Volvo S90 & V90 BLIS Sensor Failure: Fixing the 'Service Required' Warning (2017-2021)
This guide explains the common causes of blind spot system failure in the 2017-2021 Volvo S90 and 2017-2019 Volvo V90, from simple fixes to the critical need for professional calibration.
- Before assuming the sensor is broken, thoroughly clean the rear bumper corners, as dirt, snow, or ice can temporarily disable the system.
- Replacement is not a DIY job for most people because the new sensor MUST be calibrated by a dealer or qualified shop with Volvo-specific software (VIDA).
- When buying a used part, you must match the part number for the correct side (this guide is for the passenger side) or use a verified superseded number like 31665693.
- Failure is often caused by minor physical damage from a bumper impact or by water getting inside the sensor housing.
Common Failure Symptoms
When the passenger-side blind spot sensor fails on a Volvo S90 or V90, you will likely notice one or more of these specific symptoms:
- "BLIS Service Required" or "BLIS and Cross Traffic Alert Disabled" Message: This is the most common indicator. A message will appear on the driver information display, alerting you that the system is offline.
- Mirror Indicator Light Malfunction: The orange warning light in the passenger-side mirror may stay illuminated at all times, or it may fail to light up when a car is in your blind spot.
- False Alerts: The system may trigger warnings when no vehicle is present.
- System Inoperative in Bad Weather: While temporary deactivation is normal if the sensor is blocked by heavy snow, ice, or mud, a complete failure may occur if water has penetrated the sensor's housing.
Diagnosing the Problem: Is It Broken or Just Dirty?
Before assuming the sensor has failed, it's important to perform a few simple checks, as these systems can be temporarily disabled by obstructions.
- Clean the Bumper: The radar sensors are located behind the rear bumper cover, near the corners. Thoroughly wash this area to remove any dirt, road salt, snow, or ice that could be blocking the radar signal. After cleaning, drive the vehicle for a few minutes above 10 mph to see if the system resets itself.
- Inspect for Physical Damage: Even a minor bump in a parking lot can damage the sensor or its mounting bracket behind the bumper cover. Look for any cracks, scuffs, or misalignment on the bumper itself, as this could indicate a hidden problem.
- Check for Software Glitches: In some cases, a simple vehicle restart can clear a temporary electronic glitch. If the warning message disappears and does not return, it may have been a one-time fault.
- Scan for Diagnostic Trouble Codes (DTCs): If the warning persists, the system has likely stored a fault code. A professional scan tool can read codes from the Body Control Module (BCM) or Active Safety Domain module, which can pinpoint the exact nature of the failure (e.g., a communication error, an internal sensor fault, or a calibration issue).
Pro Tip: A "BLIS Service Required" message that appears intermittently, especially during heavy rain or after a car wash, often points to a failing seal on the sensor unit, allowing moisture inside. A persistent, constant warning is more likely a sign of complete sensor failure or physical damage.
Buying a Used Blind Spot Radar Sensor: What to Check
A used OEM sensor can be a cost-effective alternative to a new part, but careful inspection is crucial for these sensitive electronic modules. Aftermarket options are generally not available or recommended due to complex integration and calibration requirements.
1. Verify the Part Number
This is the most critical step. This guide covers the right (RH) passenger side sensor. The left and right sensors are not interchangeable. Over the years, Volvo has updated this part multiple times. You must match your original part number or use a verified superseded number. Known part numbers for the right-hand sensor include:
- Initial Part Number: 31499940
- Superseded By: 31687390
- Latest Known Part Number: 31665693
A seller may list any of these numbers. Parts with the number 31665693 will replace all previous versions for the specified vehicles.
2. Physical Inspection
When you have the used part in hand, check for:
- Cracks or Damage: Inspect the black plastic housing for any cracks, deep scratches, or signs of impact. A compromised housing can let in water, which is a primary cause of failure.
- Connector Pins: Look inside the electrical connector. The pins should be straight, clean, and free of any green or white corrosion.
- Mounting Points: Ensure all mounting tabs and brackets are intact. A broken bracket can prevent the sensor from being aimed correctly, rendering it useless even if it functions electronically.
Replacement Overview & Calibration
Replacing the blind spot sensor is a complex job that is not recommended for most DIY mechanics due to the calibration requirements.
- Difficulty: 4 out of 5
- Procedure: The rear bumper cover must be completely removed to access the sensor, which is bolted to a bracket in the corner of the vehicle. This involves removing screws in the wheel wells, under the car, and clips inside the trunk or hatch area.
- Calibration is Mandatory: A new or used sensor will not work correctly until it is calibrated. This process tells the vehicle's computer the exact position and angle of the sensor so it can accurately detect objects. Calibration requires specialized diagnostic software like Volvo's VIDA and may involve a specific road test procedure. Attempting to install the sensor without proper calibration will result in the "BLIS Service Required" message remaining on the dash.
Warning: Any repair to the bumper or repainting in the sensor area requires recalibration. An improperly aimed sensor can fail to detect a vehicle in your blind spot or give false warnings, increasing the risk of an accident.
Cost Comparison
The mandatory calibration is a significant part of the total replacement cost.
| Part/Service | Estimated Cost |
|---|---|
| New OEM Sensor | $550 - $750 |
| New Aftermarket Sensor | Not Available |
| Used OEM Sensor | $75 - $200 |
| Shop Labor (Bumper R&R) | $250 - $400 |
| Shop Labor (Calibration) | $200 - $400 |
| Total Professional Installation (Used Part) | $525 - $1,000+ |
| Total Professional Installation (New Part) | $1,000 - $1,550+ |
Frequently Asked Questions (FAQ)
Can I just plug in a new sensor and have it work?
No. The blind spot system requires a software calibration procedure after the sensor is replaced. Without this step, the car's computer will not recognize the new part, and the system will remain disabled.
My BLIS warning came on in the snow. Is my sensor broken?
Not necessarily. The radar sensors can be blocked by a buildup of ice, snow, or thick mud on the bumper. This will cause a temporary shutdown and a warning message. Try cleaning the rear corners of the bumper thoroughly. If the warning disappears after cleaning and driving, the sensor is likely fine.
Is the driver's side sensor the same as the passenger's side?
No, they are different parts with unique part numbers. This guide and the part numbers listed are for the right-hand (passenger side) sensor only. The left-hand sensor has a different part number, such as 31665692.
Are there any recalls for this issue?
There are no specific recalls for this BLIS sensor failure. However, some S90 and V90 models were part of a recall (NHTSA 25V908) for a software issue that could cause the rearview camera to fail. While unrelated to the sensor itself, it highlights the complexity of the vehicle's ADAS software.
Technical Specifications
OEM Part Numbers: 31499940 31687390 31665693
Radar-based sensor for Blind Spot Information System (BLIS) and Cross Traffic Alert (CTA). Located at the right rear (passenger side) corner of the vehicle, behind the bumper cover.
We Have This Part in Stock
The information in this article is provided for general reference and educational purposes only. Vehicle specifications, procedures, and part compatibility can vary by production date, trim level, and region. Always consult your vehicle's factory service manual and verify part numbers before purchasing or performing repairs. Safety-critical components such as airbags, seat belts, and braking systems should be installed by a qualified professional.
- Volvo V90:
- Volvo S90: