Running Lights Failure.
Thread Starter
Racer
Joined: Jan 2008
Posts: 464
Likes: 32
From: Azusa Ca
Running Lights Failure.
Got an issue with the rear section of my running lights: all of a sudden, they're not working when the headlights are on. The lights on the quarter panels don't work. The brake lights are not illuminated when running, but a funny thing, brake lights work when I depress the brake pedal, signal lights work when I turn the signal switch on. So I ran this scenario through Chat GPT, and this is what it came up with.
I was hoping someone knowledgeable with the C6 electrical system could tell me if this tracks?
From Chat GPT.
Since you haven't touched the wiring recently, this narrows the problem down to a component failure or a specific circuit breaker. Because the front lights work but the rear doesn't, the most likely culprit is the specific fuse that splits the rear circuit from the front.
On the 2005 C6, while there is one main relay under the hood, the power is actually split between two different fuses in the Passenger Footwell Fuse Box (located behind the carpet on the passenger side floorboard).
1. The "Smoking Gun": Fuse #14
The C6 lighting architecture separates the front and rear parking lamp loads to prevent a single short from blacking out the entire car.
• Locate the BCM Fuse Center: Pull back the carpet in the passenger footwell and remove the wooden/plastic toe board.
• Check Fuse #14 (10 Amp): This fuse is dedicated specifically to the Rear Parking Lamps.
• Note: Fuse #17 in that same box controls the Front Parking Lamps. This explains why your front lights are working while the back is dark.
• If Fuse #14 is blown: Replace it and see if the lights return. If it blows again immediately, you have a short in the brown wire leading to the rear.
2. Inspect Connector C209
If Fuse #14 is intact, the break is likely at a physical junction point. The rear lighting harness connects to the main body harness at a large connector designated C209.
• Location: This is a large white or gray connector located behind the passenger seat, usually accessible by removing the trim panel near the B-pillar or pulling up the carpet in that corner.
• What to look for: Look for a Brown wire. It is common for these pins to build up resistance and heat, eventually burning the plastic or losing tension. If that pin is scorched, power can't reach the rear of the car even if the fuse and relay are perfect.
3. The "Rear Fascia" Ground Junction
Even though your brake lights work (which means the bulbs have a ground path), the parking light circuit occasionally uses a different grounding pin in the same harness.
• On the 2005 model, there is a ground pack (G402) located on the inner frame rail near the rear passenger wheel.
• If you've been driving in wet conditions or live near the coast, this ground can corrode. However, because your turn signals and brakes share this housing, a bad ground usually makes the lights behave "erratically" (like blinking fast or dimming) rather than cutting them out completely.
Summary Recommendation:
Start with Fuse #14 in the passenger footwell. In 90% of C6 cases where the front works but the rear is dead, that 10-amp fuse has simply reached the end of its life or suffered a minor surge.
Here is the step-by-step diagnostic path to find exactly where the electricity is stopping.
4. The Fuse Box Test (Live Circuit)
Before pulling any fuses, we want to see if power is even reaching the BCM (Body Control Module).
1.Turn the headlight stalk to the "Parking Light" position.
2.Set your multimeter to DC Volts (20V range) or connect your test light to a known good ground (like a metal bracket under the dash).
3.Locate Fuse #14 in the passenger footwell. Look at the top of the fuse; there are two tiny metal test ports exposed.
4.Touch your probe to both test ports one at a time.
• Power on both sides: The fuse is good, and the BCM is sending power. The break is further back in the wiring harness.
• Power on one side only: The fuse is blown.
• No power on either side: The BCM is not "switching" the circuit on. This could mean a relay issue under the hood or an internal BCM fault.
5. Checking the "Rear Harness" Feed
If the fuse has power on both sides, we need to see if that power is making it out of the cabin and toward the rear of the car.
6.Locate Connector C209. As mentioned, this is the large 40-pin connector behind the passenger seat or under the edge of the carpet near the door sill.
7. Look for the Brown wire (Pin A or B usually, check for the thickest brown wire).
8.Back-probe that wire with your multimeter.
If you have 12V here: The problem is between the passenger seat and the rear bumper. Look for a pinched wire near the rear wheel well.
• If you have 0V here: The break is between the fuse box and this connector (rare, but possible if there is corrosion inside the BCM itself).
9. The "Last Stop" Ground Check
Even though the brake lights work, the C6 uses a dual-filament bulb (or an LED board with multiple circuits). It is possible for one ground path to be compromised while the other remains "okay" enough for high-draw signals.
1.Remove a tail light and find the Black wire.
2.Set your multimeter to Ohms (\bm{\Omega}).
3.Touch one probe to the black wire terminal and the other to a clean piece of the car's frame (the aluminum or steel chassis).
4.The reading should be very low (under 0.5 ohms). If it reads "O.L." or high resistance, the lights can't complete the circuit back to the battery.
A common C6 quirk: Check the license plate lights. On this specific car, the license plate lights are on the same circuit as the rear running lights. If your license plate lights are on, but the tail lights are off, then your fuse and BCM are fine—the issue is specifically in the rear fascia harness.
I was hoping someone knowledgeable with the C6 electrical system could tell me if this tracks?
From Chat GPT.
Since you haven't touched the wiring recently, this narrows the problem down to a component failure or a specific circuit breaker. Because the front lights work but the rear doesn't, the most likely culprit is the specific fuse that splits the rear circuit from the front.
On the 2005 C6, while there is one main relay under the hood, the power is actually split between two different fuses in the Passenger Footwell Fuse Box (located behind the carpet on the passenger side floorboard).
1. The "Smoking Gun": Fuse #14
The C6 lighting architecture separates the front and rear parking lamp loads to prevent a single short from blacking out the entire car.
• Locate the BCM Fuse Center: Pull back the carpet in the passenger footwell and remove the wooden/plastic toe board.
• Check Fuse #14 (10 Amp): This fuse is dedicated specifically to the Rear Parking Lamps.
• Note: Fuse #17 in that same box controls the Front Parking Lamps. This explains why your front lights are working while the back is dark.
• If Fuse #14 is blown: Replace it and see if the lights return. If it blows again immediately, you have a short in the brown wire leading to the rear.
2. Inspect Connector C209
If Fuse #14 is intact, the break is likely at a physical junction point. The rear lighting harness connects to the main body harness at a large connector designated C209.
• Location: This is a large white or gray connector located behind the passenger seat, usually accessible by removing the trim panel near the B-pillar or pulling up the carpet in that corner.
• What to look for: Look for a Brown wire. It is common for these pins to build up resistance and heat, eventually burning the plastic or losing tension. If that pin is scorched, power can't reach the rear of the car even if the fuse and relay are perfect.
3. The "Rear Fascia" Ground Junction
Even though your brake lights work (which means the bulbs have a ground path), the parking light circuit occasionally uses a different grounding pin in the same harness.
• On the 2005 model, there is a ground pack (G402) located on the inner frame rail near the rear passenger wheel.
• If you've been driving in wet conditions or live near the coast, this ground can corrode. However, because your turn signals and brakes share this housing, a bad ground usually makes the lights behave "erratically" (like blinking fast or dimming) rather than cutting them out completely.
Summary Recommendation:
Start with Fuse #14 in the passenger footwell. In 90% of C6 cases where the front works but the rear is dead, that 10-amp fuse has simply reached the end of its life or suffered a minor surge.
Here is the step-by-step diagnostic path to find exactly where the electricity is stopping.
4. The Fuse Box Test (Live Circuit)
Before pulling any fuses, we want to see if power is even reaching the BCM (Body Control Module).
1.Turn the headlight stalk to the "Parking Light" position.
2.Set your multimeter to DC Volts (20V range) or connect your test light to a known good ground (like a metal bracket under the dash).
3.Locate Fuse #14 in the passenger footwell. Look at the top of the fuse; there are two tiny metal test ports exposed.
4.Touch your probe to both test ports one at a time.
• Power on both sides: The fuse is good, and the BCM is sending power. The break is further back in the wiring harness.
• Power on one side only: The fuse is blown.
• No power on either side: The BCM is not "switching" the circuit on. This could mean a relay issue under the hood or an internal BCM fault.
5. Checking the "Rear Harness" Feed
If the fuse has power on both sides, we need to see if that power is making it out of the cabin and toward the rear of the car.
6.Locate Connector C209. As mentioned, this is the large 40-pin connector behind the passenger seat or under the edge of the carpet near the door sill.
7. Look for the Brown wire (Pin A or B usually, check for the thickest brown wire).
8.Back-probe that wire with your multimeter.
If you have 12V here: The problem is between the passenger seat and the rear bumper. Look for a pinched wire near the rear wheel well.
• If you have 0V here: The break is between the fuse box and this connector (rare, but possible if there is corrosion inside the BCM itself).
9. The "Last Stop" Ground Check
Even though the brake lights work, the C6 uses a dual-filament bulb (or an LED board with multiple circuits). It is possible for one ground path to be compromised while the other remains "okay" enough for high-draw signals.
1.Remove a tail light and find the Black wire.
2.Set your multimeter to Ohms (\bm{\Omega}).
3.Touch one probe to the black wire terminal and the other to a clean piece of the car's frame (the aluminum or steel chassis).
4.The reading should be very low (under 0.5 ohms). If it reads "O.L." or high resistance, the lights can't complete the circuit back to the battery.
A common C6 quirk: Check the license plate lights. On this specific car, the license plate lights are on the same circuit as the rear running lights. If your license plate lights are on, but the tail lights are off, then your fuse and BCM are fine—the issue is specifically in the rear fascia harness.
