How to Test a Fan Control Module Relay for an Always-on Condition

If your cooling fan runs nonstop even after you shut off the engine you're probably dealing with a fan control module relay that's stuck in the "on" position. This isn't just annoying. It can drain your battery overnight, wear out your fan motor prematurely, and point to a deeper electrical issue that won't fix itself. Knowing how to test the fan control module relay for always on condition saves you from replacing parts blindly and helps you pinpoint the real problem fast.

What does it mean when the fan control module relay is stuck on?

A relay is an electrically controlled switch. The fan control module uses it to turn the radiator fan on and off based on signals from the engine temperature sensor or the AC system. When the relay sticks in the closed position, it sends constant power to the fan motor. The fan never shuts off not when the engine is cold, not when the key is out of the ignition.

This is different from a fan that runs longer than expected after shutdown. Some vehicles have a normal after-run cycle that lasts a minute or two. A stuck relay means the fan runs indefinitely. If you've noticed the fan still spinning 10 or 15 minutes after turning the car off, or if your battery dies repeatedly, the relay is a strong suspect.

You can read more about why the radiator fan stays on after the car is turned off to rule out other causes first.

What tools do you need to test the fan control module relay?

You don't need expensive equipment. Here's what to gather before you start:

Multimeter for checking voltage, continuity, and resistance
Test light a quick visual way to confirm power at the relay socket
Vehicle-specific wiring diagram critical for identifying the correct relay pin layout
Replacement relay (optional) for a swap test, which is the fastest field diagnosis
Basic hand tools pliers, a small socket set, and a flashlight

A wiring diagram matters here. Different vehicles route the fan relay circuit differently. Toyota, Honda, Ford, and GM systems all have their own layouts. You can find diagrams in a Alldata subscription or through the vehicle manufacturer's service portal.

Where is the fan control module relay located?

The relay typically sits in one of two places:

Under-hood fuse/relay box most common location. Look for a black plastic box near the battery or along the fender. The lid usually has a diagram showing which relay controls the fan.
Near the radiator or fan shroud some vehicles, especially certain GM and Chrysler models, mount the fan control module directly on or near the radiator fan assembly.

Check your owner's manual or a repair database for the exact location. If the fuse box lid diagram is faded or missing, search for your vehicle's year, make, and model along with "fan relay location."

How do you test the fan control module relay for an always-on condition?

Step 1: Confirm the fan won't shut off

Start the engine, let it reach normal operating temperature, then turn the car off. Remove the key from the ignition. Wait two to three minutes. If the fan is still running, move to the next step.

Step 2: Pull the relay

With the engine off and the key removed, locate the fan relay and pull it straight out of the socket. Use pliers if it's tight, but grip the relay body not the wiring.

What happens next tells you a lot:

Fan stops when you pull the relay The relay is likely stuck closed. The circuit through the relay was providing constant power. This confirms the relay is the problem.
Fan keeps running even with the relay removed The issue is elsewhere. You may have a shorted wire, a stuck fan module, or a wiring bypass from a previous repair. This changes your entire diagnostic path.

Step 3: Test the relay out of the socket

With the relay removed, you can test it with a multimeter:

Identify the pins. Most fan relays have four or five pins. The wiring diagram labels them as 30 (battery power), 85 and 86 (coil), and 87 (output to fan). Pin 87a is normally closed (optional on some relays).
Check for stuck contacts. Set your multimeter to continuity (ohms). Measure between pin 30 and pin 87 without applying power to the coil pins (85 and 86). If you get continuity near 0 ohms the relay contacts are welded or stuck shut. A good relay should show no continuity (open circuit) between 30 and 87 at rest.
Test the coil. Measure resistance between pins 85 and 86. A healthy relay coil typically reads between 50 and 100 ohms. A reading of infinite resistance means the coil is open. A reading near zero means the coil is shorted. Both are failures, but a shorted coil can cause the relay to energize without a proper signal.
Energize the relay manually. Apply 12V to pins 85 and 86 (positive to 85, ground to 86 or reverse, depending on the relay's internal diode). While energized, check continuity between 30 and 87. It should switch on. Remove power, and it should switch off. If it doesn't release, the contacts are mechanically stuck.

Step 4: Check the relay socket for unwanted voltage

Even if the relay tests bad, check the socket too. With the relay out, use a test light or multimeter on the socket terminals. Pin 30 should have battery voltage (constant power). Pin 85 or 86 should only have voltage when the engine computer or temperature switch commands the fan on. If you see voltage at the coil pin with the engine cold and the ignition off, the control side of the circuit has a fault possibly a shorted temperature switch or a wiring issue.

What are the most common mistakes when testing a fan relay?

Skipping the pull test. Many people go straight to multimeter testing without pulling the relay first. The pull test is the fastest way to isolate the problem.
Ignoring the socket. A bad relay and a bad socket can produce the same symptom. Always check both.
Swapping relays without matching specs. Not all cube relays are the same. Amperage ratings and pin configurations vary. Using the wrong relay can create new problems.
Assuming the relay is the only cause. A stuck-on fan can also be caused by a faulty fan control module that makes the AC fan run constantly, a shorted temperature sensor, or corroded wiring.
Not checking for aftermarket wiring. Previous owners or repair shops sometimes bypass the relay with a jumper wire to keep the fan running as a temporary (and lazy) fix. If you pull the relay and the fan still runs, look for this.

What should you do after confirming a stuck relay?

If testing confirms the relay is stuck closed, replace it with an OEM-spec unit. Relays are inexpensive usually $10 to $30 and widely available at auto parts stores.

After replacing the relay:

Start the engine and let it idle up to temperature.
Confirm the fan turns on at the expected temperature.
Turn the engine off and verify the fan shuts down within a normal cycle (usually under two minutes).
Check the next morning that the battery hasn't drained.

If the new relay sticks again quickly, investigate the control circuit. A wiring short or a bad engine coolant temperature sensor can keep sending the relay a constant "on" signal, which will burn through new relays over time.

For a deeper look at the full diagnostic process, see our related article on how to test the fan control module relay for always on condition with additional model-specific notes.

Quick checklist before you start

✅ Confirm the fan runs continuously not just a normal after-run cycle
✅ Pull the relay and observe whether the fan stops
✅ Test the relay contacts with a multimeter for stuck-on continuity
✅ Test the coil resistance (expect 50–100 ohms)
✅ Check the relay socket for unexpected voltage on the coil pins
✅ Inspect for aftermarket jumpers or bypassed wiring
✅ Replace with an OEM-spec relay and verify the fan cycles normally

Next step: If the relay tests good but the fan still stays on, shift your focus to the fan control module itself and the engine temperature sensor circuit. The relay is only one part of the system the signal feeding it matters just as much as the relay itself.

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