OBD Scanner Won’t Turn On – Here’s the fix!


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 scanners 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 ca 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, plugging in the scanner to find it won’t power up is a very common fault. The OBD port or socket also known as 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 it’s 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, five minutes from now your tool will be juiced up and you’ll be back in business.

Locating the OBD fuse

OBD fuse
OBD fuse location and size

The OBD fuse usually blows for good reason, overloading of 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 lacks power also, you can bet you’ve found your problem.

Your vehicle will likely have multiple fuse boxes fitted, three is 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 power point or cigarette lighter and will most likely be located inside the vehicle.

The most efficient way to locate it is check the drivers manual. Under fuses, many good drivers manuals will list the fuse location, type, color, rating (size), and a description of what it powers.

Checking the OBD fuse

Blown fuse and good fuse

There are three common ways to check fuses:

  1. Using a test light
  2. Volt meter
  3. 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 are 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 the correct size. 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. A 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 a solid contact with the tool won’t power it.

Checking OBD socket damage

To check OBD for damage this type issue we’ll perform two checks. 

First we’ll check the port, examine the port terminals closely using a light. Look at the terminals and see if terminal 4 and 16 appear larger than any of the others.

Checking DLC terminal tension

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 use a test probe to drag test tension on the terminals which checks for spreading. If the tension 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 obvious terminal spread.

The second check will require a test light or volt meter. You can find both here on the Auto electrical repair tools page.

DLC Power and ground check

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 paths, 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.

4 Damaged OBD Scanner Plug

OBD 2 Plug pins

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, 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.

John Cunningham

John Cunningham is an Automotive Technician and writer on Rustyautos.com. I've been a mechanic for over twenty years, I use my knowledge and experience to write articles that help fellow gear-heads with all aspects of classic car ownership, from tires to roof aerials and everything in between.

Recent Posts