Ford F-150/F-250: Why Do My Vents Blow Only Hot or Cold Air?

When you're too hot or too cold, you expect your vents blow cold on the blue and hot on the red. Here is why your Ford F-150 or Super Duty's vent is only blowing hot or cold air.

By Thom Cannell - September 15, 2014

This article applies to the Ford F-150 (2004-2014) and F-250 (2005-2014).

Whether you are in the midst of a 110° heat wave or the arctic chill to end all arctic chills, you expect your HVAC vents to blast you with the desired dose of cold or hot air. But sometimes your vents betray you, often at the worst possible times. Have you done something to deserve this abuse? Probably not. But it's a problem you can troubleshoot and fix yourself, saving you time and precious dollars.

To understand and possibly fix the problem, a decision tree helps guide your thoughts before smashing knuckles against reluctant fasteners. First you need to know what and where the problem originates, then possible repair options, followed by DIY instructions. We’ll give advice, hints, and links to repairs to guide your money saving and self-satisfaction enhancing efforts.

Step 1 - Do you need a back flush?

Figure 1. Check the heater core.

There are three systems governing comfort: the radiator and heater core, A/C system and evaporator, and the control system. Any one of these systems could be the culprit. To make matters more complex, modern trucks run heat and A/C systems at the same time (unless turned off manually), blending hot and cold air

Does hot coolant make its way to the heater core? The heater core is buried deep, but you can feel the entry and exit hoses. Hot water in, hot water out, then everything is fine. If not, your heater core may need back flushing.

(Related Discussion: Before replacing heater core -

Step 2 - Does the A/C work?

Figure 2. Check the coolant temperature sensor.

Again, is there coolant flowing in and slightly warmer coolant flowing out? Check the coolant temperature sensor. Most auto parts stores sell coolant replacement with built-in test meters.

Figure 3. Coolant replacement.

Step 3- Does the blend door work?

Figure 4. Check the blend door.

The blend door mixes the hot air with the cold air. If your blend door is broken you could only get cold or hot air.

If the HVAC controls change temperature, wherever the air is discharging, the blend door likely works.

Step 4 - Check for vacuum leaks

Figure 5. Check for vacuum leak.

Last, much of the HVAC system is vacuum controlled and the ETAC (electronic automatic temperature control) controller can leak, as can vacuum hoses.

Pro Tip

Checking for vacuum leaks is very difficult. It is best to take your truck into a repair shop.

Featured Video - EATC Rebuilt

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