Plugging in your scanner and finding it doesn’t work is slightly alarming; some of these tools are expensive. But don’t panic; we’ll get this figured out. This has happened to me a ton of times, and it’s never the scanner’s fault.
The most common cause of an OBD scanner that won’t turn on is a blown fuse on the vehicle’s OBD socket power supply circuit. If the check engine light fails to aluminate, you can be sure the fuse has blown. Checking the vehicle’s fuse box and replacing the blown fuse fixes the issue.
In this post, you’ll learn all the most likely causes of a scanner that won’t turn on, how to diagnose them, and how to fix them.
Blown OBD Socket Fuse
I’ve been a mechanic for over twenty-five years, and plugging in the scanner to find it won’t power up is a very common fault. The OBD port, socket, or DLC (Data Link Connector) provides power to your scan tool.
When your tool has no power, it usually means the vehicle’s OBD socket is missing its power supply. When the check engine light is missing, too, you can bet you found the reason your scan tool won’t power up.
The most common cause of the lack of power at the socket is, as you know, a blown fuse, and I’ll bet it will be the same for you. If that is the case, your tool will be juiced up five minutes from now, and you’ll be back in business.
Locating the OBD fuse
The OBD fuse usually blows for a good reason, overloading the circuit. That’s because the OBD fuse is commonly shared with the cigarette lighter or 12 v socket or other consumers. Plugging in a damaged accessory like a phone charger or using a high-consuming device such as a fan may cause the fuse to blow.
So if your 12 v port/cigarette lighter also lacks power, you can bet you’ve found your problem.
Your vehicle will likely have multiple fuse boxes fitted; three are common. One under the hood, one in the trunk, and another behind the dashboard.
The OBD fuse may be listed on the fuse cover as OBD II, DLC, Accessory port, 12 v powerpoint, or cigarette lighter and will most likely be located inside the vehicle.
The most efficient way to locate it is to check the driver’s manual. Under fuses, many good driver manuals will list the fuse location, type, color, rating (size), and a description of what it powers.
Checking the OBD fuse
There are three common ways to check fuses:
- Using a test light
- Volt meter
- Remove the fuse and check the conductive strip
The fastest way is to pull the fuse. Most fuse boxes will use a blade-type fuse which is easily removed. However, they’re small and awkward to grip. A handy tool is often clipped to the rear of the fuse box cover.
Hook the tool over the fuse body and pull. Holding the fuse to the light reveals the strip; if it’s broken, replace the fuse.
Check the rated fuse in the driver’s manual; never assume the removed fuse size is correct. They are often not; fitting a fuse that’s rated below spec will cause the fuse to blow again.
Fitting a fuse that’s larger than spec could damage components or, at worst, start an electrical fire. The common fuse size is 15-20 amp but use the size recommended by the manufacturer.
Open DLC Power Or Ground
When the fuse checks out Okay, the next most likely cause of no power is a damaged OBD socket. It’s common in older vehicles. And that makes sense; more OBD port use over time causes the delicate terminals to wear or, more commonly, lose tension.
A DLC power terminal that doesn’t make solid contact with the tool won’t power it.
Checking OBD socket damage
We’ll perform two OBD port checks.
First, we’ll check the port and examine the port terminals closely using a light. Look at the terminals and see if terminals 4 and 16 appear larger than any of the others.
Terminal 4 is chassis ground, and 16 is power. These two terminals are used to power up your scan tool and, for now, are the most important terminals to examine.
In the workshop, I used a test probe to drag test tension on the terminals, which checks for spreading. If the tension is low, I’ll use a fine mental tool to tighten the terminal legs. I don’t advise poking anything into the front of the DLC unless you see an obvious terminal spread.
The second check will require a test light or voltmeter. You can find both here on the Auto electrical repair tools page.
Testing as per the diagram above will test both power and ground paths. If you don’t get a result on the voltmeter or test light, then go ahead and test each in isolation to assess if power or ground is missing.
It is best to back probe the DLC terminals, but I know that can be a pain in the ass to access. Alternatively, use a paperclip or similar to gently probe.
If you find an issue with either path, check under the dash panel for damaged wiring. The most likely area of damage is just under the dash, where objects may impact the wiring.
Damaged OBD Scanner Plug
Scanner damage is last on the list because I’ve been using them for years, and they are surprisingly durable; I have yet to break one. That said, it is possible. What’s more likely is a damaged or worn-out scanner cable.
Checking the scanner plug
Check the scanner plug carefully; the pins should be straight and aligned correctly. A pin that’s bent won’t make contact with the vehicle’s port. The good news is the cable is universal and is easily replaced.
If the cable checks out okay, go ahead and plug it into a donor vehicle and check if it works. This helps confirm the problem is definitely the scanner.
Check the scanner fuse
All scanners are fused, and bottle type is common. Access may not be as straightforward as checking a car fuse. The scanner may need to be opened to check and swap out the fuse. Check the process with the maker.
If your scanner is toast and you need to buy an inexpensive scanner that gets the job done, check out the Topdon vs Autel code reader review, and check out the scanners I recommend here on the “Mechanics tools page.”
About the Author
John Cunningham is a Red Seal Qualified automotive motive technician with over twenty-five years of experience in the field. When he’s not writing about car repair, you’ll find him in his happy place – restoring classic cars.
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John Cunningham is an Automotive Technician and writer on Rustyautos.com. He’s been a mechanic for over twenty-five years and has worked for GM, Volvo, Volkswagen, Land Rover, and Jaguar dealerships.
John uses his know-how and experience to write fluff-free articles that help fellow gearheads with all aspects of vehicle ownership, including maintenance, repair, and troubleshooting.