Cooling Cycle Fault Zone Failed to Complete
The THERMO-KING-23 (Thermo King) diesel fault code means: Cooling Cycle Fault Zone Failed to Complete. This is a critical severity code.
- Keep driving?
- Yes, but fix soon
- DIY difficulty
- moderate
- Estimated cost
- DIY basic checks (coil cleaning, filter inspection): $0-$50. Refrigerant leak repair and recharge by a certified tech: $200-$600 depending on leak location and refrigerant volume. Expansion valve replacement: $150-$400 parts and labor. Compressor replacement: $1,200-$2,500 parts and labor at a Thermo King dealer.
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Common Symptoms
- Alarm code 23 appears on the Smart Reefer 4 (SR-4) HMI display
- Zone temperature rises and does not recover to setpoint
- Unit runs continuously in cooling mode but box temperature keeps climbing
- Unit may shut down or switch to a STOP condition after repeated failed cooling attempts
- Discharge air temperature sensor reads warmer than expected during a pull-down
- Condenser or evaporator fans may be running but refrigeration is not occurring
- Cargo may warm to unsafe temperatures if fault is not resolved quickly
Probable Causes (Ranked by Likelihood)
- Low refrigerant charge due to leak in the refrigeration circuit Very Likely
- Refrigerant system restriction (liquid line, filter-drier, or expansion valve blockage) Very Likely
- Compressor failure or loss of pumping capacity Likely
- Condenser coil fouled or condenser fan not operating, preventing heat rejection Likely
- Expansion valve (TXV or electronic) stuck closed or failed, blocking refrigerant flow to evaporator Possible
- Incorrect zone temperature sensor reading causing false cycle monitoring failure Possible
- Evaporator coil heavily iced over, blocking airflow and refrigerant heat exchange Less Likely
Step-by-Step Diagnostic Procedure
Navigate to the Alarm Menu on the Smart Reefer 4 (SR-4) panel and confirm code 23 is active. Note whether any companion codes are stored alongside it, particularly refrigerant pressure or sensor-related codes. Multiple codes together will narrow the fault faster.
Check the condenser coil from the front of the unit. A plugged condenser with dirt, bugs, or debris is one of the most common reasons a cooling cycle fails on a road unit. Clean it with compressed air or water if it is visibly fouled, then clear the alarm and attempt to run the unit.
Inspect the evaporator coil inside the trailer or box. If the coil is blocked with ice, the unit will not move air or transfer heat effectively. Run a manual defrost cycle from the SR-4 panel, let the coil fully drain, and attempt cooling again.
With the unit running in cooling mode, observe the sight glass in the liquid line (if accessible). A continuous stream of bubbles or a completely empty sight glass indicates a low refrigerant charge. This is a strong indicator of a refrigerant leak and requires an EPA-certified technician with manifold gauges to verify pressures and locate the leak.
Listen to the compressor while the unit is running. A compressor that is cycling on and off rapidly, making a rattling noise, or producing very low discharge pressure alongside code 23 likely has internal wear or valve failure. This requires manifold gauges and professional evaluation.
Check that all condenser and evaporator fan blades are spinning freely and in the correct direction. A reversed fan motor or seized bearing will cripple heat transfer on either side of the system. Spin each fan blade by hand with the unit OFF to check for bearing drag.
Inspect the liquid line filter-drier. If the unit has high hours since the last service or if moisture contamination is suspected, a clogged drier will cause a restriction that starves the evaporator. A noticeable temperature difference (frost line or condensation drop) across the drier body indicates blockage. Replacing the filter-drier requires a certified refrigeration technician with recovery equipment.
If all physical checks pass and the fault returns, connect the Smart Reefer 4 (SR-4) to a laptop running Thermo King diagnostic software or use EZcal to pull extended data logs and review suction and discharge pressure trends during a cooling cycle. This step requires specialty tools and is best handled by a certified Thermo King dealer or mobile reefer tech.
Frequently Asked Questions
What does Thermo King code 23 mean?
Code 23 means the controller tried to cool the zone, measured the box temperature over a timed window, and determined the unit did not pull the temperature down fast enough or at all. It is a monitored performance failure, not just a sensor trip. The unit detected that the refrigeration system could not do the job it was supposed to do in that zone. The root cause is almost always a refrigerant problem, airflow problem, or mechanical failure in the refrigeration circuit.
Can the unit still run with code 23 active?
It depends on how the controller handles repeated failures. The unit may continue to run in a degraded state trying to reach setpoint, but if the controller logs multiple failed cooling cycles it can escalate to a shutdown. Either way, the cargo temperature is not being maintained properly. You should not rely on the unit to protect temperature-sensitive loads until the fault is diagnosed and cleared.
How much does it cost to fix code 23?
If it is just a fouled condenser coil or an iced evaporator, cleaning the coil yourself costs almost nothing. If there is a refrigerant leak, expect $200 to $600 for a certified tech to locate the leak, repair it, and recharge the system. A failed expansion valve runs $150 to $400 in parts and labor. A compressor replacement is the worst case at $1,200 to $2,500 at a Thermo King dealer.
Will my reefer unit protect the load the next time it cycles?
Not reliably. Code 23 tells you the refrigeration system already failed to maintain the zone once. Until you find and fix the underlying cause, the same failure will happen again. If you are hauling perishable freight, get the unit checked before loading. A load of produce or frozen product lost to a warm trailer will cost far more than any repair bill.
Sources
This page is built from documented references. Verify against your own service info before repair work.