2020 Jeep Gladiator Check Engine Light with No P-Codes: Diagnosing a Hidden Brake Light Fault
Introduction
Every now and then, you run into a vehicle that throws you a curveball—something that doesn’t quite add up at first glance. This case involved a 2020 Jeep Gladiator that came in with a check engine light on, but no obvious powertrain codes to explain it.
At first, it looked like a classic network issue with multiple U-codes. But after clearing everything and starting fresh, the real problem revealed itself—and it wasn’t where most people would expect.
Vehicle Concern
Vehicle: 2020 Jeep Gladiator
Complaint: Check engine light illuminated
Customer concern: Recently came on, no noticeable drivability issues
Initial Inspection & Symptoms
When I first scanned the vehicle, there were multiple U-codes across several modules, mostly related to lost communication with the ABS system.
That typically points you toward:
CAN bus issues
Voltage problems
Module failures
But here’s the key:
After clearing all codes and rescanning, only one code returned—and it wasn’t in the PCM.
Meanwhile, the check engine light (MIL) was still on, which didn’t seem to match what I was seeing.
Diagnostic Trouble Codes
After clearing and retesting, the only code that came back was:
B16B7-15 — Center Stop Lamp Control Circuit Short to Battery or Open (Active)
Module: Body Control Module (BCM)
No P-codes. No active ABS codes. Just a body code… with a check engine light still illuminated.
Diagnostic Process
This is where understanding how modern vehicle networks work really matters.
Step 1: Don’t Chase Old Codes
The initial flood of U-codes looked serious, but they were history codes—likely triggered by a temporary fault.
Clearing them and seeing what comes back is critical. Otherwise, you end up chasing ghosts.
Step 2: Focus on What’s Active
The only active code was:
B16B7-15 — Center Stop Lamp Circuit Fault
That tells me:
The BCM is actively detecting a problem
The issue is present right now—not intermittent
Step 3: Verify the Concern Physically
Instead of overthinking it, I went straight to the basics.
👉 Had someone press the brake pedal
👉 Checked the third brake light (center high mount stop lamp)
Result:
The third brake light was NOT working
That immediately confirms the fault is real—not a false code.
Step 4: Understand System Behavior
Here’s the part that throws people off:
On this platform, the BCM can request the check engine light from the PCM.
So even though there are:
No P-codes
No engine faults
The MIL can still be on due to a body-related safety issue.
That’s exactly what was happening here.
Step 5: Narrow Down the Circuit
On the Gladiator, the third brake light is mounted in the tailgate, which means the wiring runs through:
Tailgate harness
Hinge area (flex point)
Body-to-tailgate connector
This is a known weak point.
Step 6: Check Power and Ground
Next step was to verify the circuit:
Remove tailgate trim
Access CHMSL connector
Check for:
Power when brake is applied
Solid ground
This tells you whether the issue is:
The light itself
Or the wiring feeding it
Step 7: Inspect the Harness
This is where experience pays off.
The tailgate hinge harness is a very common failure point on these trucks.
What I look for:
Broken wires inside insulation
Pinched or stretched wiring
Previous repairs or aftermarket additions
In many cases, the wire is internally broken—you won’t see it until you flex the harness.
Key Findings
Third brake light was completely inoperative
BCM detected an open circuit condition
Check engine light was triggered via BCM request
Likely fault located in:
Tailgate harness
Or failed CHMSL assembly
Final Diagnosis / Recommendation
Root Cause:
Open circuit in the center stop lamp circuit (most likely in the tailgate hinge harness or lamp assembly)
Recommended Repair:
Inspect and repair wiring in tailgate hinge area
If wiring checks good, replace CHMSL (third brake light)
Clear codes and verify operation
Once the circuit is restored, the BCM will stop requesting the MIL, and the check engine light will turn off.
Common Causes of This Issue
This isn’t just a one-off problem. I’ve seen this pattern multiple times on Gladiators and similar platforms.
Here are the most common causes:
Broken wires in tailgate hinge harness
Failed third brake light assembly
Aftermarket LED brake light accessories
Trailer wiring splices
Corrosion in tailgate connectors
Poor previous repairs
Tools Used
Professional scan tool (bi-directional capability preferred)
Digital multimeter
Test light
Trim removal tools
Tips for DIYers or Buyers
If you’re tackling this yourself:
✔ Always verify the concern physically (don’t rely only on codes)
✔ Check simple things first—like whether the light actually works
✔ Inspect harnesses in moving areas (hinges, doors, tailgates)
✔ Be cautious with aftermarket wiring—it causes more issues than people realize
If you’re buying a Gladiator:
Check that all brake lights function properly
Look for signs of wiring repairs in the tailgate
Scan for BCM codes—not just engine codes
When to Refer to a Professional
If you don’t have:
A wiring diagram
Proper testing tools
Experience tracing circuits
It’s easy to misdiagnose this and replace parts unnecessarily.
Electrical issues require a methodical approach, not guesswork.
Conclusion
What looked like a complex network issue turned out to be a straightforward wiring fault in the brake light circuit.
The key was:
Clearing old codes
Focusing on what’s active
Verifying the problem physically
Understanding how modules communicate
Modern vehicles can blur the lines between systems, but if you stay grounded in fundamentals, the diagnosis becomes much clearer.
Call to Action
If you're buying a used vehicle, having a solid inspection strategy can save you thousands. Check out my Used Car Inspection Guide to help you make a smart purchase.
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