U0171Generic code

U0171 Code: Lost Communication With Restraint System Sensor B

The U0171 code means other modules in the vehicle stopped receiving messages from restraint system sensor B. The airbag light comes on and some airbags may not deploy. Start with the battery, the sensor power and grounds, and the network wiring before you replace anything.

Severity
High
Can I keep driving?
Yes, but with caution
Estimated cost
$0 to $1,500
Difficulty
Medium
System
Communication Network
Applies to
All OBD2 vehicles

Updated Reviewed by Renzo Chacca

On this page

What the U0171 code means

The modules in your vehicle constantly exchange information over a data network, normally CAN. Restraint system sensor B detects impacts so the system can decide when to deploy the airbags. When the other modules stop receiving its messages for a set amount of time, they store the U0171 code.

The code is stored by the module that is missing the information, not by the module that is failing. That is why it often shows up in several modules at once. The problem can be in sensor B itself (no power, no ground or an internal failure), in its connector, or in the network wiring that runs to it.

Typical location: at the front of the vehicle, in the pillars or in the doors. If that area is exposed to water or was worked on recently, start there.

Diagnostic order: 1) battery and alternator (low voltage is the number one cause of bursts of U codes); 2) the module list in the scan tool, to see which module is not responding; 3) power, fuses and grounds of the module that is not responding; 4) network resistance at the OBD2 connector (about 60 ohms between pins 6 and 14 with the battery disconnected); 5) module connectors and wiring, especially in damp areas (under carpets, in doors, in the trunk).

The CAN Network and U Codes guide explains in detail how to measure the network and isolate the module that is bringing it down.

U0171 code symptoms

  • Warning lights on in the dash.
  • Airbag light on, and some airbags may not deploy.
  • Communication codes stored in several modules at the same time.
  • The fault can be intermittent (for example, in rain or over bumps).

Likely causes of the U0171 code

  1. Sensor power or ground. A blown fuse, a corroded ground or a loose power connector.
  2. Weak battery or low voltage. Modules reset and lose communication.
  3. Sensor connector with water or corrosion. Very common on parts mounted in damp areas, such as doors and the front of the vehicle.
  4. Damaged network wiring. A CAN high or CAN low wire that is cut or shorted.
  5. Failure of restraint system sensor B. Only after everything above has been ruled out.
  6. Poorly installed accessory. Alarms or trackers wired into the network.

How to diagnose the U0171 code step by step

  1. Measure the battery voltage at rest (12.4 to 12.7 V) and with the engine running (13.5 to 14.7 V).
  2. Check in the scan tool which modules respond. If restraint system sensor B does not show up, the fault is in it or in its connection.
  3. Check the sensor fuses and grounds using the wiring diagram.
  4. Measure the network at the OBD2 connector. About 60 ohms between pins 6 and 14 with the battery disconnected, and roughly 2.5 V on each line with the key on.
  5. Inspect the sensor connector for moisture, corrosion and bent pins.
  6. Measure CAN high and CAN low at the sensor connector. They should have continuity back to the OBD2 connector.

How to fix the U0171 code

  • Repair the sensor power, ground or connector.
  • Repair the network wiring.
  • Replace and program the sensor if it has failed (it almost always needs programming).
  • Clear the codes in all modules and verify the repair.

Technical reference data

  • CAN network resistance between OBD2 pins 6 and 14: about 60 ohms.
  • CAN high: 2.5 to 3.5 V. CAN low: 1.5 to 2.5 V.
  • These are reference values only: check the manufacturer data for your vehicle.

Brand-specific notes

After the repair, clear the codes in every module. Many of them keep old U codes from the time the fault was present, and those can leave a warning light on or confuse the next scan.

Accessories installed after purchase (alarms, GPS trackers and OBD2 adapters left plugged in all the time) are a frequent cause of network problems, so unplug them first when you chase a U0171 code. Airbag circuits are sensitive, so leave any work on the airbag wiring itself to a qualified shop.

Frequently asked questions about U0171

Can I drive with a U0171 code?
It depends on the part. This sensor helps decide when the airbags deploy, so some of them may not fire in a crash. Drive with caution and have the vehicle checked at a shop soon.
Does U0171 mean restraint system sensor B is bad?
Not necessarily. The most frequent causes are its power supply, its ground, its connector or the battery, and the sensor itself is replaced last.
Why are so many codes showing at once?
Every module that was waiting for information from the affected part stores its own fault. One problem on the network can therefore produce a long list of codes.
How much does it cost to fix U0171?
From $0, if it was only the battery or a fuse, up to about $1,500 if the sensor has to be replaced and programmed.
What does the U0171 code mean?
It is a generic OBD2 network code that means the vehicle lost communication with restraint system sensor B. It points to a power, ground, connector or wiring problem as often as to the sensor itself.
Will a U0171 code clear itself?
If the cause was a brief voltage dip, the code may stop returning after the fault goes away, but the stored code stays until it is cleared. A loose connector or a damaged wire will bring it back.
What is the most common cause of U0171?
Low battery voltage and problems with the sensor power, ground or connector are the most common causes. Check those before you suspect the sensor.

Related codes

Questions about the U0171 code

There are no questions about this code yet. Tell us about your case: vehicle, symptoms and what you have already checked.