U0671: Lost Communication With Fuel Temperature Sensor B

What it means

The vehicle bus reports loss of communication with smart fuel temperature sensor B. Density and viscosity correction for injection quantity switches to a substitute value.

At a glance

• Usually lights the warning light of the system that lost communication
• Affects emissions; the car can fail an emissions test

How it works

Fuel temperature sensor B reports fuel temperature over the vehicle network so injection control can account for changes in fuel density and viscosity. The control module sets U0671 when it stops receiving messages from sensor B, leaving those fuel corrections to use a substitute value.

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 for network faults and review freeze-frame data to see whether sensor B stopped communicating alongside other devices.
2. Check scan-tool data for fuel temperature sensor B's online status and reading, and compare it with sensor A if fitted.
3. Inspect sensor B's connector and nearby fuel-system harness for looseness, corrosion, fuel contamination, or damage.
4. Using the manufacturer's wiring information, verify sensor B's power and ground and test its network wires for opens, shorts, or poor connections.
5. If its supplies and wiring are sound, test communication at sensor B and determine whether the sensor module has failed.

Common fixes

• Reconnect or clean the fuel temperature sensor B connector and repair damaged wiring.
• Restore a missing or poor power or ground connection at sensor B.
• Repair an open or short in the network wiring to sensor B.
• Replace sensor B if its power, ground, and network connections are good but it remains offline.

Can I keep driving?

If the vehicle runs normally, a short gentle trip is generally reasonable, but avoid heavy loads if drivability or fuel economy has changed because fuel corrections are using a substitute temperature. Stop if the engine begins to stumble or stall.