When your trailer temperature starts climbing, the clock is running on your load. A refrigerated unit that won't pull down or won't hold setpoint can cost thousands in spoiled product and a rejected delivery. The good news is that a fair number of "not cooling" calls trace back to something simple a driver can spot in a few minutes. The rest need a tech with gauges and refrigerant certification. This guide walks through the common causes on both Thermo King and Carrier Transicold reefers, what you can safely check yourself, and the point where you should stop guessing and get help before the load turns.
First, Figure Out Which Problem You Have
The single most useful thing you can do is decide which of two situations you're in, because they point at completely different systems.
- The unit won't run at all (engine off, screen dark, or engine cranks but won't start). This is almost always a diesel engine, fuel, or electrical problem, not a refrigeration problem.
- The unit runs but won't cool (engine running, fans turning, but box temperature is above setpoint and climbing or stuck). This points at the refrigeration side: airflow, coils, refrigerant, the compressor, or a mode/defrost issue.
Knowing which camp you're in tells you what to check and helps a tech help you faster over the phone. Before anything else, look at the display and note the setpoint, the actual return-air temperature, the operating mode, and any alarm codes. Write the codes down exactly. They are the fastest route to a diagnosis.
Simple Things a Driver Can Check Safely
Start with the checks that need no tools and carry no real risk. A surprising share of service calls end here.
- Setpoint and mode. Confirm the setpoint is actually where it should be for the load. It's easy to bump a setpoint or leave the unit in the wrong mode. If the unit is set to heat, or the setpoint got changed at a stop, the box will drift the wrong way.
- Continuous vs. cycle-sentry mode. Frozen loads generally want continuous run. If a unit is in a start-stop mode when it should be running continuously, temperatures can creep, especially in heat.
- Is it in defrost? During a defrost cycle the unit intentionally stops cooling for a few minutes and can even feel warm. That's normal. If the display shows defrost, give it several minutes to finish and return to cooling before you assume the worst.
- Fuel level. A reefer engine that won't run because it's low or out of fuel is one of the most common no-cool calls there is. Check the reefer's own fuel gauge, not the tractor's. Low fuel can also pull air into the fuel system, which is a harder restart.
- Airflow inside the box. Walk the trailer if you can. Product stacked against the front bulkhead or piled over the return-air path chokes the unit. Air has to circulate around the load, not just sit on top of it. Boxes that are overloaded, loaded warm, or blocking the return air will run forever and never hold temperature.
- Doors and seals. A door left cracked, a bad seal, or a curtain that's torn lets warm humid air pour in and can also ice the coil. On a hot day this alone can defeat the unit.
If you correct one of these and the unit starts recovering, you've likely found it. Watch the return-air temperature trend down over the next 20 to 30 minutes to confirm.
Runs but Won't Cool: The Refrigeration Side
If the engine is running, fans are turning, and you've ruled out mode, defrost, and airflow, the problem is deeper in the system. These are the common culprits, and most need a tech.
- Iced-up evaporator coil. A coil packed with ice or frost can't transfer heat, so the box stops cooling even though the unit is running hard. This often comes from a failed defrost, a stuck defrost timer, a bad damper, or too much moisture from open doors. Sometimes a manual defrost clears it temporarily, but a coil that ices right back up has an underlying fault.
- Dirty condenser coil. The condenser sheds the heat the unit pulls out of the box. Pack it with road grime, pollen, or debris and the system can't reject heat, so it struggles in warm weather. A visibly filthy condenser is worth mentioning to your tech.
- Low refrigerant or a leak. If the charge is low, the unit runs continuously but can't pull the box down, especially under load. Refrigerant doesn't get "used up" in normal operation, so low charge means a leak somewhere. Checking and correcting this requires gauges and EPA 608 certification. This is not a driver fix.
- Failed or weak compressor. The compressor is the heart of the system. A worn, seized, or valve-damaged compressor may run but won't build the pressure needed to cool. This is a shop-level or advanced mobile repair.
- Stuck defrost, damper, or heat fault. If a defrost cycle won't end, a damper sticks, or a heating component fails on, the unit can actively fight itself and never settle at setpoint.
When It Won't Run at All: Engine and Electrical
A dark screen or an engine that won't start is a different animal. Common causes include low or contaminated fuel, a dead or weak battery, blown fuses, a tripped high-pressure or oil-pressure switch, a bad glow plug or start circuit in cold weather, or a failed contactor or relay on electric standby units. You can safely check the reefer's fuel level and glance for an obviously loose battery cable or a blown main fuse. Beyond that, cranking, fuel-system, and electrical diagnosis belongs with a tech. Don't keep grinding the starter on a unit that won't catch, and don't ignore a unit that shut itself down on a protection alarm. It stopped for a reason.
Alarm Codes and Knowing When to Call
Thermo King and Carrier units log alarm codes for a reason, and they range from a harmless reminder to a shutdown fault that stopped the engine to protect itself. Read the codes off the display and note them. Some are informational and clear on their own. Others, especially anything that shut the unit down, need attention before you run again. Don't just clear codes and keep driving with a perishable load riding on it.
Here's the honest threshold. If you have a frozen or perishable load and the box temperature is out of range and not recovering within about 30 minutes of your first checks, call for service now rather than waiting to see if it sorts itself out. Temperature-sensitive freight doesn't give you a second chance, and a rejected load costs far more than a service call. Northeast Refrigeration Services runs mobile reefer repair 24/7 across Vermont, New Hampshire, Maine, New York, Massachusetts, and Connecticut on Thermo King and Carrier Transicold units. If your reefer is down and the clock is running, call (802) 243-9373.