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How to Troubleshoot Alarm Codes 10 and 20 on a Thermo King Unit

I have been a diesel truck mechanic since 1978 and have worked on both Thermo Kings and Carriers since 1985. I know how hard it is to get good information or help—even service manuals may not do a good job of guiding you.

Here are my tips for troubleshooting common alarm codes on Thermo King trailer units.

Refrigerated truck climbing a hill in Denmark

Refrigerated truck climbing a hill in Denmark

Thermo King Alarm Code 10: High Discharge Pressure

This shutdown alarm code can be difficult to troubleshoot. Here are a few common tips to make the process a little easier:

  1. Check for broken fan belts.
    • This is the easiest item on the list. Open up the lower and upper front doors and check for missing or broken belts.
    • Replace as needed.
    • Be sure to check the idler gear box for frozen or worn bearings.
  2. Check for a defective 12-volt battery.
    • This is more common than you would expect.
    • When you turn on the unit, it loses power before engaging the starter. The screen may go blank or blink on and off.
    • Use a voltmeter and verify that the battery voltage is correct (12V).
    • Jump the starter and observe the battery voltage.
    • If the starter fails to engage and the voltage falls to 1V or 0V, then you have a bad battery. Replace the battery.
    • Warning: Any attempt to jump-start the battery may cause it to explode.
  3. Check for a defective high-pressure cutout (HPCO) switch.
    • The HPCO is located on the top of the compressor.
    • Disconnect the harness and bypass the HPCO switch.
    • If the unit starts, then the HPCO switch is defective.
  4. Check the wiring harness on the 8D circuit.
    • If the unit still won't start, then verify voltage to the HPCO switch.
    • Do a voltage drop test to check for an open circuit.
    • You will need a wiring schematic for the Thermo King unit you are working on.
    • Trace the wiring circuit and check any fuses or switches connected to the circuit.

Thermo King Alarm Code 20: Engine Failed to Start

This is one of the most common alarm codes you will encounter. Minus a few exceptions, you likely have a fuel system problem. The following troubleshooting tips should solve this problem quickly.

  1. Yanmar 486 fuel solenoid failure
    • Clear any alarms.
    • Position yourself in front of the unit so that you can touch the side or back of the Yanmar engine's fuel solenoid. The fuel solenoid is located on the back of the injection pump.
    • Turn the unit on and keep a finger on the solenoid. The unit will buzz, indicating a startup.
    • The fuel solenoid will clunk, and the starter will engage. If the solenoid clunks, then it has energized and is okay.
  2. Plugged fuel system
    • Check the inline fuel filter, located in the transfer pump inlet hose fitting.
    • Remove the bolt and washers.
    • Remove the filter from inside the bolt and check for debris clogging the screen.
    • Clean out the bolt, the screen, and the banjo fitting.
    • Reinstall the bolt and washers. You can change the spin-on fuel filter if needed now.
  3. Air in fuel system
    • Loosen the hand primer on the transfer pump.
    • The fuel tank needs to have at least a three-quarter tank of fuel. If not, you will need to add some fuel to the tank.
    • Locate the breather fitting on top of the tank next to the supply and return fuel lines.
    • Pressurize the fuel tank. I use an old Freon bottle with compressed air and one of my Freon hoses for this. Watch the front of the tank to ensure you don't overpressurize it.
    • Bleed the air out of the fuel lines.
    • Locate the bleeder bolt on the top of the injection pump and open it to allow any air in the fuel system out. On Yanmar Tier II engines, there is a banjo fitting that will have to be loosened.
    • Turn the unit on. It should start up. If not, continue pressurizing the tank and priming the fuel lines to get all of the air out.
  4. Confirm that return fuel is getting back to the fuel tank.
    • Locate the return fuel line.
    • Loosen the hose fitting and carefully pull out the line to see if fuel is coming out.
    • If there is no fuel, then you still have air somewhere in the system.
    • A good steady stream indicates that the fuel system is functioning properly.

Tip: If you check the return fuel line every time you work on a fuel system, you can prevent expensive warranty claims.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

Question: How can I troubleshoot my car's sudden auto stop and auto start failures?

Answer: The most common cause for a unit not to cycle has to do with charging. Check the amp meter in your gauge menu if it is above 5 amps the unit will not shut off. If this is true your battery may be starting to fail.