PTC Thermistor: Blower Resistor Symptoms, Testing & Replacement Guide

It is the middle of winter. You get into your car, turn the heater knob to speed “1,” and… nothing. You try speed “2” and “3”—still dead silence. Frustrated, you crank it all the way to “4” (High), and suddenly a blast of air hits your face.

Congratulations, you have just diagnosed a classic blower resistor failure.

As a PCB and electronics engineer, I often see this component treated as a simple “swap-and-forget” part. However, the blower motor resistor is actually a fascinating study in thermal management and load regulation. It is essentially a high-power heating element living inside a plastic air duct, constantly battling the laws of thermodynamics.

Whether you are dealing with a traditional wire-wound resistor, a ceramic-coated unit, or a modern PWM module, understanding the physics behind the failure will save you from replacing the same part twice. This guide will walk you through the symptoms, the multimeter testing procedures, and the correct replacement protocols.

What is a Blower Resistor?

The blower resistor (or blower motor resistor) is an electrical component used to control the speed of the fan motor in your vehicle’s Heating, Ventilation, and Air Conditioning (HVAC) system.

The Physics of Speed Control

In standard (manual) climate control systems, the blower motor is a simple DC brush motor. To change its speed, we must change the voltage supplied to it.

High Speed: The switch connects the motor directly to the 12V car battery. No resistance. Maximum current.

Lower Speeds: We cannot easily “lower” the battery voltage. Instead, we insert a resistor in series with the motor. The resistor absorbs some of the electrical energy, turning it into heat, and allows less voltage to reach the motor.

The Resistor Network:

Most blower resistors are actually a “pack” of 3 or 4 different resistors combined into one physical unit.

Speed 1: Current flows through three resistors in series (High Resistance $\rightarrow$ Low Speed).

Speed 2: Current flows through two resistors.

Speed 3: Current flows through one resistor.

Speed 4: Bypass the resistor entirely.

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The PTC Factor: Protection Mechanisms

You might see the term PTC (Positive Temperature Coefficient) associated with these devices.

While older resistors were simple coils of wire (nichrome), modern units often use thick-film technology or ceramic construction.

The Thermal Fuse: Every blower resistor has a safety device called a thermal cutoff. If the resistor gets too hot (because the airflow stopped), this fuse melts and cuts the power to prevent a fire. This is a one-time failure.

PTC Protection: Some advanced resistor designs and the control circuits protecting them utilize PTC thermistor properties. As the current spikes (due to a seized motor), the PTC heats up, its resistance skyrockets, and it chokes the current. Unlike a fuse, a PTC can theoretically reset when it cools down, offering a “self-healing” circuit protection, though in automotive applications, the thermal fuse is still the standard failsafe.

Symptoms of a Bad Blower Resistor

How do you know if the resistor is the culprit or if the motor itself is dead? The symptoms are usually quite specific.

1. Fan Only Works on “High”

This is the “smoking gun” of resistor failure.

The Engineering Logic: The “High” setting typically bypasses the resistor block entirely and sends direct 12V to the motor. If the fan works on High but dies on Low/Medium, the motor is fine, but the resistive path is broken (Open Circuit).

2. Fan Works on Some Speeds but Not Others

If Speed 1 and 2 are dead, but 3 and 4 work, a specific section of the resistor coil has burned out. Because the resistors are often wired in series, a break in the “Speed 1” coil will often kill Speed 1 but leave the others functional, depending on the internal topology.

3. Fan Stops Working Completely

While this usually points to a fuse or the motor itself, a blown thermal fuse on the resistor assembly can cut power to all lower speeds. If the High-speed relay is also routed through the logic of the resistor assembly (rare but possible), you might lose everything.

4. Burning Plastic Smell

The resistor converts electrical energy into heat. That is why it is mounted inside the air duct—to be cooled by the moving air. If the motor bearings are stiff, the motor draws excess current. The resistor heats up beyond its rating, melting its wire insulation or the nearby plastic housing.

Types of Blower Controllers

Before ordering parts, verify what technology your car uses.

Type	Technology	Application	Failure Mode
Wire Wound	Coiled resistance wire (like a toaster element).	Older cars, Basic Trucks.	Physical wire break or rust.
Ceramic / Thick Film	Resistive paste printed on a ceramic slab.	Modern Economy Cars.	Cracking or Thermal Fuse failure.
Blower Control Module	MOSFET / Transistor (PWM).	Automatic Climate Control.	Electronics short circuit (Fan stuck on).

Engineer’s Note: If your car has “Auto Climate Control” (where you set a specific temperature like 72°F), you likely do not have a traditional resistor. You have a Power Transistor Module (often called a “Final Stage Unit”). These fail differently—often causing the fan to run continuously even when the car is off.

Testing a Blower Resistor

You don’t need a mechanics degree to test this; you just need a multimeter.

Tools Needed:

Digital Multimeter (DMM)

Flashlight

Nut driver set (usually 5.5mm or 7mm)

Step 1: Visual Inspection

Remove the resistor (see the replacement section below). Look at the coils or the ceramic coating.

Signs of Failure: Broken coils, “popcorn” texture on the ceramic coating, melted plastic connector pins, or signs of corrosion (green fluff).

Check the Connector: Crucial Step. Often, the resistor is fine, but the plug that connects to it has melted due to high resistance contact. If the plug is black or melted, you must splice in a new “pigtail” connector.

Step 2: The Continuity Test

Set your multimeter to the Resistance (Ohms / $\Omega$) setting.

Locate the common pin (Ground or Input).

Touch the probes between the common pin and the other speed pins.

Reading: You should see low resistance values (typically 0.5$\Omega$ to 4.0$\Omega$).

Failure: If you see “OL” (Open Loop) or Infinite Resistance between any pins, the internal path is broken. The part is trash.

Step 3: Check the Motor (The Root Cause)

This is the step most DIYers miss. Resistors rarely die of old age; they are murdered.

They are murdered by a blower motor that is drawing too much current.

Spin the blower fan by hand. It should spin freely and coast for a second.

If it stops instantly or makes a grinding noise, the bearings are shot.

Result: If you replace the resistor but keep the bad motor, the new resistor will burn out within a week. Replace the motor first.

Replacement Guide: Step-by-Step

Replacing the blower resistor is usually a 20-minute job. It is almost always located near the blower motor fan, inside the HVAC ducting.

1. Locate the Component

Location: In 95% of cars, it is located under the passenger side dashboard, right behind the glovebox.

Why there? It needs to be inserted into the air stream of the fan to keep cool. Look for a wire harness plugging into a rectangular plate held by two screws.

2. Remove and Inspect

Disconnect the negative battery terminal (Safety practice).

Unplug the electrical harness. Press the locking tab and pull. Inspect the harness side for melting.

Remove the two or four screws securing the resistor.

Pull the resistor straight out. It might be hot if you just ran the car!

3. Install the New Unit

Compare the new part to the old one. The physical shape of the ceramic/coils might look different (updated design), but the bolt pattern and connector pins must match.

Slide the new resistor into the duct.

Secure the screws. Do not overtighten; you are screwing into plastic.

Reconnect the harness.

4. The Smoke Test

Reconnect the battery.

Turn the ignition to “On.”

Test all fan speeds (1, 2, 3, 4).

Note: It is normal for a new resistor to emit a very faint “hot” smell for the first minute as the factory coating cures, but smoke is bad.

Resistor vs. Control Module: Understanding the Difference

If your car has a digital display for the AC, you are likely dealing with a Blower Motor Control Module.

Unlike the resistor which uses passive resistance to waste energy, the Control Module uses a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) to pulse the power on and off thousands of times a second (PWM – Pulse Width Modulation).

Why it matters:

Resistor Failure: Fan works on High only.

Module Failure: Fan typically behaves erratically. It might pulse, stay on when the key is removed, or not run at all.

Replacement Cost: Resistors are cheap ($15-$40). Modules are expensive ($50-$200).

Do not buy a “Resistor” if your car requires a “Module.” Check your VIN number.

Useful Resources

For engineers and DIYers looking for part numbers and wiring diagrams:

RockAuto / DigiKey: Excellent databases for comparing aftermarket vs. OEM part numbers. Search for “Blower Motor Control Module” or “Resistor”.

The 12 Volt: A great forum and resource for finding wiring diagrams for specific vehicle years/models.

Standard Motor Products (SMP): Their technical library often contains PDF guides on diagnosing HVAC electrical faults.

Frequently Asked Questions (FAQ)

1. Why does my new blower resistor keep burning out?

You have treated the symptom, not the disease. The root cause is almost certainly a failing blower motor. As the motor bearings wear out, the motor requires more amperage to spin. This excess amperage flows through the resistor, overheating it and popping the thermal fuse. Replace the blower motor.

2. Can I just bypass the thermal fuse on the resistor?

Absolutely NOT. That fuse is the only thing preventing your car from catching fire. If the resistor overheats and there is no fuse to cut the power, the red-hot coils can ignite the plastic HVAC housing and the leaves/debris inside the duct.

3. Where is the blower resistor located?

It is almost always on the passenger side, under the dash, inserted into the black plastic heater box. You may need to remove a kick panel or the glove box to see it.

4. How hot does a blower resistor get?

During normal operation on low speed, the resistor coils can reach temperatures of 200°F to 300°F (90°C – 150°C). This is why it must be mounted in the airflow. Never run the fan with the resistor hanging out of the dashboard; it will overheat in seconds.

5. Is a “Blower Regulator” the same as a “Blower Resistor”?

Usually, “Regulator” refers to the solid-state Control Module found in automatic climate control systems, while “Resistor” refers to the passive coil type found in manual systems. However, parts stores often mix up the names. Look at the part: does it have big coils (Resistor) or a heatsink with fins (Regulator/Module)?

Conclusion

The blower resistor is a deceptively simple component that performs a critical job. It sacrifices itself to give you control over your comfort. While a failure is annoying, it is also a valuable warning light—a signal that your blower motor is likely nearing the end of its life.

For the engineer or the mechanic, diagnosing this part is a straightforward exercise in Ohm’s Law. If the circuit is open, the fan won’t spin. If the path is bypassed (High Speed), it will. By understanding the relationship between the motor’s current draw and the resistor’s thermal limits, you can ensure that your next repair lasts for the life of the vehicle.