From the Driver's Seat—Featured Story

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Some cases drag on not because the fault is tricky—but because the people around it are.

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This one stretched on for weeks. Every time I’d circle back to the shop, the Service Manager had a fresh complaint ready for me.

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At one point, he even called my boss on the phone, complaining to him about how “This guy still hasn’t fixed it yet.” Which is ironic, given that it was his shop asking for my help because they couldn't figure it out on their own.

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Other times, he’d scream at me directly, demanding answers.

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The pressure was real. So was the toxicity.

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The customer's concern was that some early mornings [while still dark out] the interior panel lights would turn on and off, and the auto headlamps would also turn on and off. The cluster and radio would stay illuminated but would change in brightness—like a moody teenager.

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It took a couple of visits before I was finally able to verify the concern myself because it was so elusive.

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You can see in the video snippets above that the dash was torn apart—the shop had already spent countless hours chasing what they guessed was a pinched dash harness. Not to mention, they had thrown enough parts at it to fill a warranty return crate.

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The technician had previously noticed the ambient light sensor data parameter (a voltage value) was changing when the fault was present previously. This odd-natured issue got me thinking of a ground issue.

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Since this ambient light sensor had already been replaced along with the BCM, I decided to follow the wiring schematic to the ambient light sensor's ground source. On this vehicle, the sensor was grounded through the HVAC module.

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Interesting, I thought. Time for a loaded voltage drop test.

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I loaded the sensor's circuits and measured the voltage drop—and I found...

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Bupkis. No issues there.

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Next, I confirmed the ambient light sensor voltage reading would change on the scan tool when blocking light to the sensor vs. when shining a light directly on the sensor.

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All good here. I didn't have anything to go on at this point, so I scheduled a return trip.

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On my next visit, I was able to recreate the fault and confirm the Ambient Light sensor voltage reading would fluctuate between 1.8 and 4.0 volts when the fault was present.

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In between visits, I had also consulted with my peers. Luckily, a very kind individual heard my plea for help and reached out.

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"Hey Ted, I saw something similar a couple years back. Check for aftermarket (LED) headlamp bulbs."

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Huhh?? I thought for a moment. This didn't seem at all related to the ambient light sensor, but I was out of ideas.

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So, I opened the hood and popped off the back cover of the left headlamp assembly.

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NO WAY! I was floored. Sure enough, there were aftermarket LED bulbs installed.

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Before removing them, I again verified the concern to ensure the fault was then present. I confirmed with the scan tool that these aftermarket bulbs were affecting the BCM in such a way that the ambient light sensor voltage reading was fluctuating.

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This shop had new OEM bulbs in stock, so I installed them and verified the fault was no longer present.

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With the true cause finally surfaced, it wasn’t the manufacturer at fault. It wasn’t the BCM, nor the grounds, nor any overlooked schematics. It was customer-installed aftermarket LED headlamps—throwing off the ambient sensor voltage, tricking the BCM into switching between day and night modes.

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In the end, I explained to the Service Manager that the root cause wasn’t a manufacturer defect at all—it was due to aftermarket headlamp bulbs. Once those were replaced with OEM parts, the issue was resolved. While it had been a challenging case, I was glad to provide clarity and closure for both the shop and the customer.

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Here's what I learned from this case:

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1. Aftermarket parts change the game. LEDs don’t always play nice with OEM circuitry. What looks “better and brighter” to the customer can wreck signal integrity for sensors that rely on precise readings.
2. Don’t stop at the symptom. Fluctuating sensor voltage isn’t always the sensor. Sometimes it’s what’s around the sensor, or even seemingly unrelated.
3. Stay Professional Under Pressure. Even when emotions run high, staying calm and patient goes further than reacting. Let the facts speak louder than frustration.
4. Your community is immensely valuable. You never know who's seen what over the years.

That's why we've created a free online Skool community where fellow troubleshooters, thinkers, and diagnostic detectives can swap stories, wisdom, and wiring diagrams. 'Torque Authority' isn't just a newsletter, it's the start of a movement. Come be a founding member by joining the party here:

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Join Torque Authority Hub: Where Automotive Technicians Transform into Diagnostic Daredevils. Embrace your worth, master your craft, and drive towards financial autonomy with a community that gets it.

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I hope you enjoyed the story! Please share this article with a friend if you did.

In this section each week we highlight a person—an innovator, subscriber, engineer, or even a business—who has made a positive impact in our industry. This is the place in Torque Authority where their authority in the Automotive Industry is highlighted.

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This week's highlight is on Professor John D. Kelly, Professor in the Automotive Department at Weber State University:

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John D. Kelly, has been shaping the minds of diagnostic wizards and hybrid-electric tech specialists for over 34 years at Weber State University

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Some of his contributions:

• Hands‑on hybrid/EV training pioneer — Kelly has taught dozens of five-day Hybrid & Electric Vehicle Bootcamps, equipping dealership techs and college instructors with EV diagnostic skills that matter on the shop floor
• DIY‑style video masterclass — For decades he has been posting detailed automotive repair videos on YouTube. Today, WeberAuto has amassed millions of views and a global audience tuning in for his clear, high‑definition breakdowns of everything from drivetrain systems to EV high‑voltage components
• Curriculum catalyst — Kelly’s courses don’t just teach—they build workstations, overhaul foundational electrical classes, and integrate real tools.

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John Kelly is an amplifier of automotive know-how. His boot camps empower trainers who spread that knowledge across shops nationwide. And those YouTube videos? They’re extremely precise and helpful, with a touch of professor-level clarity that makes even the most complex system feel approachable.

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So, here’s to Professor Kelly—for teaching with passion, producing content with care, and building a legacy that has diagnostics techs thinking sharper and diagnosing smarter—worldwide.

One quote of wisdom for you to consider this week...

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This is where we share one practical resource: a tool, app, book, strategy, or event which can have a positive impact in YOUR line of work.

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This week's Toolbox Tip is a simple way of looking at A/C refrigerant flow. Do you struggle with trying to remember the state of refrigerant as it passes through the system? When is it a liquid or a gas? When is it high-pressure or low-pressure? What about temperature???

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The simplest method is by drawing two perpendicular lines on a piece of paper to make a cross or 'plus' symbol. At the top of the vertical line draw a compressor. At the bottom of the vertical line draw a TXV or Orifice Tube (restriction device). At the far left of the horizontal line draw a condenser, and at the far right of the horizontal line draw an evaporator.

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The horizontal line represents the state of the refrigerant—the refrigerant above this line is in a gas state, refrigerant below is a liquid. The vertical line represents the pressure of the refrigerant—the refrigerant to the left of this line is high pressure, the refrigerant to the right is low pressure.

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This is an easy way to remember the state of refrigerant as it passes through the A/C system. The compressor takes low-pressure gas and compresses it into a high-pressure gas. As it goes through the condenser, the refrigerant loses heat and condenses into a high-pressure liquid. The restriction device (TXV or Orifice Tube) causes the pressure of the refrigerant to drop so that it enters the evaporator as a low-pressure liquid. As it passes through the evaporator, the refrigerant absorbs heat from the cabin and evaporates from a liquid to a gas. This low-pressure gas then flows back into the compressor, and the cycle starts all over again.

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I hope you found this helpful! Let us know if it resonated with you.

Ready to unlock the potential within you and become a more confident automotive diagnostician? Check out Hotwire Your Skills: The 10-Step Playbook to Six-Figure Success for Automotive Technicians

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