P0644 Module communication: circuit fault

By: Diagno editorial team · Updated: 2026-10-15

P0644 means: Module communication: circuit fault. The car's control modules talk to each other over the CAN bus. Drive gently and have the car checked soon. The most common repair costs about €45–90 in Estonia, depending on make and model. The most common cause: connector loose, damp or corroded.

What it means

The car's control modules talk to each other over the CAN bus. One module isn't getting messages from another, or the data it gets is faulty. The computer found a fault in the electrical circuit: the part, a wire, a connector or a fuse.

What you may notice

• Several warning lights at once
• Some function doesn't work
• The car may not start or may go into limp mode

What could be wrong

1. 1Connector loose, damp or corroded · Wiring or connector repair €45–90
2. 2Wire broken or chafed · Wiring or connector repair €45–90
3. 3Faulty module or wiring · Control unit check and software €150–300
4. 4Engine computer fault (rare) · Control unit check and software €150–300

Repair difficulty: Moderate

What you can check yourself

First look at the connector and wiring: anything loose, melted or green with corrosion? Check the battery and terminals first. A great many communication codes appear after a weak battery or a jump start. Best left to a workshop. It needs a diagnostic tool and experience.

How we fix it

1. We read the fault data on the diagnostic tool and check whether the fault is constant
2. We measure the circuit's supply, ground and signal
3. We repair the wire or replace the connector
4. If the wiring is fine, we replace the module or wiring

Standard SAE description: Driver Display Serial Communication Circuit