Thermo King APU not starting — common causes

A Thermo King APU that won't start is usually one of six things, in this rough order of frequency: battery state of charge, glow plug or controller fault, fuel supply, starter or starter solenoid, controller software fault, or the safety lockout sequence triggered by a prior fault that's still active. Most no-start calls resolve in 30-90 minutes with a structured diagnostic approach. Here's the flow we follow on a TriPac or TriPac Evolution.

Step 1 — Battery state of charge

Probably 40% of no-start calls come down to battery condition. The APU draws from the tractor battery bank (or its dedicated APU battery on Evolution units with the optional setup). Low voltage at start attempt causes the controller to abort with a vague fault. Check voltage at the APU controller input before anything else — minimum spec is typically 11.8V cold-cranking. Below that and the diagnosis is the battery, not the APU.

• Voltage above 12.4V at rest → batteries probably OK, look further
• Voltage 11.8-12.3V at rest → marginal, may start in summer but not in winter
• Voltage below 11.8V at rest → batteries first, then re-test APU

Step 2 — Glow plug system

TriPac engines are small diesels (Kubota or similar) with glow plugs for cold starts. A failed glow plug or glow plug controller stops cold starts cold — literally. The unit will crank but not light off. Diagnosis: pull the glow plug controller fuse, measure resistance to ground on each glow plug at the engine. Spec is typically 0.6-1.2 ohms per plug. Infinity = failed plug. Same ohms across all plugs but no starting = controller or wiring.

Step 3 — Fuel supply

APUs pull fuel from the main tractor tank (in most installations). Common fuel-supply failures:

• Plugged fuel filter — APU has its own small inline filter that gets ignored
• Failed APU electric fuel pump — usually audible (click on key-on or silent)
• Air leak in the fuel pickup line from tractor tank — diesel will start hard then die
• Empty fuel tank (yes, this happens — driver thinks he has half a tank, APU's pickup is higher than the tractor's)

Step 4 — Starter and solenoid

If you hear a click but no crank, the starter or solenoid is most likely. Test starter draw with a clamp meter on the positive cable — healthy draw is typically 80-150 amps. Excessive draw (over 200 amps) = mechanical bind in the starter. No draw = solenoid or wiring.

Step 5 — Controller software fault

TriPac Evolution controllers occasionally lock up after a power event (tractor battery disconnect, jump start, etc.) and need a manual reset. The reset sequence is in the operator's manual but commonly involves disconnecting APU power for 30 seconds, then reconnecting with all switches off, then powering up with the APU disable switch off. After a successful reset, fault history clears and the controller will attempt start normally.

Step 6 — Active fault lockout

If the APU detected a critical fault on a prior run (high coolant temp, low oil pressure, alternator fault, high inverter temp), it locks out further starts until the fault is cleared. The dash will usually display the active fault code. Looking up the code in the TriPac service manual tells you what needs to be addressed before a restart is permitted.

Carrier ComfortPro — different unit, similar logic

Carrier ComfortPro APUs follow a similar failure pattern but use different fault codes and a different controller. Battery, glow plug, fuel, starter, and lockout faults all manifest similarly. The diagnostic flow above applies in concept; specific tests differ. We carry ComfortPro service literature on the truck.

What APU maintenance prevents most no-starts

• Change APU oil at the manufacturer-specified interval (typically 1,000-1,500 run hours) — APUs run a LOT of low-load idle hours that load up the oil
• Replace APU fuel filter annually even if it looks fine — small filter, big consequences when plugged
• Test APU batteries monthly — load test, not just voltage check
• Run the APU through its cooling and heating cycles weekly even when not needed, to keep refrigerant circulating and seals lubricated
• Inspect the drive belt every PM — they're cheap and break expensively