Why Both Headlights Stopped Working on a 2020 Jeep Grand Cherokee (And the Simple Reason Behind It)
Introduction
Here’s a scenario that’ll test even experienced technicians: both headlights suddenly stop working, but everything else—fog lights, marker lights, halo rings—still operates normally. That’s exactly what showed up on a 2020 Jeep Grand Cherokee that came through the shop.
At first glance, this kind of failure raises red flags. You start thinking about control modules, wiring faults, or even a failed body control module (BCM). But as with many real-world cases, the symptoms can lead you down the wrong path if you don’t stay grounded in your diagnostic process.
This is a perfect example of why proper automotive troubleshooting matters—and how skipping simple checks can cost time and money.
🔧 Recommended Diagnostic Tools
When you're diagnosing modern vehicles, having the right tools makes all the difference. A basic code reader will only get you so far—you need tools that let you see what’s really going on and test components properly.
Autel MaxiSys MS906 Pro TS Scanner (TPMS + full system diagnostics)
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This is a solid professional-level scan tool that allows full module access, live data, and bi-directional controls—critical for diagnosing electrical issues like this.
Power Probe 3 Circuit Tester
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Power Probe ECT3000 Short Finder
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Vehicle Overview
The vehicle in question was a 2020 Jeep Grand Cherokee, a platform that uses advanced lighting control through the BCM. These systems don’t rely on traditional relays like older vehicles—they use solid-state drivers and electronic controls.
Vehicle Concern
The complaint was straightforward:
Headlights inoperative (both high and low beams)
Halo rings working
Fog lights working
Dash indicator showed headlights were ON
Right away, this tells you something important: the system is partially functioning. That’s a key detail when diagnosing car problems like this.
Initial Inspection
The first step was a full system scan. On modern vehicles, you always want to see what the modules are reporting before tearing anything apart.
The scan revealed multiple codes—but none related to headlamp circuits. Most were communication-related or tied to unrelated systems like blind spot monitoring.
This is where many techs get sidetracked. Just because codes are present doesn’t mean they’re relevant to the problem.
Codes
No active or stored diagnostic trouble codes pointed to:
Headlamp circuit faults
BCM output issues
Short or open conditions
That absence of codes is actually a clue in itself.
Diagnostic Process
Now the real automotive troubleshooting begins.
First, I verified that the BCM was receiving the correct inputs. The dash indicated the lights were on, and through the scan tool, I could see the BCM responding to commands. That means the switch and input side of the system were working.
Next step: actuator tests.
Using the scan tool, I commanded:
Left low beam
Right low beam
Left high beam
Right high beam
The BCM responded and showed the commands were being executed—but the lights did not come on.
This is where you separate command from output.
At this point, we know:
The BCM is receiving the command
The BCM is attempting to execute the command
The lights are still not functioning
So now we move to the circuit side.
Power Supply Check
Pulled up the wiring diagram and located Fuse 40 in the underhood power distribution center. This fuse supplies power to the headlamp circuits when the run/start relay is active.
Checked the fuse:
Power present
Circuit active with engine running
So the system has power at the source.
But that doesn’t mean it’s reaching the load.
Load-Side Testing
Next step was to go directly to the headlight connector.
This is where a lot of technicians waste time—they stay in the diagram instead of going to the component.
At the connector, I verified:
12 volts present
Good ground
Proper conditions with lights commanded ON
At this point, everything electrically checked out.
So now you’re left with one logical conclusion:
Either the headlamp assembly has failed… or something inside it is missing.
The Turning Point
At this stage, instead of continuing to chase wiring or suspect modules, I made a simple decision:
Pull the bulb and check it.
And that’s where everything changed.
There was no bulb.
Not burned out. Not damaged. Just completely missing.
Key Findings
Power and ground present at headlamp connector
BCM functioning correctly
No related fault codes
Headlight bulb physically missing
This wasn’t an electrical failure at all—it was a parts issue.
Final Diagnosis
The root cause of the inoperative headlights was missing headlight bulbs.
No wiring faults.
No module failures.
No programming issues.
Just missing components.
Common Causes
Believe it or not, this isn’t as rare as you’d think—especially in used car environments.
Some common causes include:
Parts removed before sale
Previous repairs not completed
Theft of components
Vehicles being transferred between departments without verification
This is why used car inspection tips are so important.
Lessons for Buyers / DIYers
This case is a perfect reminder of a few critical principles:
First, always verify the basics. Before diving deep into wiring diagrams and module diagnostics, make sure the component you’re testing actually exists.
Second, don’t let modern systems intimidate you. Yes, vehicles today are complex, but the fundamentals still apply: power, ground, and load.
Third, don’t rely solely on scan data. Just because there are no codes doesn’t mean there isn’t a problem—and just because there are codes doesn’t mean they matter.
Finally, in the world of diagnosing car problems, never assume anything. Every step should be verified.
Don’t Get Burned on Your Next Used Car
If you’re thinking about buying a used car—or even if you already have one—this is exactly the kind of problem that can slip through the cracks and cost you big money later.
After 35+ years in the automotive field, I’ve seen firsthand how often people end up with vehicles that have hidden issues just like this.
That’s why I put together a simple Used Car Guide to help you understand what to look for and how to avoid making a costly mistake.
It’s a quick, practical guide designed to give you the right mindset and approach before you buy.
👉 https://stan.store/BobbyWhiteside
Affiliate Disclaimer
Disclaimer: Some links in this post may be affiliate links. I may earn a small commission at no extra cost to you. I only recommend tools I personally use and trust in real-world diagnostics.
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