⚠️ IMPORTANT KIT VARIANT SELECTION:
Before purchasing or installing the L5P CAN BUS Plug Kit, please note that the required harness color variant depends entirely on your truck's Plug 2 connector color. Click here to jump to the Plug 2 location photo to inspect your factory connector before placing an order.
For certain aftermarket 3-plug and 4-plug harnesses used on 2017-2019 Chevrolet Silverado HD and GMC Sierra HD trucks with the 6.6L Duramax L5P, this guide labels four connection areas: one under the hood, two around the front passenger-side frame, and one inside the frame behind the passenger-side rear wheel. Plug 1 through Plug 4 are photo-navigation labels for this guide, not GM connector IDs and not a count of every CAN connection on the truck.
Key Takeaways
The correct install depends on matching the truck, harness instructions, connector shape, and photo location before any plug is disconnected.
- Use these photos only for 2017, 2018, and 2019 L5P trucks unless the harness manufacturer documents additional fitment.
- Plug 2 is shown inside the front passenger-side frame; Plug 3 is outside that frame, so they are easy to confuse.
- A 3-plug harness may intentionally use fewer locations than a 4-plug harness; never choose the omitted connection by guesswork.
- Power the truck down as directed, support it with rated equipment, and release each CPA or secondary lock before separating a connector.
- After installation, scan every module and inspect terminal seating before blaming the tuning device or replacing a control module.
2017-2019 L5P Harness Location Photos
Use the five original photos in order: the first two establish the underhood reference, followed by the front inner frame, front outer frame, and rear inner frame locations.
Important Variant Selection Note: Before ordering an aftermarket CAN Bus plug kit, inspect the connector shell color at the Plug 2 location. The color of your factory Plug 2 connector determines which harness variant your truck requires for proper fitment.
Plug 1-4 Location Reference
The fastest search order is underhood first, front passenger-side inner frame second, front outer frame third, and rear passenger-side inner frame last.
| Guide Label | Photo Location | Orientation Check | Common Install Error | Shop Verification |
|---|---|---|---|---|
| Plug 1 | Upper area of the engine bay as pictured | Match the photo background and connector shell | Using visual left or right without accounting for camera angle | Compare both overview and close-up before releasing the latch |
| Plug 2 | Inside the front passenger-side frame | Aim the work light into the rail | Searching only along the outer face of the frame | Confirm the large shell and the harness label |
| Plug 3 | Outside the passenger-side frame | Move from the inner rail to its outer face | Confusing it with Plug 2 | Verify inside-versus-outside location before seating |
| Plug 4 | Inside the frame behind the passenger-side rear wheel | Look behind the tire line with a mirror or phone camera | Missing a connector covered by mud, brine, or undercoating | Clean the shell, then compare latch and cavity layout |
What Plug 1-4 Actually Mean
Plug 1-4 are working labels tied to these photos, so the correct connector must be confirmed by the exact harness instructions rather than by the number alone.
GM uses formal connector identifiers, circuit numbers, and service views that can vary by harness and option content. An aftermarket manufacturer may use its own Plug 1, A/B, or color-label system. That is why a connector that physically reaches the harness is not automatically the right connection.
Before opening anything, lay the harness out on a clean bench and compare every branch length, keyway, latch, terminal count, and label. We have found during real parts test fitting that branch length is a useful warning: if a lead must be stretched tight or folded into a hard loop to reach, stop and recheck the route.
Fitment, Power-Down, And Lift Safety
A correct 2017-2019 L5P install starts with VIN-level fitment, a complete module scan, controlled power-down, and rated support under the truck.
- Confirm a 2017-2019 Chevrolet Silverado HD or GMC Sierra HD with the 6.6L L5P, then verify the exact harness part number and its instructions.
- Run a full vehicle scan before the mod. Save the module list and all current, history, and communication DTCs so you have a clean baseline.
- Set the parking brake, chock the wheels, work on level ground, and use a rated lift or jack stands. Never work under a truck supported only by a floor jack.
- Turn the ignition off and follow the harness or GM service power-down procedure. These trucks commonly use dual batteries; if battery disconnection is required, isolate both negative cables as directed and keep them from springing back to the posts.
- Keep keys and the remote away from the truck while connectors are open so an accidental wake-up does not energize modules mid-install.
Do not disconnect a module or network connector while a flash is running. For programming work, use the battery support method specified by the tool provider; a low-voltage event can interrupt communication and create a much bigger job than the harness install.
3-Plug Vs 4-Plug L5P Harnesses
A 3-plug kit can be correct even though this guide shows four reference locations, because aftermarket harness architecture and required connection points are not universal.
Do not decide which location to skip based on convenience. Match the printed labels, connector keys, branch lengths, and current manufacturer instructions. If the documentation does not match the truck, stop and contact the kit supplier before applying power.
Owners comparing flash tools, unlock methods, and external harnesses should first understand what changes during the L5P unlock and setup process. That context matters because an OBD workflow, an unlock service, and an external bypass harness do not use the same physical steps.
How To Release And Inspect The Connectors
Release the CPA or secondary lock first, hold the connector housing, and separate the shell without pulling on the wires.
If the connector will not move with normal hand pressure, clean the latch area and study its lock path. Do not drive a screwdriver into the mating seam or use the wire bundle as a handle. A cracked latch may look seated in the driveway and open later when the frame twists under payload.
- Check for bent, spread, recessed, or pushed-back terminals.
- Lightly tug each wire from behind the seal to identify poor terminal retention; do not yank the conductor.
- Look for green corrosion, white residue, water tracks, damaged cavity seals, swollen gaskets, and broken CPA tabs.
- Confirm that no pin is folded over and that both connector halves sit square before the lock is engaged.
- Use dielectric grease only as the connector or kit instructions permit. A light film on an intact seal can help exclude moisture, but grease is non-conductive and should not be packed into terminal cavities.
Corroded or loose terminals need a proper connector, terminated-lead, or pigtail repair. Grease cannot restore terminal tension, remove oxide, or repair a damaged seal.
Diagnosing No-Communication And U-Codes
A no-communication condition after installation should be diagnosed from the last connection touched outward: seating, terminal position, harness fitment, power and ground, then network testing.
| Symptom | Likely Area | Physical Check | Next Diagnostic Step | Do Not Do |
|---|---|---|---|---|
| Tool will not identify the truck | Power-up sequence, DLC, tool cable, or unsupported workflow | Confirm key state, cable seating, battery voltage, and tool account status | Return to the tool maker's connection procedure | Start unplugging random underbody connectors |
| One or more modules disappear after the harness install | Recently opened connector or terminal pushout | Inspect every mating face and compare the post-install module list with the baseline | Power down, reseat, rescan, and isolate one change at a time | Replace a control module before proving powers, grounds, and network integrity |
| Multiple U0xxx communication codes appear at once | Network branch, low voltage, or a connector left partially latched | Check both batteries, grounds, CPAs, branch tension, and recessed pins | Use the GM wiring diagram and a high-impedance DVOM or scope with the specified test setup | Probe CAN circuits with an incandescent test light |
| Fault returns only while towing or on rough roads | Harness strain, abrasion, heat, or loose lock | Inspect witness marks near frame edges, exhaust, suspension, and tire travel | Repair the damaged section and add strain relief with a service loop | Zip-tie the harness tighter across a moving gap |
| Flash stops partway through | Voltage, cable, network interruption, computer, or account connection | Do not cycle power until the tool provider's recovery instructions say to | Follow the supported recovery workflow and document the exact message | Repeatedly disconnect batteries or attempt another file blindly |
For a broader fault tree, review why communication failures happen across different workflows. A network code is evidence of lost communication, not automatic proof that the tuning device, ECU, or CAN harness has failed.
Routing For Towing, Payload, Jobsites, And Winter
A frame-mounted harness needs abrasion clearance, heat clearance, water protection, and enough service loop to survive axle travel and frame flex under real HD-truck loads.
Route the loom away from exhaust components, steering parts, the driveshaft, suspension travel, brake and fuel lines, sharp frame holes, and the full tire envelope. Use existing protected paths where possible, support the harness at stable points, and avoid crushing the loom with overtightened ties.
Heavy fifth-wheel towing loads the chassis differently than an empty highway run. Jobsite mud can hold salty water against a connector, off-road articulation can pull a branch tight, and winter brine can wick under damaged loom. After the first loaded trip, inspect all four areas for shiny rub marks, stretched ties, displaced seals, and a latch that no longer sits fully home.
Match The Hardware To The Intended Workflow
Choose parts by model year, connector architecture, supported calibration process, and lawful vehicle use rather than by a similar-looking plug shell.
Selecting the Correct SPELAB CAN BUS Plug Kit
The L5P CAN BUS Plug Kit is the closest hardware match to the locations shown in this guide. However, because GM utilized different connector configurations across model years, you must select the correct connector color variant when ordering:
- Check Plug 2 First: Locate Plug 2 inside the front passenger-side frame rail.
- Match the Connector Color: Inspect the outer shell color of your factory Plug 2 OEM connector before placing your order. Select the corresponding kit color variant on the product page to ensure keyway and latch compatibility.
Broader options include an L5P tuner and unlock package and an EZ LYNK tune package. Verify current instructions and supported model years before ordering; a 2017-2019 location photo does not prove compatibility with a later truck.
For a wider fitment check, compare the 2017-2019 L5P Duramax parts collection and read how the major workflows differ before purchase. Any diagnostic, security-access, or calibration hardware must be used according to its manufacturer documentation and all applicable emissions, anti-tampering, and road-use requirements.
Frequently Asked Questions
These answers cover the fitment, orientation, connector handling, and diagnostics that most often stop a 2017-2019 L5P harness installation.
Q: Does a 2017-2019 Duramax L5P have only four CAN bus connectors?
A: No. This article shows four photo-reference locations used by certain aftermarket harnesses; it does not count every CAN connector or network node on the truck.
Q: Are Plug 1, Plug 2, Plug 3, and Plug 4 official GM connector numbers?
A: No. They are navigation labels for this guide. Match the exact connector ID, harness label, keyway, and manufacturer instructions for the job you are performing.
Q: Why does my kit have three plugs when the guide shows four locations?
A: Different harness designs use different branches and connection strategies. A 3-plug kit may be correct, but only its current instructions can identify which locations it uses.
Q: Where is the large front frame connector shown as Plug 2?
A: It is shown inside the front passenger-side frame rail. Put the light inside the rail and confirm the shell against the photo and kit label before disconnecting it.
Q: Should I disconnect both batteries before opening an L5P connector?
A: Follow the exact kit and GM service power-down procedure. When battery isolation is required on a dual-battery truck, both negative cables must be handled as directed so one battery cannot keep the circuit energized.
Q: Can I fill the connector with dielectric grease to prevent corrosion?
A: No. Use only the amount and location allowed by the connector or kit instructions, generally a light film on an intact seal. Do not pack terminals or cavities; repair corrosion, loose tension, or damaged seals correctly.
Q: What should I check first when the scan tool loses communication after installation?
A: Compare the module list with the pre-install scan, then inspect the last connectors touched for incomplete latching, a backed-out terminal, wrong harness fitment, low battery voltage, or branch strain before replacing parts.
Q: How do I know which color option to select for the L5P CAN BUS Plug Kit?
A: Your choice depends entirely on the OEM connector shell color at the Plug 2 location (inside the front passenger-side frame rail). Visually inspect your factory Plug 2 connector first, then match that color option when ordering your kit to guarantee the proper keying and latch alignment.
About the Author
John Lee
Mechanical Engineer | 10+ Years Experience
John has spent the last decade engineering and testing high-performance automotive components. Specializing in drivetrain durability and thermal management across Powerstroke, Cummins, and Duramax applications, he bridges the gap between OEM limitations and aftermarket performance. His philosophy: "Factory parts are just a starting point."
