Last Updated on October 22, 2025
As temperatures rise, renting an air conditioner can be a great solution to keep your project site cool and comfortable. In fact, climate control is especially important for food processing facilities, data centers, and healthcare facilities where warm, humid air can damage products and equipment.
Portable air conditioners are the perfect solution to keeping your facility cool during the occasional heat wave or when the central AC goes out.
However, even the most reliable portable AC units can run into issues. Luckily, modern air conditioners will display error codes that give you clues as to what’s wrong. Use this article to learn common portable air conditioner error codes and how to troubleshoot them.
What Are Air Conditioner Error Codes?
Both central AC and portable AC units work by cooling the air using complex components and sensors. An error code appears when something goes wrong or a sensor malfunctions.
Error codes give you (or a technician) a general idea of what is going wrong with the AC unit. AC error codes appear on the unit’s display screen as either one or two letters or a combination of a letter and a number.
When users address the issue as soon as possible, they can potentially prevent further malfunctions or costly damage.
General Tips for Troubleshooting Portable AC Error Codes
While many AC codes are standardized across brands, they may still vary slightly from brand to brand. It’s a good idea to check the manual for your unit. Know that even if you recognize the error code, the manual might have important information about resolving the issue in your specific machine.
In addition, when troubleshooting AC error codes, investigate the following:
- Has a circuit breaker been tripped? Poor electrical connections can trigger a variety of malfunctions and AC error codes. Try resetting the AC unit and call in an experienced electrician to troubleshoot electrical issues.
- Are the filters clean? Portable AC units, especially on construction sites, can get dirty very quickly. Dust and debris can clog up the filters in a hurry. Dirty filters and external units can cause malfunctions, including overheating.
- Is there a clog or leak? Look around the unit for any leaks (liquids or gases) or clogged drain pipes. Clean the unit and call in an HVAC professional to repair leaks.
10 Common Codes: What Portable AC Error Codes Mean
In this list, you’ll find the most common air conditioner error codes and tips for troubleshooting them.
Remember: Your model may use different letters or numbers for the same code. Review the manual to confirm that you understand the code.
AF: Anti-Freezing
If you see the code “AF” and the Error LED light is on, then this likely means there is an anti-freezing error. The evaporator is too cold to operate correctly because the AC unit is running in an area where temperatures that are too cold.
- What It Means: The temperature of the evaporator is below 28°F.
- Troubleshooting Tips: Do not use the unit if the ambient temperature less than 72°F. Check the refrigerant level.
AL: Fire Alarm Signal
Portable AC units can be connected to the building’s fire alarm system. When you see “AL” — possibly AL1 or AL2 — displayed, check the fire alarm 1 or fire alarm 2 connection. When the fire alarm system goes off, the AC unit will turn off automatically until it has been reset.
- What It Means: The fire alarm signal has been triggered.
- Troubleshooting Tips: Reset the unit by pushing and holding the “OFF TIMER” button for 5 seconds.
FL: Full Drain Tank
Portable air conditioners remove water from the air. This water needs somewhere to go, and that holding spot is the drainage tank. When the tank is full, the machine will shut off to prevent overflowing. “FL” typically means the tank that holds condensation from the machine needs to be drained.
- What It Means: Operators must empty the water in the drain tank.
- Troubleshooting Tips: Take the tank out of the machine, empty it in a safe place away from the machine — such as down a sink, replace the tank and press the “Start/Stop” button to resume air conditioning.
E1: RTS Thermistor Sensor Problem
“E1” will be displayed and the Error LED will be on, indicating that there is a problem with one of the sensors, likely the room or return temperature sensor (RTS) thermistor. This RTS thermistor measures ambient air temperature outside of the machine to balance the temperature of the cold air coming out of the unit. This thermistor is located near evaporator coils.
- What It Means: The RTS thermistor is not able to measure ambient temperature or is not able to adjust the cool air coming out of the machine.
- Troubleshooting Tips:
- Clean filters and coils.
- Check and clean filters evaporator/condenser and evaporator/condenser coils.
- Check the refrigerant level and fix any refrigerant leaks.
- Check for faulty thermistor cables.
E2: ODS Thermistor Sensor Problem
“E2” will be displayed and the Error LED will be on the digital panel, indicating a problem with one of the sensors, likely the oxygen depletion sensor (ODS) thermistor. This ODS thermistor is a safety device that measures when oxygen levels in an environment drop below a necessary threshold. That threshold is typically around 19% to 20%. In a serious case, this error code could mean that the oxygen levels in the room are not high enough. In a less dire context, it could simply mean there is an issue with the sensor.
- What It Means: The ODS thermistor has detected local oxygen levels or is simply unable to properly measure oxygen levels in the room.
- Troubleshooting Tips:
- Clean filters and coils.
- Verify power (ensure the correct voltage/amps or call in an electrician).
- Check and clean filters evaporator/condenser and evaporator/condenser coils.
- Check for faulty thermistor cables.
E3, E4 or E5: CTS Thermistor Sensor Problem
This AC code usually means there is a problem with the unit’s temperature sensor, which may cause it to operate at incorrect times.
- What It Means: The temperature sensor is short-circuited or has been cut.
- Troubleshooting Tips: Replace the temperature sensor.
F5: Fan Motor Problem
“F5” will be displayed on the unit and usually indicates a problem with the AC unit’s fan motor. Cool air can’t circulate properly if the fan isn’t functioning correctly.
- What It Means: Over current from the fan motor is detected, or there’s a faulty capacitor.
- Troubleshooting Tips: Verify power (voltage/amps) and check the flex for restrictions. This may require a professional HVAC technician to fix.
F6: Blower Motor Problem
“F6” will be displayed and can indicate an issue with the blower motor, similar to the F5 Fan Motor error.
- What It Means: Over current from the blower motor is detected.
- Troubleshooting Tips: Verify power (voltage/amps) and check flex for restrictions.
HP: High Pressure
“HP” will be displayed, the Error LED will be on, and the pressure switch will be open. This usually means that there is a blocked air filter or duct that is causing too much pressure in the system. Ambient temperatures that are too hot can occasionally trigger this code.
- What It Means: The pressure in the compressor is too high, opening the pressure switch.
- Troubleshooting Tips: Ensure that the exhaust duct is not blocked or kinked. Do not use the air conditioner if the ambient temperature is higher than 113°F.
TP: Tank Position
As mentioned in the previous section, portable AC units draw water out of the air and into a condensate tank. If the tank is off balance or not positioned correctly, it could spill. Another concern of a poorly positioned tank: water damage to the unit.
If “TP” is displayed and the Align Drain Tank LED is on, then the water tank is out of alignment.
- What It Means: The water tank is not in the correct position.
- Troubleshooting Tips: Place the water tank in the right position.
Cool Off with Reliable Portable Air Conditioners from Herc Rentals
No one wants to work in a hot and humid facility. Renting a portable air conditioner is a great way to keep your facility properly climate controlled, especially in an emergency when the central AC system malfunctions.
Being aware of common air conditioner error codes and knowing how to address them, will keep your portable AC unit running smoothly and efficiently. AC error codes are also helpful indicators for when to call in the experts.
At Herc Rentals, we have a fleet of efficient and reliable portable air conditioners ready for the next heatwave. All our spot cooling units are simple and quick to install. We’ll help you find the right sized unit for your needs and are ready to assist if an error code does pop up.
Browse our selection of climate control solutions and portable air conditioners or call 1-800-432-6215 to speak with an expert today.
