Refrigerated trailer faults: common Thermo King and Carrier alarms, protecting the load and ATP

A transport refrigeration unit is a self-contained machine bolted to the front of a trailer or the roof of a rigid body, with its own engine or electric drive, its own fuel tank and its own controller. When it stops in July with a load of chilled meat inside, the clock that matters is the temperature curve in the cargo space, not the repair time.

What kinds of reefer unit are there?

Most trailer units are diesel-driven, with an electric standby motor so the unit can run from a mains supply when parked at a depot or on a ferry. Newer designs, including hybrid and all-electric units, use a diesel generator or the tractor's power to drive an electric compressor, and some run entirely from battery packs or an axle generator. Rigid bodies commonly use a smaller nose-mounted unit driven by the vehicle engine or a separate power pack, and multi-temperature layouts add evaporators and bulkheads inside the body. Knowing which type you have determines what a driver can check and what a technician needs to bring.

How do Thermo King and Carrier units report faults?

Both manufacturers use a controller on the unit, and often an in-cab display or telematics, to log alarms. Thermo King units show numbered alarm codes; Carrier Transicold units show alphanumeric codes prefixed AL. On both, alarms are graded: an informational or log-only alarm records an event, a check alarm asks for attention at the next opportunity, and a shutdown alarm stops the unit to protect it and needs a technician. The code numbers differ by model and generation, so do not rely on a list from another vehicle; read the unit's own manual or telematics app, or ask the technician to interpret the code. Common alarm categories across both brands:

Category	Typical cause
Low fuel or fuel supply	Empty reefer tank, waxed fuel in winter, blocked filter, air in the line
Engine faults	Fails to crank or start, low oil pressure, high coolant temperature, stopped for unknown reason
Refrigerant and pressure	High discharge pressure from a blocked condenser or hot ambient, low suction from a refrigerant leak or a valve fault
Sensors	Return or supply air sensor out of range, box temperature reading implausible
Defrost	Frequent or failed defrost cycles, iced evaporator, poor airflow
Electrical	Low battery, alternator or charging fault, standby power missing, blown fuse
Belt and drive	Belt failure, clutch fault, generator fault on hybrid units
Door and temperature out of range	Door open too long, set point not reached within the expected time

What can a driver safely check?

• Fuel. The reefer tank is separate from the tractor. Check the level and, in winter, whether the fuel has waxed.
• Standby power. If the trailer is on mains standby, check the plug, the socket and the supply.
• Set point and actual temperature. Confirm the controller is set correctly, the mode is right (continuous rather than start-stop for sensitive loads) and note what the return-air reading is doing.
• Airflow. Make sure the load is not blocking the air chute or the return-air path at the front, that pallets are not against the bulkhead and that the evaporator is not iced solid.
• Doors and seals. A door left ajar or a damaged seal defeats the unit; check the strip curtains.
• The alarm display. Read and write down every code, in order, and whether the unit is running or shut down.
• A single restart of a check alarm if the manual allows it; repeated restarts of a shutdown alarm can damage the unit and destroy the evidence a technician needs.

Do not open the unit housing, work near the belts or handle refrigerant. The unit can start automatically, the belts and fan run without warning, and refrigerant burns.

How do I protect the load?

Start a written or photographed temperature log from the first alarm, with times, set point and actual readings. Keep the doors closed and stop opening them to look. If the unit is running but struggling, a shaded parking spot, a pre-cooled empty compartment or reducing door openings buys time. Tell the customer, your traffic office and, if the load is high value, the insurer early rather than after the goods have gone out of range. Ask the technician for an honest estimate of restoration time and, if it is long, plan a load transfer to another reefer, a cross-dock into cold storage or a generator hire. Preserve seals and chain-of-custody records, because whether the goods are accepted will be decided on evidence.

Food loads are subject to the temperature requirements in UK food hygiene law and to the customer's own limits; pharmaceutical loads under good distribution practice have tighter rules still. A driver's judgement is not a substitute for the customer's release decision.

What is ATP and why does it matter?

ATP is the international agreement on the carriage of perishable foodstuffs. It classifies insulated, refrigerated and mechanically refrigerated equipment by its insulation and cooling performance, and equipment used for international carriage of perishable goods must hold a current ATP certificate and plate, renewed after inspection at an approved test station. In the UK the test station is Cambridge Refrigeration Technology. Domestic UK movements do not legally require ATP, but many customers specify it, and an expired plate can stop a load at a port or a customer's gate. Check the plate date as part of trailer maintenance.

What does a reefer technician need to know?

Unit make, model and serial number; whether it is diesel, standby, hybrid or electric; the alarm codes in order and whether the unit is running; the set point and the actual temperature now and at the time of the alarm; the cargo and its temperature limits; the trailer's location and access, including whether it can be moved; and whether the tractor can stay with it. Most reefer specialists are mobile and carry common parts, but refrigerant work needs certified engineers, so ask.

What HGV247 does

HGV247 lists transport refrigeration specialists and trailer repairers with their published capabilities and coverage so you can call the nearest suitable one directly or send a single incident request. It is a directory, not a repairer, and it cannot make a decision about the load; that stays with you and your customer.