U0671 Code Symptoms, Causes, Diagnosis and Repair Guide Steps

U0671 Code Symptoms and Solutions – How to Diagnose and Repair the Issue

When the Powertrain Control Module (PCM) can no longer talk to Fuel Temperature Sensor B, the engine‑control strategy loses a key data point. Drivers typically see the check‑engine light come on and may experience a rough idle, hesitation when the throttle is applied, or a measurable decline in miles‑per‑gallon because the PCM is forced to use default fuel‑temperature assumptions. These signs appear early enough that a timely scan can prevent more serious drivability loss.

Symptoms

• Check‑engine illumination – the MIL (malfunction indicator lamp) flashes or stays solid as soon as the PCM detects a loss of sensor data.
• Rough idle – engine speed wavers between 600–800 rpm under no‑load conditions.
• Hesitation on acceleration – a brief lag of 0.5–1 second before the engine responds to throttle input.
• Reduced fuel efficiency – fuel‑consumption numbers can drop 5‑10 % because the PCM defaults to a richer mixture.

These symptoms are directly tied to the PCM’s inability to read accurate fuel‑temperature information, which influences ignition timing, fuel‑injection pulse width, and emissions control.

Why Communication Fails

Faulty Wiring or Connector Issues

Corrosion, broken strands, or loose pins in the harness that runs from the PCM to Sensor B interrupt the signal. Heat cycles and vibration accelerate connector wear, especially in the engine bay where exposure to coolant and oil is common.

PCM Internal Fault

The communication controller inside the PCM can develop open or short circuits, or its microcontroller may experience memory corruption. When the internal UART (or CAN) interface fails, the PCM cannot poll the sensor, triggering U0671.

Sensor‑Circuit Interference

Electromagnetic interference from nearby high‑current devices (e.g., fuel‑pump driver, ignition coils) can mask the sensor’s voltage signal. A compromised ground path also creates a noisy circuit that the PCM interprets as a loss of communication.

Outdated or Corrupt PCM Software

Factory software revisions sometimes contain bugs that misinterpret legitimate sensor data under specific temperature ranges. If the PCM’s calibration tables are out of sync with the sensor’s output range, the module may flag a loss of communication even though the wiring is intact.

Diagnostic and Repair Procedures

1. Retrieve the code with a PCM‑compatible scanner

– Use a Level 2 or dealer‑grade scan tool. Confirm U0671 and note any pending or stored codes that could indicate related faults (e.g., B1220 – sensor circuit high).

– Cost of a professional scan: $100‑$150.

1. Perform a live data check

– Access the “Fuel Temp Sensor B” data stream. A flat line or erratic values confirms loss of communication.

– Verify that Sensor A (if present) shows normal readings; this helps isolate the problem to the B circuit.

1. Inspect wiring and connectors

– Visually examine the harness for frayed wires, cracked insulation, or corroded pins.

– Use a multimeter to check continuity (should read near 0 Ω) and resistance to ground (typically < 1 Ω).

– Repair any damaged sections with heat‑shrink tubing and proper crimp connectors. Typical repair labor: $80‑$120.

1. Test sensor power and ground

– Measure the sensor’s supply voltage (usually 5 V) while the ignition is on. Low voltage may indicate a power‑supply fault rather than a PCM issue.

– While a faulty sensor could cause the symptom, the issue may stem from the PCM not communicating correctly. A diagnostic scan and module communication test will determine if module replacement or reprogramming is needed.

1. Execute a PCM communication test

– Many scan tools offer a “PCM Self‑Test” that cycles the communication lines. Failure of this test points to an internal PCM fault.

1. Re‑program or update PCM software

– If the hardware checks out, load the latest PCM calibration file from the manufacturer. Re‑flashing typically costs $150‑$250 plus labor.

1. Replace the PCM when communication cannot be restored

– When continuity, power, and software updates fail to re‑establish a signal, the PCM itself is the likely culprit. Replacement units vary depending on production date and software version, so the correct module is matched by VIN before programming.

Typical cost breakdown

• Wiring repair: $120‑$250 (parts + labor).
• PCM re‑programming: $150‑$250.
• PCM replacement (including VIN‑matched unit and programming): $800‑$1,200 for the part plus $200‑$300 labor.

When Replacement Makes Sense

If the PCM communication test fails after thorough wiring inspection and software updates, the probability of an internal PCM fault exceeds 70 %. Continuing to chase wiring repairs often results in repeated failures and higher shop time. Replacing the PCM eliminates the internal hardware defect and restores reliable sensor communication.

Modern control modules are complex and integrated with security and immobilizer systems. That’s why choosing a replacement isn’t only about the hardware—it’s about correct programming and compatibility. Flagship One specializes in VIN‑matched control modules, providing a plug‑and‑drive solution backed by warranty. Their units arrive pre‑programmed to your vehicle’s specifications, ensuring immediate compatibility and reducing installation time for qualified technicians.

Preventive Maintenance

• Regular visual inspections of the sensor harness during routine service (e.g., oil change) catch early signs of wear.
• Clean connector terminals with dielectric grease to prevent corrosion, especially in humid climates.
• Maintain proper coolant and oil levels; leaks can seep onto wiring bundles and accelerate degradation.
• Schedule PCM software updates at manufacturer‑recommended intervals (often coinciding with major service campaigns).
• Use OEM‑specified wiring repair kits if any damage is found; aftermarket kits lacking proper shielding can re‑introduce communication noise.