Mastering Geotab
★ Geotab Harnesses · Make-Specific and Custom Harnesses
The HRN-CT20T1 is the Geotab harness for first-generation Tesla Model 3 cars from model year 2018 and earlier, linking the car to a GO9. Later Model 3 years use different part numbers.
The HRN-CT20T1 is a Geotab custom cable built for the earliest Tesla Model 3 production, covering cars of model year 2018 or older, and it lets a GO9 device connect where a normal OBD-II socket would be on another car. If your Model 3 came off the line in the first production years, this is the harness Geotab points to.
Those early cars are now arriving in fleets as second-hand purchases and lease buyouts, which is why we see this part number come up more often than its age suggests. A used 2018 car that joins a fleet in a later year still needs the 2018 harness, because the part follows the car's build, not the date it was bought.
According to Geotab's harness identification table, the HRN-CT20T1 is meant for Tesla Model 3 vehicles from 2018 or older, and the device it needs is a GO9. The harness measures 30 cm (12 in). The total cable length connected to the device should not pass 2 m (6.5 ft). Geotab shows no part it replaces, no base harness requirement and no extra kit pieces.
That narrow fit is the whole point. A Model 3 from 2019 onward moves to the HRN-CT20T11, and the 2024 redesign moves again to the HRN-CT06K1 kit. The Model S, Model X and Model Y each have their own Tesla harnesses too, so the HRN-CT20T1 should only ever be ordered for one car and one band of model years.
Consider a property management firm that bought three used 2018 Model 3 cars for its field supervisors. The rest of its vehicles report into MyGeotab, but these three show nothing, and the manager has no way to confirm site visits or mileage claims. Fitting this harness and a GO9 closes that gap without touching the car's wiring.
The harness also answers a question we hear from fleet buyers: can an older electric car be tracked as well as a new one? For the 2018 Model 3, Geotab built a dedicated part, so the answer is yes, as long as the right year harness is used.
Every Tesla job we do follows the same discipline: verify first, then open panels. The work itself on an early Model 3 tends to run in this order.
Geotab's limits still hold here: the full length of harness on the car is capped at 2 m (6.5 ft), and one T-harness is not to be joined to a second.
| Spec | Detail |
|---|---|
| Part number | HRN-CT20T1 |
| Origin | China |
| Category | Make-specific custom harness |
| Fits | Tesla Model 3, 2018 and older |
| Device needed | GO9 |
| Harness length | 30 cm (12 in) |
| Total connected length limit | 2 m (6.5 ft) |
Used cars hide their history. A car sold as a 2018 on a spreadsheet may be a 2019, and the paperwork from an auction does not always match the metal, so we check the VIN on the car itself where a J in the tenth place means 2018 and a K means 2019. Old trim clips on high-mileage cars break easily, so carry spares. On a car that has had dash work before, look for loose factory connectors near the install point and make sure the vehicle connector itself is fully seated before blaming the harness for a no-data result.
The car has to be model year 2018 or earlier. We confirm that from the VIN, where a J in the tenth position marks a 2018 car, before the part is ordered.
Geotab specifies a GO9 for this harness. It is the only device Geotab lists against this part number.
Geotab does not list a required base harness or add-on kit for this part, so the harness and the GO9 are the core of the order, plus cable ties for mounting.
Our technicians bring the Geotab harness and GO device to your yard, fit them on site and leave each car reporting before we go. With 35 years in twelve-volt work, we treat an early Model 3 with the same care as a new one. To set a date, phone 803-513-1009, or send the VINs through Get a Quote.
Sources: Geotab harness identification table (HRN-CT20T1 entry) and Geotab overview of telematics harnesses. GEOTAB is a registered trademark of Geotab Inc. API Signal is an independent installer and is not Geotab.
Tell us how many vehicles and what they are. We'll quote the hardware and the install together.