U0171

Universal

Lost Communication With Restraints System Sensor B

Definition source: General OBD-II reference catalog
Definition verification: Not independently verified for display.
Time: Varies by diagnosis and vehicle | Diagnostic difficulty: Advanced
High Severity

The restraint system monitors communication with Sensor B. This code means the system did not receive the expected communication from it. Depending on the vehicle and the role of that sensor, some airbag or seat-belt pretensioner functions may not work as intended. The code does not by itself prove that Sensor B has failed.

Report an error with this code

⚠️ Before You Begin (Safety First)

Treat this as a safety-related fault. If the airbag/SRS warning light is on, have the system checked promptly because occupant protection may be reduced. Do not probe or disconnect airbag, pretensioner, or restraint-system connectors. If an airbag has deployed or there is visible damage to restraint-system wiring, do not drive the vehicle; arrange professional assistance.

Common Symptoms

  • The airbag/SRS warning light may illuminate if the system detects the communication loss.
  • A dashboard message or stored restraint-system fault may appear, depending on the vehicle.
  • Some restraint functions may be unavailable or may not operate as intended; the specific effect depends on the vehicle and Sensor B's role.
  • There may be no obvious driving symptoms, because this fault concerns the restraint system rather than normal engine operation.

Common Causes

  • An open, damaged, corroded, or poorly connected wire or connector in the communication path.
  • A power or ground problem affecting Sensor B or another related restraint-system component.
  • A fault in the vehicle communication network or in a related control module.
  • A fault within Sensor B, which should be considered only after wiring, power, ground, and network checks.
  • Previous repairs, impact damage, or moisture intrusion affecting restraint-system wiring or components.

Diagnostic Steps

  1. Note whether the airbag/SRS warning light is on and record when it appeared; this helps describe the fault without disturbing the system.
  2. Check the vehicle owner's manual for the meaning of any restraint-system warning and follow its safety guidance.
  3. With the vehicle parked, visually check for obvious damage or moisture around accessible wiring without touching, unplugging, or probing SRS connectors.
  4. If you have an appropriate scan tool, record restraint-system and related-module codes before clearing anything; code patterns can help distinguish a network issue from an isolated fault.
  5. Arrange diagnosis by a qualified technician for electrical testing or any work on SRS components. Do not bypass the system or probe airbag or pretensioner circuits.

When to Call a Professional

Use a scan tool that can access the restraint system to check current and stored codes across related modules, then follow the vehicle-specific communication and wiring diagnosis. A qualified technician can check power, ground, connectors, and network communication using the correct SRS precautions. Avoid replacing Sensor B based on this code alone.

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Commonly Asked Questions

Does U0171 prove that Sensor B needs replacement?

No. It reports lost communication, which can result from wiring, power, ground, network, or module problems as well as a sensor fault. Diagnosis is needed before replacing parts.

Can the vehicle still be driven with this code?

The code may not affect normal driving, but restraint protection could be reduced. If the SRS warning light is on, arrange prompt professional diagnosis and follow the vehicle owner's manual; do not drive if restraint-system damage or an airbag deployment is evident.

Why might there be no noticeable symptoms?

The fault is in restraint-system communication, so it may not change engine or transmission behavior. The system may still log the code or show an SRS warning.

Can I check the Sensor B connector myself?

Do not unplug, probe, or handle restraint-system connectors. A visual check for obvious external damage is reasonable, but electrical testing and SRS repairs should be performed using vehicle-specific safety procedures.

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