high discharge pressure, which makes troubleshooting difficult.

The first option you can do is to check any broken fan belts. Check for faulty or missing belts by opening the upper and lower front doors. If necessary, replace the belt that has worn out. While you are at it, make sure there are no loose bearings.

The second option you can do is to check if the 12-volt battery is defective. A typical sign of a worn-out battery is if the unit loses power even before engaging the starter. Using a voltmeter, jump the starter and check if the battery voltage is 12V.

If unsuccessful and the voltmeter reading is 0V or 1V, it indicates a bad battery. The only way to fix this is to replace the bad battery with a new one.

The third option you can do is to check if the high-pressure cutout switch is working correctly. You can find the HPCO on top of the compressor. If you bypass the HPCO and the unit starts, it means the HPCO switch is defective.


Thermo King code 37 has a green status, which indicates that it is OK to run your Thermo King. When you encounter it, start by checking the engine coolant level. If it is within normal levels, check the sensor.

Residues stuck inside the reservoir cause inaccurate engine coolant level readings. Carefully pull out the sensor and wipe it with a clean cloth. If the error is still there, check pins 14 and 37 at the microprocessor and ensure all the pins are correctly attached.

If you just repaired a leak, you need to refill the coolant. After doing so, press the TK button. Afterward, press and hold the equal sign button.


Code 84 has a green status, which indicates you can report the alarm at the end of the day. It usually happens as a result of other alarms in your Thermo King. It is a temporary condition and should go away by itself.

Your Thermo King unit should shut down for about 15 minutes. After 15 minutes, your Thermo King should restart. If it does not, it might have to wait for the condition to reset. In most cases, the other error codes that came with this should also go away.

If it does not seem to work, repeat it two more times. If the error persists on the third attempt, the reason that caused your Thermo King to turn off will appear on your control panel. The unit will then automatically shut down. Once it’s fixed, the error should automatically clear after a restart.


Code 17 has a yellow status, which needs checking as specified. It indicates the engine failed to crank when the microprocessor requested a start. It is a frequent thing experienced by drivers and technicians.

Check the:

  • battery
  • cables
  • and starter.

In most cases, troubleshooting these components can lead to knowing the problem and fixing the situation you’re in. 

Check the battery to see if it receives the proper amount of voltage. Load testing the battery checks if it can hold the right amount of charge. Next, check the relay interface circuit board. It controls the fuel shut-off solenoid.

When doing this, be sure to wear a wrist-ground strap to avoid internal damage to the circuit board. Once you’ve checked the battery and the relay interface circuit board, it’s time to check for broken wires and if the pin connectors are attached appropriately. Take note of any broken wires and reattach loosely attached connectors.


Code 26 has a yellow status, which needs checking as specified. It indicates you need to check the refrigeration capacity of your Thermo King. The refrigeration capacity measures the effectiveness of the cooling capacity.

While in use, manually monitor the temperature. A consistent drop in temperature could indicate that your Thermo King is low in freon, or it’s losing freon. If possible, monitor the temperature in equal time intervals and check the results at the end of the day. Doing so will make it easier for you to spot the temperature levels at different times.

If the issue doesn’t go away, report the alarm to Thermo King at the end of the day. Present the data you gathered to spot the problem easier and faster.


Code 61 has a yellow status, which needs checking as specified. It indicates there is a low battery voltage in your Thermo King reefer.

The design of the in-cab controller lets the voltage go through the on/off switches before the in-cab controller.

In most cases, corroded switches cause a drop in battery voltage. This results in low battery voltage. You can find the on/off switches on the nearside of the fridge unit.


Code 25 has a yellow status, which needs checking as specified. It indicates the alternator needs checking. The alternator is a device that converts electrical energy from mechanical energy by alternating current.

Start by going through the small things, such as checking the tightness of the connections. Checking the amps, it is pulling, and checking the battery are also important things to do.

Clear it and run in continuous mode after going through the wires and connections. Did it disappear? If the troubleshooting steps you took did not fix the issue, this could indicate a faulty alternator.

A great option is to replace the alternator with a new one. If you just had it replaced, either you got a defective alternator, or the battery has a bad cell.


Code 35 has a red status, which indicates immediate action is needed. The unit will not be able to operate and should remain shut down.

It indicates a problem with the run relay circuit. The first possible fix for this issue is a software update. A quick software update to your unit may fix it. If it does not, check the alternator is properly working. Also, check the battery is in good health and that the connections are securely in place.


Code 66 has a red status, which indicates immediate action is needed. The unit immediately needs repair if it is off.

It indicates your Thermo King has a low engine oil level. To fix this issue, you need to refill your engine with oil. If you recently filled your engine with oil, check the oil level sensor. Speaking of oil, you might want to read how to reuse waste oil instead of discarding it right away.

The oil level sensor is on the passenger side. To access it, open the side door and look for the oil pan. You can find the oil level sensor by looking for the oil pressure switch. It has one wire going to it and two wires on the plastic piece on top of the oil pan.

If the problem is still there, this could indicate a defective oil level sensor. Before replacing the oil level sensor, disconnect and clean the sensor first.


Code 23 has a red status, which means immediate action is required. It indicates a faulty cooling cycle.

It is an indication that your Thermo King is low on freon or has run out of freon. It can also indicate that the compressor is defective or discharge pressure.

Start by making sure all your belts are in place. If they are secured, check the freon. The freon needs replacing if it is low level or has run out. The freon is on the lower front cover of your Thermo King’s APU AC system.

The ports to look for are the ones coming from the a/c compressor. Your Thermo King’s engine should be running in a/c mode while doing this procedure.


Code 18 has a yellow status, which means it needs checking as specified. If the unit is running, you can check on the alarm at the end of the day. Report it. If your Thermo King is off, the best action is to repair it immediately.

This error indicates that the engine has a high coolant temperature, which could cause the engine to overheat in a shorter period than usual. It is tricky to troubleshoot as it may involve several factors, such as a low coolant level or a leak.

Start by checking your Thermo King’s thermostat. It may be defective or covered with dirt. Clean it off and check if the same error is appearing. Next is to check if the water pump is working correctly.

Suppose you’ve checked the thermostat and water pump and have determined they are working correctly. In that case, you may want to pressure wash the radiator. A dirty radiator may hinder the circulation of air, which may cause overheating.

If the above steps did not work, try checking if the engine oil is within the usual level. While you are at it, check if the oil is in its normal consistency. The diesel can mix with the engine oil if there is leaking.


Code 50 has a green status, which means it’s good to continue using your Thermo King. It has something to do with resetting the time of your unit.

It usually comes up whenever the main power gets disconnected from the microcontroller. Turn on the unit and wait for the setpoint temperature and box temperature to appear to address this issue.

Once it appears, press and hold the TK button until you see “Pre-Trip” on your control panel screen. Then, press and hold the cycle button until you see the time/date. Continue through the following screens and make sure to press the equals button.

Once you have completed the above steps, it should go away. Note that the above steps are for alarm checking and will not affect the reefer on how it operates.


Code 32 has a red status, which indicates immediate action is necessary. It appears when the refrigeration capacity is low. The refrigeration capacity is a measure of the cooling capacity effectiveness of your Thermo King.

If your unit is running, immediately turn it off. A compressor running in low freon may cause it to burn out. You will need to add freon into the unit. Be sure you use a set of gauges to monitor getting proper pressures. Before restarting the unit, check for leaks.


Alarm code 20 has a yellow status, which means it needs checking as specified. It indicates the vapor motor failed to start. It is a common occurrence that you might encounter with their products.

There is likely a problem with the fuel system, causing the vapor motor to fail. A problem with the fuel system involves several factors.

To check for solenoid failure, switch on the unit while keeping a finger or two on the solenoid. If the unit buzzes, that indicates a startup. The solenoid should clunk before the starter engages. If you hear a clunk, it means the solenoid has energized. It is working as expected. 

Check the inline fuel filter by removing the bolt and washer for a clogged fuel system. Remove the filter and clean it well. Reinstall the bolt and washer that were removed.

For air in a fuel system, start by loosening the hand primer found on the transfer pump. See if the fuel tank is at least ¾ full. Otherwise, add fuel to the tank. Next, look for the bleeder bolt on top of the injection pump. Let air inside the fuel system out by opening it. If the unit doesn’t turn on, continue to pressurize the tank.


Code 89 has a green status, which indicates it is good to run. You need to check the electronic throttling valve circuit if you see it.

The electronic throttling valve (ETV) circuit extends maintenance intervals and reduces fuel costs. It also improves temperature control on Thermo King products.

The ETV circuit is on the top middle of the compressor. Check for loose plugs and clean the ground wires as these are the usual causes of your system indicating something is wrong.


Code 13 has a yellow status, which indicates it needs checking as specified. The sensors need checking as one or more sensor is not working as expected.

To check if the sensors are working, you need to monitor the temperature manually. The sensors that need inspection are:

  • the discharge air temperature sensors
  • return air temperature sensors
  • and coil temperature sensor.

Thirty minutes after a defrost is complete, the sensors should read within 30 °F of each other. You will have to observe and compare all three sensors to determine which sensor is faulty.

Once you have determined the sensor at fault, apply lube to the micro connection found on the back of the microprocessor. If it does not work, you will have to replace the sensor with a new one.


Code 44 has a red status, which requires immediate action. It tells you to check the fuel system.

In most cases, it indicates that the system lacks fuel. Aside from adding fuel, look for the banjo fitting on the transfer pump line. Unscrew the screen and clean it. Also, make sure that you are getting fuel from the transfer tanks using such systems.


You’ll need to manually monitor the temperature when you see this check return air sensor code.


It requires immediate action and has a red status code with input shutdown.


It is a status yellow, and the back pressure regulator must be checked. It requires you to monitor the temperature manually.


No action is necessary and you can proceed. However, it means that the sensor grades are not set. The situation should be reported.


It’s a status yellow, and it should be reported as it may require repair. It indicates a heater fan failure.


The vapor motor is experiencing high RPM, which is a serious issue. It causes it to shut down as a consequence.


It’s caused by a failure with the discharge air sensor and gets a yellow warning label. It’s important that you monitor the temperature manually when you experience this.


A high evaporator temperature is experienced, and the unit may require immediate attention and repair.


The evaporator coil sensor is having issues and needs to be checked. The temperature must also be monitored.


Performing a cooling cycle check is required, and the status is yellow. Make sure to stay on top of the temperature.


A heating cycle check is needed, similar to code 21. The temperature must be manually monitored.


Manually monitoring the temperature is important, as is a defrost damper circuit check.


Perform an evaporator coil inlet temperature sensor check, and continue to monitor the temperature manually.


Repair may be necessary if the zone is having issues and a line solenoid circuit check is needed.


An immediate repair may be needed as the unit may have shut down. Proceed with an evaporator coil temperature check.


An immediate repair could be necessary. It depends on the hot gas or hot gas bypass circuit that needs to be checked.


This red status code requires immediate attention, where you ensure the condition of the evaporator coil outlet temperature.

When either of these codes shows up on display, it’s important to investigate. It could hinder or limit the operation of the truck. Parts may need to be changed.

How does Thermo King work?

Thermo King works by using liquid carbon dioxide as a coolant. Liquid carbon dioxide alone won’t produce cold air as it needs to undergo a process to use it.

Cooling is not needed all the time since the user sets the temperature. As a result, the liquid carbon dioxide has to be stored securely. A refillable tank mounted under the vehicle chassis stores liquid carbon dioxide. When the microprocessor controller demands cooling, the liquid carbon dioxide can flow from the tank to the evaporator coils.

The coils alone won’t cool the cargo space. Remember the liquid carbon dioxide that is in the evaporator coils? The liquid carbon dioxide evaporates as air pushed by electric fans passes through the coils. It cools the air that passes over it.

The product of this entire process is cold air maintains the temperature set by the user. The cold air circulates through the whole cargo space.

Here are some different units.

Precedent® S-750iSingle-temperature
Precedent® S-600Single-temperature
Precedent® S-700Single-temperature
Precedent® C-600Single-temperature
SLXi Local TrailerSingle-temperature
Precedent® C-600MPrecedent®S-610DE
Heat King® 450 SeriesHeating

What engines are used?

The engine used by Thermo King is specific to its applications. It focuses on high efficiency, which means high performance without sacrificing emission levels. Before the engine makes it to a Thermo King product, it goes through a rigorous testing and validation process to ensure compliance with standards.

It uses the GreenTech engine, which is known to have the lowest emissions and highest performance. Unlike its competitor brands, the GreenTech engine reduces emissions and pollution without sacrificing overall performance.

It’s a top priority! The GreenTech engine exceeds all environmental requirements because of its commitment to the environment.

It is quite common for high-performance engines to have high emissions. That is not the case for GreenTech engines. Its engines are high-performance without sacrificing their commitment to the environment.

GreenTech engines have the cleanest engines on the refrigeration market. It is also compatible with B5 fuels, making it an environmental-friendly step forward. While we’re on the topic of fuel, we have an article about fuel transfer tanks you might be interested in.

Let us first discuss how to clear and reset the system.

A red diamond warning indicates an alarm code, which may occur for many reasons. Don’t panic because it’s easy to clear and reset so that you can get back on the road!

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