U0678: Lost Communication With Fuel Level Sensor C

What it means

The vehicle bus reports loss of communication with smart fuel level sensor C. Tank gauge may freeze and low-fuel warnings may be incorrect.

At a glance

• Usually lights the warning light of the system that lost communication

How it works

Smart fuel level sensor C monitors fuel in its section of the tank and sends its reading over the vehicle network. U0678 sets when the receiving module loses messages from sensor C, potentially leaving the fuel gauge or low-fuel warning incorrect.

Common causes

Most likely first.

1. Bus wiring open or shorted
2. Module without power or ground
3. Module failed

Symptoms

• Multiple warning lights may illuminate
• Affected modules may go into limp mode or shut down
• Specific functions inoperative depending on module
• Diagnostic tools may show communication faults

How to diagnose it

1. Scan all modules and save freeze-frame data, noting whether sensor C's communication loss accompanies other bus faults.
2. Check live data for sensor C and compare it with the gauge and the known tank level to establish whether its reading is absent or stuck.
3. Inspect sensor C's connector and accessible harness for corrosion, looseness, chafing, or damage around the tank.
4. Check sensor C's power and ground against the manufacturer's specification.
5. Test the network wiring serving sensor C for an open or short using the manufacturer's procedure.
6. If the circuit checks pass, use the manufacturer's diagnostic procedure to identify a failed sensor C or its communication module.

Common fixes

• Reconnect or clean the sensor C connector and repair damaged wiring.
• Restore a faulty power or ground connection to sensor C or its module.
• Repair an open or short in the network wiring to sensor C.
• Replace sensor C or its communication module if testing confirms failure.

Can I keep driving?

You can generally drive if the vehicle operates normally, but treat the fuel gauge and low-fuel warning as unreliable; refuel early and track mileage. A false reading could leave you stranded when the tank runs empty.