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✅ Try this yourself

1. Take the front cover off. Find the A/C pressure switch — it's under the fuse box, on the silver canister (the A/C drier). It has two red connectors.
2. Push both red connectors on firmly. If they look dirty or corroded, clean them. A loose connector here is the #1 cause of this message.
3. Put the cover back on and try it again.

✅ Try this yourself

1. 🔦 Shine a flashlight through the condenser from behind. Can you see light through it? If not — it's clogged, and that's your problem.
2. Clean it. Coil cleaner and a regular garden hose. Never a pressure washer — it bends the fins and you'll ruin it permanently.
3. With the A/C running, check the condenser fan is spinning. It should run non-stop in A/C mode.
4. Fan not spinning? Check the 25A condenser fan fuse and swap it for the same rating.

✅ Try this yourself

1. Check the 10A compressor fuse. Replace with the same rating if blown.
2. Make sure your set temperature is well below the cab temperature — if it doesn't think you want cooling, it won't engage.

✅ Try this yourself

1. Check the cabin air filter — it's in front of the HVAC box inside the cab. If it's packed with dust, clean or replace it. Cheap, common, and fixes this more often than you'd think.
2. Check the condenser is clean (shine a light through it) and the fan is spinning.

✅ Try this yourself

1. Give it 5–10 minutes. The blower comes on by itself once it's warm.
2. Want to watch it? Diagnostics → Basic → General and watch WTMP climb. At 130°F the blower kicks in.
3. If WTMP never reaches 130 — or it looks nothing like your truck's temp gauge — come back and call us.

✅ Try this yourself

1. Turn your set temperature up — it needs to be a couple degrees above the current cab temp before it will call for heat.
2. Check Diagnostics → Basic → General and look at ATMP (cab temp). Does it match how it actually feels in there?
3. If ATMP is way off, make sure the controller isn't sitting in the airflow from a vent — air blowing on the sensor gives a false reading.

✅ Try this yourself

1. Turn the APU on, set the temp all the way up, Heat Only mode.
2. Take the cap off your truck's coolant reservoir, then start the truck.
3. Run both the truck and the APU for 5+ minutes.
4. Shut the truck off. Let the APU keep running 20 minutes.
5. Shut the APU off, top up the coolant, put the cap back on. That fixes it most of the time.
6. Also check your coolant hoses for kinks — a pinched hose kills heat.

✅ Try this yourself

1. Check the 150A main fuse on your truck batteries — the big one.
2. Check the 2A controller fuse — inside the APU's fuse box.
3. Check the 175A ground fuse — bottom of the APU frame.
4. Look at your battery cables. They must connect across all four batteries — never both positive and negative on one battery. If anyone worked on the truck recently, suspect this first.
5. Make sure the harness plug at the top-right of the APU is fully seated. It gets bumped loose.
6. Screen faint but readable at an angle? That's just contrast — Diagnostics → Advanced → Adjust Contrast, set it to 08.

✅ Try this yourself

1. 🔴 Is your truck tank above 1/4? The APU's fuel pickup does not reach the bottom of the tank. Below a quarter tank it starves — even though your truck runs fine. This is the #1 cause of this call.
2. Fuel up above 1/4 and try again.
3. Change the fuel filter if it's old — pre-fill the new one with diesel and it'll start easier.
4. Check the air filter while you're in there.

✅ Try this yourself

1. Go to Diagnostics → Last Shutdown. It shows the last fault and a voltage.
2. If it says Run Failure → hit "Start over" below and pick "It shuts down on its own."
3. If it says Overheat → check your coolant level first.

✅ Try this yourself

1. Check your batteries. A healthy one rests at 12.5–12.8V. Old or weak batteries cause exactly this.
2. Clean the battery terminals. Corrosion is a very common cause.
3. Make sure the cables land across all four batteries, tight.
4. Check the air filter and exhaust aren't blocked.

✅ Try this yourself

1. Check the 30A starter fuse.
2. Check the starter cable connections — corroded or loose is common. Clean them up.
3. Make sure the exhaust and air filter aren't blocked.

✅ Try this yourself

1. If the truck was running: you're fine. Nothing to fix.
2. If the truck was OFF and it still shut down — then a sensing wire is likely damaged or on the wrong terminal. Give us a call.

✅ Try this yourself

1. 🔴 Check your oil level. Pull the dipstick. If it's low, top it up — HD synthetic diesel, 10w30 or 15w40. This is the most common cause, full stop.
2. Check the 25A fuel solenoid fuse while you're there.
3. Top up the oil and try it again.

✅ Try this yourself

1. 🔴 Is the truck tank above 1/4? The APU pickup doesn't reach the bottom of the tank. Most common cause.
2. Change the fuel filter — pre-fill the new one with diesel.
3. Change the air filter.

✅ Try this yourself

1. 🔴 Let the truck cool before you switch over. Idle it 5–10 minutes before shutdown — otherwise you dump the truck engine's heat straight into the APU. This alone fixes a lot of these.
2. Check your coolant level.
3. Purge the air out: APU on, temp all the way up, Heat Only. Cap off the truck reservoir, run the truck 5 min, shut it off, let the APU run 20–30 min. Top up coolant, cap it.
4. Check the coolant hoses for kinks or rubbing.
5. Check your truck's radiator cap.

✅ Try this yourself

1. 🔴 Battery Monitoring has to be turned on manually, every single time you want it. Select it from the menu and press the knob. It does not stay on. This is the #1 reason for this call.
2. It only kicks in below 12.6V. If your batteries are above that, it's supposed to sit there doing nothing.
3. Engine preheat only starts below 40°F coolant.
4. Check your battery cables are tight and across all four batteries.

✅ Try this yourself

1. Test each battery. A healthy one rests at 12.5–12.8V. Old or weak ones need replacing — usually this is the answer.
2. Clean the terminals and make sure the cables land across all four batteries, not just one.
3. Think about anything on the truck that could be draining them overnight.

✅ Try this yourself

1. There's a 2-hour run limit — if it hasn't hit that yet, some running is expected.
2. Test your batteries. If they won't accept a charge anymore, it will never reach the set point and will just keep running.

✅ Try this yourself

1. Check your oil level first — low oil trips the oil pressure circuit.
2. In cold weather, thick oil can trip it too. Let it warm up and try again.
3. Check your coolant level.

✅ Try this yourself

1. Check the 25A blower fuse. Replace with the same rating. This is usually it.
2. Test the blower yourself: Diagnostics → Basic → Blower Test — turn the knob up (0 to 255) until you hear and feel air. Nothing at any setting? The motor or relay is bad.
3. Check the wiring connectors for corrosion.