Curt 4 Flat Wiring Tester 56040 Running Lights/Tail Lights LED Illuminates when Lights are Off?

Updated 06/05/2025 | Published 06/03/2025

Question:

Curt 56040 When I attach a 4 prong tester to the vehicle end, is it normal for the tail lights/running lights LED to illuminate when the vehicles lights are turned off?

asked by: Joan C

Expert Reply:

Hello Joan,

If you’re seeing the tail light or running light LED on your 4-way tester # I26 light up even when your 2025 Subaru Outback Wagon’s lights are turned off, it might seem strange, but don’t worry. While this behavior isn’t typical for older vehicles, it’s actually pretty common and harmless on newer ones like yours.

Modern vehicles (including your Outback) use sophisticated electrical systems such as CAN bus and pulse-width modulation (PWM). These systems are designed to monitor your vehicle’s lighting circuits and can send out small pulses of voltage, even when the lights are technically "off." These pulses are usually just your vehicle checking for things like a connected trailer or bulb continuity.

What’s happening here is that these tiny signals, although not strong enough to power real trailer lights, are enough to slightly illuminate the very low-draw LEDs inside your tester. That’s why you’re seeing a faint glow, it’s not a malfunction, it’s just the system doing its job.

Some vehicles, Subaru included, may also use what’s called “phantom voltage.” This is a small amount of standby power that can be present in the wiring to help the car monitor the circuit’s status. It’s not a short or a wiring issue, just another side effect of a modern electrical system keeping tabs on everything.

What are you towing with your Subaru?

expert reply by: Kate F

Joan

6/5/2025

I'm towing Harbor Freight flat trailer with kayak or other supplies for home projects.

Kate F.

6/5/2025

@Joan We have quite the group of kayakers here at etrailer, and even carry a lot of kayak transporting accessories too. Nothing better than a day out on the water! If you ever have any questions in the future Joan, you know where to find us =)