Symptom
Diagnostic tool cannot connect
A code reader or scan tool plugged into the diagnostic socket cannot establish communication, or reaches some modules and not others. Worth distinguishing from a stored communication code, because the two mean different things: if a tool can read a lost-communication code from another module, the network is working well enough to hold a conversation and the problem is narrower than it appears. A tool that cannot connect at all points at the socket's own power and earth, the gateway that arbitrates the connection, or a bus jammed by one failed module.
Also described as: scanner wont connect, cant read codes, no communication with vehicle, obd port not working, scan tool no link, reader wont connect, no comms, دیاگ وصل نمیشود, دستگاه دیاگ اتصال ندارد, maquina no conecta, no conecta escaner, obd no comunica, no lee centralita, جهاز الفحص لا يتصل, الكمبيوتر ما يقرأ, لا يوجد اتصال مع الكومبيوتر
Some causes mean you should stop driving
9 of the 100 codes that produce this symptom indicate a fault where continuing to drive risks damage or is unsafe. If the symptom is sudden or severe, treat it as one of those until a scan says otherwise.
What this usually means
Worth distinguishing from a stored communication code, because the two mean different things. If a tool can read a lost-communication code from another module, the network is working well enough to hold a conversation and the problem is narrower than it appears. A tool that cannot connect at all is a different matter.
That case points at three things. The socket's own power and earth — the diagnostic connector has a permanent supply and a ground of its own, and a blown fuse there leaves a car that runs perfectly and cannot be talked to. The gateway that arbitrates access between the tool and the vehicle's networks, which on modern vehicles may also require security authorisation before it will pass anything through. Or a bus jammed by one failed module holding the network in a state where nothing can communicate.
Check the socket first because it is quick and it is the commonest. Measure supply and earth at the connector's pins; a cheap tool that shows no lights at all has already told you the supply is missing.
Then narrow by scope. A tool that reaches some modules and not others is a network segment or a gateway problem, and which modules are missing is the clue — look for what they share, a fuse, a connector, or a segment of the bus.
Two ordinary causes get overlooked. A device left plugged into the diagnostic port — an insurance dongle or a tracker — can occupy the port or disturb the network. And a low battery produces communication failures that look like network faults, because modules misbehave rather than stop when voltage sags.
Working through it
| What you find | Likely cause | How to confirm it | What fixes it |
|---|---|---|---|
| Tool shows no power at all when plugged in | The socket's own supply or earth — commonly a blown fuse | Measure supply and earth at the connector pins. A dead tool has already narrowed it before any software runs. | Replace the fuse and find why it opened. The car runs perfectly with this fault, which is why it surprises people. |
| Reaches some modules and not others | A network segment or the gateway — not a whole-vehicle failure | List which modules are missing and look for what they share: a fuse, a connector, a bus segment. | Repair the shared item. A single failed module can also take a segment down, so the missing one is not automatically the faulty one. |
| Another dongle or tracker is plugged into the port | The device occupying the port or disturbing the network | Remove it and retry. This is quick, free, and increasingly common. | Remove the device permanently if it recurs. A device on the port is on the network by definition. |
| Battery weak or the car has electrical oddities elsewhere | Low voltage producing communication failures that mimic network faults | Test the battery before diagnosing the network. Modules misbehave rather than stop when voltage sags. | Repair the supply, then retry. Many apparent network faults disappear. |
| A stored lost-communication code IS readable | Not this fault — the network is working well enough to be read | Being able to read the code is itself the finding, and it narrows the problem considerably. | Diagnose the named module rather than the network. Different problem, different scope. |
| Modern vehicle refusing the connection with a security message | Gateway authorisation — the vehicle requires the tool to be authorised before passing traffic | Establish whether the vehicle uses secure gateway access and whether the tool is enrolled. | Use an authorised tool or account. Not a vehicle fault, and no wiring will change it. |
-
Tool shows no power at all when plugged in
- Likely cause
- The socket's own supply or earth — commonly a blown fuse
- How to confirm it
- Measure supply and earth at the connector pins. A dead tool has already narrowed it before any software runs.
- What fixes it
- Replace the fuse and find why it opened. The car runs perfectly with this fault, which is why it surprises people.
-
Reaches some modules and not others
- Likely cause
- A network segment or the gateway — not a whole-vehicle failure
- How to confirm it
- List which modules are missing and look for what they share: a fuse, a connector, a bus segment.
- What fixes it
- Repair the shared item. A single failed module can also take a segment down, so the missing one is not automatically the faulty one.
-
Another dongle or tracker is plugged into the port
- Likely cause
- The device occupying the port or disturbing the network
- How to confirm it
- Remove it and retry. This is quick, free, and increasingly common.
- What fixes it
- Remove the device permanently if it recurs. A device on the port is on the network by definition.
-
Battery weak or the car has electrical oddities elsewhere
- Likely cause
- Low voltage producing communication failures that mimic network faults
- How to confirm it
- Test the battery before diagnosing the network. Modules misbehave rather than stop when voltage sags.
- What fixes it
- Repair the supply, then retry. Many apparent network faults disappear.
-
A stored lost-communication code IS readable
- Likely cause
- Not this fault — the network is working well enough to be read
- How to confirm it
- Being able to read the code is itself the finding, and it narrows the problem considerably.
- What fixes it
- Diagnose the named module rather than the network. Different problem, different scope.
-
Modern vehicle refusing the connection with a security message
- Likely cause
- Gateway authorisation — the vehicle requires the tool to be authorised before passing traffic
- How to confirm it
- Establish whether the vehicle uses secure gateway access and whether the tool is enrolled.
- What fixes it
- Use an authorised tool or account. Not a vehicle fault, and no wiring will change it.
Codes that produce this symptom (100)
Listed with the codes that most typically present this way first — the likeliest place to start.
- U0001 High Speed CAN Communication Bus Typical of this code Stop driving High
- U0003 High Speed CAN Communication Bus (+) Open Typical of this code Stop driving High
- U0004 High Speed CAN Communication Bus (+) Low Typical of this code Stop driving High
- U0005 High Speed CAN Communication Bus (+) High Typical of this code Stop driving High
- U0006 High Speed CAN Communication Bus (-) Open Typical of this code Stop driving High
- U0007 High Speed CAN Communication Bus (-) Low Typical of this code Stop driving High
- U0008 High Speed CAN Communication Bus (-) High Typical of this code Stop driving High
- U0009 High Speed CAN Communication Bus (-) Shorted to Bus (+) Typical of this code Stop driving High
- U0028 Vehicle Communication Bus A Typical of this code High
- U0030 Vehicle Communication Bus A (+) Open Typical of this code High
- U0031 Vehicle Communication Bus A (+) Low Typical of this code High
- U0032 Vehicle Communication Bus A (+) High Typical of this code High
- U0033 Vehicle Communication Bus A (-) Open Typical of this code High
- U0034 Vehicle Communication Bus A (-) Low Typical of this code High
- U0035 Vehicle Communication Bus A (-) High Typical of this code High
- U0036 Vehicle Communication Bus A (-) Shorted to Bus A (+) Typical of this code High
- U0037 Vehicle Communication Bus B Typical of this code High
- U0039 Vehicle Communication Bus B (+) Open Typical of this code High
- U0040 Vehicle Communication Bus B (+) Low Typical of this code High
- U0041 Vehicle Communication Bus B (+) High Typical of this code High
- U0042 Vehicle Communication Bus B (-) Open Typical of this code High
- U0043 Vehicle Communication Bus B (-) Low Typical of this code High
- U0044 Vehicle Communication Bus B (-) High Typical of this code High
- U0045 Vehicle Communication Bus B (-) Shorted to Bus B (+) Typical of this code High
- U0063 Vehicle Communication Bus D (-) Shorted to Bus D (+) Typical of this code Moderate
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Others with this symptom
Be the first to write this up
Nothing posted about this yet. Whatever you found is worth writing down — even a dead end saves someone the same afternoon.
What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
Sign in or create an account to add what you found about Diagnostic tool cannot connect.