OBD Won’t Connect To ECU – Solved

Computers are great most of the time, but when they don’t work, they’re a pain in the jacksie. Hooking up the scan tool only to be greeted with no communication is frustrating!!! 

OBD scanners fail to connect for 3 common reasons:

  1. Faulty OBD plug/socket
  2. Scanner not compatible
  3. CAN network fault

In this post you’ll learn about each of the common reasons an OBD scan tool won’t connect to a vehicle, you’ll learn how to diagnose why your OBD tool won’t connect and how to fix it.

When writing this post I’ve assumed your scan tool powers up when you plug it into your vehicle’s OBD port but it just won’t connect. 

If however your scan tool won’t even power up when you hook it up, then check out this post “OBD scanner won’t turn on” it covers the simple fix in detail. A blown fuse is often the cause of a scan tool that simply won’t power up.

1 Faulty OBD Plug or Socket

This may seem like a simplistic answer and you are correct but it’s worth exploring for two reasons. Number one it only takes a few moments and number two, while not being the most common cause of a scan tool that won’t communicate, it is by far one of the easiest fixes.

So what’s the problem with the OBD plug/socket?

I’m a mechanic and while we’re known for our MacGyver type skills we may not be generally prized for our finesse. That’s not to say we disrespect kit or customers’ vehicles, but working on a workshop floor is all about speed.

In a long winded way I’m saying your vehicle’s OBD connector also known as the DLC (Data link connector) may be damaged from possibly years of OBD scanning and dare I say it “Front probing”.

Reaching into a vehicle, blindly fitting or pulling the scanner cable is a normal everyday occurrence. This type repeated activity will eventually cause scan tool pin wear or damage and spreading of the vehicles female OBD socket terminals.

How to check the scan tool plug?

Scan tool plug pin check

Examine your scan tool plug and check the pins. There are 16 pins. (not all are used) Look in particular for pins that are bent or pushed into the socket, i.e. a lot shorter than the others.

Check for corrosion or debris in the plug. Check the cable itself for damage, and check the plug at the scanner too. Finding a problem with the plug or cable is easy to fix, most good scanners will have a detachable and therefore replaceable OBD cable. Just go ahead and buy a new cable, they’re not expensive. 

Of course a fast way to check your scan tools is simply plug into another vehicle, if it works great! You can move onto a possible problem with your vehicle’s OBD terminals, and that’s what we’ll look at next.

How to check the OBD socket (DLC)?

DLC pin check

Checking your vehicle’s socket is a little more work but not much. A visual test may reveal the issue but more likely you’ll need to perform a pin drag test. 

Your vehicle’s DLC terminals are known as female terminals. Wear and tear and front probing causes the terminal legs to lose tension. This causes resistance and often is the root cause of scan tool communication issues.

For this test we’ll do what I said was rarely a good idea, well front probe the pins and compare tension.

A visual test comes first, examine closely the DLC terminals, they should look similar. Note: not every DLC slot will contain a terminal. Pay particular attention to terminal 6 and 14, they are the communication terminals your scan tool uses to access the vehicle’s CAN network.

Next we’ll pin drag test the terminals. We’ll place a fine metal tool between the pins and feel the resistance when we remove it. This isn’t a measurable result, we’re using our ability to feel the difference in tension from one terminal to another.

In a workshop. We have very precise tools for this job, but you can still nail this test by improvising a fine paper clip or such. Do not place objects like a multimeter probes into the terminals, that will damage them.

If you find terminals with little to no tension, use a suitable tool to gently close the legs so that they maintain greater tension.

2 Scanner Not Compatible

Scan tools

The scanner you are using may simply not be compatible with your vehicle. Of course if you have used this same scanner previously and it worked, then this whole section doesn’t apply to you. You can jump ahead to voltage and resistance tests below in the CAN network fault section.

If it’s a new or new to you scanner, go ahead and check your scanner’s capability. Check that it contains the latest software, many tools offer free software upgrades. Scan tools often have a useful website with a troubleshooting and forum section dedicated to solving scan tool issues. Check them out, common scan tool problems will be front and center.

3 CAN Network Fault

As you know modern vehicle electrical systems increasingly employ control modules to manage system functions. Your vehicle will employ several, each system will have its own dedicated control module also called controllers. 

CAN Network diagram

Vehicle computers are known by several names, and acronyms, and to make things even more exciting manufacturers use their own names. But don’t let any of this confuse you; they are all just control modules. Some of the common controller names include – Control modules, Controllers, Computer, CU, CM, Modules, Microprocessor. 

To identify individual controllers manufacturers generally use acronyms like PCM (Powertrain control module), ECM (Engine Control module), TCM (Transmission control module), BCM (Electronic brake control module) or EBM (Electronic brake module), or BCM (Body control module), some manufacturers call also called EIPCM (Instrument panel control module) you get the idea.

What does a controller do?


I’m not going deep into the weeds here, I realize you simply want your OBD to connect, but a little background info is useful for later when we’ll be running some tests. 

As said, vehicles employ lots of controllers, the more sophisticated your vehicle is the more controllers it will have. All modern vehicles for example, will have an ABS brake system. This is a sophisticated system that only a computer could manage. 

Your vehicle’s dedicated brake system controller, commonly called a EBM (Electronic brake module), gathers data from various sensors on your car. Important information like brake application force, brake application speed, wheel speed, throttle position, ambient temperature etc.

The information is processed by the EBM and action is taken according to its software parameters. 

Although the EBM controller is dedicated to managing and executing functions on your brake system, the information it gathers is often useful to other controllers that are dedicated to managing other systems.

Controllers therefore share information, they don’t work in isolation, they are part of a network of information.

Controllers Share Data:

The PCM (Powertrain controller – manages engine and transmission) uses basic data like vehicle speed in order to perform timely fueling or gear change decisions. As the BCM already gathers speed data, it shares it with the PCM.

But the BCM may also share information about ABS activation, the PCM may use this information to cut power to the engine when the vehicle is skidding.

Many controllers gather data that may be useful to other controllers, and as all this information needs to be shared and shared at lightning speed, the controllers are all connected.

Controllers are connected via multiplexers which facilitate communication and the sharing of information. Modern vehicles typically use a few different types of communication systems depending on how important the message is. The most common type system is known as CAN (Controller Area Network). 

Most systems use conventional wiring, each module in the system is connected together using a twisted pair of wires, however some systems use fiber optic.

How do they communicate?

CAN networks as you know connect all the controllers together using a twisted pair of wires known as the CAN lines. They are twisted because that helps reduce interference from other wiring circuits.

CAN network twisted wires
Twisted CAN wiring

The two wires are identified as CAN high and CAN low, so called because the range of volts they use to communicate on the network. The CAN high uses a voltage range of 2.5 to 3.5 volts and the second wire known as CAN low uses a 1.5 to 2.5 voltage range.

Every controller on the system is recognized and monitored by all the other controllers on the system. It’s eyes on eyes. Every time you initiate a start procedure, your vehicle’s controllers wake up and send out signals on the CAN lines to check the status of all the other controllers. 

They in turn, signal back, so that all the controllers talk to each other, it’s known as the hand shake. Each controller has a time to communicate, this is important, if all controllers talk at once messages are lost.

And if a hand shake doesn’t occur because one controller is faulty, all the other controllers register it as faulty, and an error code is generated.

The fault could also of course be a wiring issue between the suspected faulty controller and the network.

Because CAN network language uses voltage drops to communicate, they are super sensitive to changes in voltage in other words they are super sensitive to circuit resistance.

To help stabilize resistance, the CAN circuit employs two 120 ohm resistors in parallel, this using Ohm’s law makes the measurable resistance of 60 ohms. That’s important to know for later when we’ll be testing circuit resistance.

The resistors are commonly control module integrated but may also be embedded in the circuit. 

Common communication issues: 

Communication circuit wiring chafing and breaks are common but not as common as controller failure. Controllers usually fail in two common ways:

  1. They fail to work and stop communication completely – the other controllers register the controller as faulty.
  2. They contaminate the CAN network with constant erratic voltage. The other controllers and scan tools can’t understand this constant gibberish and simply don’t get a chance to shake hands with the other healthy controllers as the CAN lines are always busy.

It’s not uncommon for one controller to become corrupt and to pull down the whole network, this causes a no start issue. Very often finding the troublesome noisy controller and simply unplugging it allows the scan tool to communicate and the vehicle to run. 

Common reasons control modules fail:

Control module water

By far the two most common causes of control module failure in my experience is old age and moisture. Moisture caused by water leaks is a common cause of failure. Many controllers that are fitted under the hood are pretty durable and can withstand moisture but controllers fitted inside the cabin aren’t built to withstand moisture, since they aren’t expected to get wet.

It is possible to have controllers repaired but it’s a specialist job and a dealer won’t entertain such a repair they will quote for a new control plus software plus labor to fit, and we’re talking hundreds here, $800 upwards.

My best advice – investigate constant misting up windows, damp carpets, water sloshing noises, musty smell, sweet smell, keep your window cowl drains and fire wall drains clear, check those sunroof drains and take care of water leaks immediately, they can do a surprising amount of damage.

How to check the CAN System

Controllers are expensive and complex units, calling one failed without doing the proper test is foolish. Many the good tech has been caught out by jumping the gun and calling a perfectly good controller as faulty only to find the problem is still present with a shiny new spendy one installed.

I was an apprentice to an old but wise mechanic and the way he explained controller testing to me, made good sense and has served me well. 

I’m passing it on. Controllers are like black boxes. We don’t know what’s going on inside them and we don’t really care.

What we do care about is verifying the following:

  • Good voltage supply
  • Good ground path
  • Good inputs

Once we have checked the voltage, ground and inputs it’s easy to condemn a failing or faulty controller.

The following tests aren’t complicated and most of the checks are easy to do. Some may be more challenging than others, not because of complexity but because some controllers may be difficult to access.

To test the circuits and controllers we’ll need to check voltage when the system is at rest (static voltage check) but also need to test the circuit when the circuit is live (volt drop test).

In addition we’ll check the resistance of the CAN network. We’ll cover all these tests below, but before we do we’ll need proper tools to make these tests.

Tools we’ll need:

DVOM and wiring diagram

Because CAN networks use changes in voltage to communicate they are super sensitive to resistance which of course changes voltages. CAN networks communicate in a predictable square wave pattern, however they do it at lightning speed, trying to catch a glitch in an information packet would require pure luck when using a simple voltmeter.

For this reason a pro shop will use a scope to capture, slow down and check visually, glitches in a graph. Having a scope does make capturing intermittent issues easier.

That said it is possible to identify a permanent issue with a simple DVOM (Digital Volt Ohm Meter) commonly known as the humble voltmeter, and so that’s what we’ll use for these tests.

A wiring diagram will be needed for some of these tests as how the CAN network is wired will predict some of the readings we should expect.

Tools we’ll need:

  • Wiring diagram
  • DVOM 
  • Probes
  • Scope would be nice

If you need wiring diagrams or workshop manuals check out eManualonline.com, I’ve been using them for years.

Check out the Auto repair tools page where I list all the repair tools I use.

Check CAN Voltage

This is a simple voltage check using a voltmeter and fine probes, remember back probing terminals is advised unless you have fine non invasive probes. Here’s a pin out of your vehicle’s DLC you can use to follow along. 

DLC pin out

As said at the beginning of this post, I’ve assumed you have power in the scan tool, but it isn’t communicating. If that isn’t the case, check voltage terminal 16 (scan tool voltage) and at terminal 4 (chassis ground). Check terminal 16 and terminal 5 (signal ground) also. A blown fuse is often the cause of a scan tool that won’t power up.

DLC voltage check
My battery is low, expect 12.5 volts minimum

A strong battery is critical, low voltages may distort your results. A battery with min 12.5 volts should be maintained. 

Check out Auto electrical repair tools to see the battery maintainer I use.

Check voltage at CAN High

Vehicle ignition should be turned on (Pos II). Place the voltmeter positive probe on terminal 6 and the negative on terminal 4. Use the min max voltmeter function to capture the max voltages and min voltages. The readings between 2.5 and 3.5 volts indicate normal.

CAN High volt check

Check voltage at CAN Low

Vehicle ignition should be turned on (Pos II). Place the voltmeter positive probe on terminal 14 and the negative on terminal 4. Use the min max voltmeter function to capture the max voltages and min voltages. The readings between 1.5 and 2.5 volts indicate normal.

CAN Low voltage check

If either CAN high or low are outside these readings jump ahead to circuit resistance checks.

Ground side CAN voltage drop test

Vehicle ignition should be turned on (Pos II). With voltmeter set to volts. Place the negative probe on terminal 4 and the positive probe on chassis ground and note the reading Repeat this for pin 5. 

CAN voltage drop test diagram

Both tests should read less than .2 volts. More than this means there’s excessive resistance in the circuit. Ground should be inspected first.

Check CAN Resistance

CAN Circuit resistance check

With the battery disconnected, and volt meter set to ohms. Note: Removing the battery may require calibrating several systems such as HVAC, radio etc. In many cases this may require a bidirectional scan tool. I wrote a post about the systems it effects which is included in this post –“How hard is it to change car battery”

With the battery disconnected (ground terminal). Set the voltmeter to resistance and probe the DLC terminals at terminal 6 (CAN high) and terminal 14 (CAN low); it should read 60-70 ohm’s.

If you have a reading outside this window, you’ll need to check a wiring diagram to verify where controllers are and how they are configured, and where the resistors are located.

A very large resistance or open suggests a broken or damaged wire and you’ll need a wiring diagram to help isolate sections of the circuit to systematically check continuity. 

If on the other hand resistance is lower or higher than the expected but not open then a faulty controller is a likely suspect. See checking Controller diagram below.

Resistance test using wiring diagram

Depending on how your CAN network is wired. A process of elimination is often fruitful, simply start by removing a controller that’s easy to access and repeat the network resistance test again. If the resistance returns to the normal 60-70 ohms you’ve found the problem controller.

Most controllers are wired in series, with sub controllers (Side streets off main street) knowing which module is on main street and which is a side street is crucial to efficient resistance testing.

Controllers at the start of the CAN network and the end often employ an integrated 120 ohm resistors.

Removing a main street midpoint controller block connector effectively breaks the system in two and allows you to isolate the problem side of the circuit.

CAN Network Resistance split check
Split circuit and test resistance

With the system in two halves, each end will contain a 120 ohm (Ω) resistor and since they are not connected and no longer in parallel, they will measure 120 ohms or close to it.

Now, measuring resistance not at the DLC but instead at “CAN In” and “CAN Out” (at the disconnected controllers block connector) will identify which side of the circuit is at issue. See diagram above.

This process can be repeated again and again until the problem controller or wiring fault is isolated.

This of course is only possible if you have a good working wiring diagram. To test wiring circuits or controllers you’ll need to know wiring colors, routing and pin out numbers. Finding and fixing problems is straight forward once you have the power of knowledge.

I use emanualsonline.com and you can check out their link here on my Auto electrical repair tools page.

Controller test

Making repairs to the CAN network

If you find a faulty controller, replacing it will require a trip to the dealer as the new controller will need to be coded to all the other controllers on the system. (Introduced if you like)

Chafed wire

You’ll often find chafed wiring under the hood where a loom turns direction sharply or around hot components like manifolds or EGR valves. Finding a simple chafed wire is much less problematic. Using a good heat shrink insulator and solder will repair the wiring to NASA standard. If a wire needs to be replaced, be sure to use the same size wire and maintain the twist in the wiring and you’ll be golden.

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.

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