12-Volt Truck Fridge or Cab Heater Not Working?

Disclosure: Some links on this site may be affiliate links; see our disclosure. Always follow your unit's manual. Fuel-fired heaters must be installed and vented as the manufacturer specifies. Never run an unvented fuel heater in a sleeper. Carbon monoxide kills.

A dead cab fridge means truck stop food. A dead bunk heater on a January night is a safety problem. Here's what to check.

Part 1: 12-volt fridge or cooler

Most cab fridges are either compressor units (cool like a home fridge) or thermoelectric coolers (cheaper; only cool a set amount below ambient temperature).

Thermoelectric cooler "not cold enough"

That's often normal. Thermoelectric coolers only cool a limited amount below the surrounding air temperature, so on a hot day the inside can't get fridge-cold. Check your unit's rated temperature drop in the manual. If you need real refrigeration, a compressor fridge is the upgrade.

Compressor fridge shuts off or won't start

Low-voltage cutoff (battery protection). Most compressor fridges have a cutoff setting so they don't drain the starting batteries. If it's set high, the fridge shuts off as soon as the engine stops. Check the setting in the manual.
Voltage drop in the wiring. Cigarette-lighter plugs and thin wire can lose enough voltage under load that the fridge sees "low battery" even when the battery is fine. Measure voltage at the fridge while the compressor runs. A dedicated, properly fused circuit with heavier wire is the common fix.
Loose 12V plug. Lighter sockets wiggle loose on rough roads, so try a locking socket or hardwired connection.
Fuse in the plug tip or on the unit.
Ventilation. The compressor needs airflow. Don't bury it under bags in the bunk.
Error codes. Many units flash a code; your manual decodes it.

APU or inverter users

If you run a 120V fridge from an inverter, an inverter's own low-voltage shutoff can cut power. Check its display and battery health.

Part 2: Diesel bunk or parking heater

Diesel air heaters burn fuel from the truck's tank and vent exhaust outside.

Fuel level: Many installs draw from above the bottom of the tank, so the heater can starve at low fuel before the engine does.
Battery voltage: Low voltage at startup (the glow pin draws heavily) can cause failed starts. Check at the heater.
Error code on the controller: Most report a code. Look it up in the manual.
Intake and exhaust: Check for snow, mud or road debris blocking either one.
Gelled fuel in winter: Use winter-blend fuel or an additive per your fuel supplier's guidance.
Repeated failed starts can carbon up the glow pin and burner. That's service territory.
Altitude: Some heaters need an altitude setting or kit for mountain passes. Check the manual.

Carry one regardless of heater type.

Part 3: Electric blankets and heaters on inverters

Resistive heaters draw a lot of power. A small inverter or tired battery bank can run them only briefly. Check your inverter's rated continuous watts against the heater's label.

Tools to keep in the cab

Multimeter
Spare fuses (blade types matching your truck and devices)
12V socket tester
Zip ties and electrical tape

When to replace vs. repair

Thermoelectric cooler with a dead fan: Usually replace.
Compressor fridge with a power problem: Usually wiring, so fix the wiring before buying a new fridge.
Diesel heater with persistent faults: Have it serviced. Fuel and exhaust systems aren't a parking-lot repair for most drivers.

FAQ

Why does my truck fridge stop when I shut the engine off? Usually the low-voltage cutoff is set too high, or voltage drop in the wiring makes the fridge think the battery is low.

Can I run a 12V fridge all night off my truck batteries? Often yes, with the battery-protection setting right and healthy batteries. Check your fridge's power draw and your batteries' capacity.

Why won't my diesel parking heater start? Common causes are low fuel at the pickup, low voltage, blocked intake or exhaust, and gelled fuel in cold weather. Check the error code.

Is a 12V cooler the same as a fridge? No. Thermoelectric coolers only cool a set amount below ambient temperature. Compressor fridges actually refrigerate.

Is it safe to sleep with a bunk heater on? Only with a properly installed, outside-vented heater and a working CO detector. Follow the manufacturer's instructions.

General guidance; follow your manufacturer's manual. Last checked 2026-10-21.