OBD-II trouble code
B009A: Left Side Restraints Sensor 4 (Subfault)
The restraints module has a fault with what it calls left side sensor 4. Before anything else, note the number: most vehicles do not have four side sensors on one side, and confirming that this one does is the step that decides whether you are chasing a component or a configuration.
Quick facts
- System
- Body
- Category
- Airbag / SRS Restraints
- Severity
- High severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $100 – $700
- DIY difficulty
- Shop recommended
What does B009A mean?
B009A is stored by the airbag control module and refers to a satellite crash sensor on the left flank of the vehicle. The character after B009 is the letter A rather than a digit — the code numbering here is hexadecimal, which is why the range runs B0099, B009A, B009B and so on rather than stopping at nine.
The useful information is in the count. A large number of vehicles carry one or two side impact sensors per side, mounted in the B-pillar or low in the door, and that is enough for the module to answer a simple question: is a side impact happening, and is it hard enough to deploy. A vehicle numbering as far as sensor 4 on a single side is doing something more ambitious. It has a distributed sensing array along the flank, and the reason to build one is discrimination — not merely detecting that the side has been struck, but working out roughly where along the vehicle the strike landed and how fast the intrusion is developing, so the module can choose which restraints to fire and in what order. A hit at the front door and a hit at the rear wheel arch call for different responses, and a single sensor cannot tell them apart.
That leads directly to the first thing worth checking, and it is not electrical. A code naming sensor 4 on a vehicle that was only ever built with three sensors per side is not a sensor fault at all. It is the module expecting hardware the vehicle does not have, which happens after a replacement module has been fitted without being configured to the vehicle's build, after a module has been coded from the wrong donor vehicle's data, or after a repair has used a part from a higher trim level. The symptom byte stored with the code will often say the circuit is open, which reads convincingly as a wiring fault, when in fact there is nothing at the end of the wire because there was never meant to be. Establishing from build data how many left side sensors this vehicle actually has, before opening a single trim panel, is worth the ten minutes it takes.
Where the sensor genuinely exists, the code means the module is not getting valid data from it. The vehicle drives normally and nothing about the powertrain is affected, but side impact protection on the left may not behave as designed, and the module will generally have disabled part of the system rather than risk acting on data it does not trust. Restraint circuits contain pyrotechnic devices, so the system must be safely powered down before anything is probed or unplugged.
Common causes
- Control module configuration that does not match the vehicle's actual sensor count, common after a replacement or wrongly coded module
- Sensor mounting bolt loose or the sensor not fastened to a rigid structural surface, so it cannot read intrusion correctly
- Corroded or water-intruded connector at the sensor
- Open, short or high resistance in the sensor's circuit back to the module
- Failed satellite sensor
- Previous side impact repair that disturbed the sensor, its bracket or its harness routing
- Aftermarket accessory installation that disturbed a pillar or rocker harness
- Damaged harness where it passes through a door or pillar and flexes with use
Symptoms
- Airbag or SRS warning light illuminated and staying on
- B009A stored in the restraints module with a symptom byte, frequently reported as an open circuit
- No effect whatsoever on how the engine or transmission behaves
- Code returning immediately after being cleared, which is characteristic of a configuration mismatch rather than an intermittent fault
- Other left side sensor codes stored at the same time
- Fault appearing shortly after a control module was replaced or reprogrammed
Diagnostic steps
- 1.Establish how many left side restraints sensors this vehicle was actually built with, using build data or the manufacturer's parts information. A code for sensor 4 on a three-sensor vehicle is a configuration problem and no amount of wiring work will resolve it.
- 2.Check whether the restraints module has been replaced, reprogrammed or coded recently. A module configured for a different build will report sensors that were never fitted.
- 3.Record the full symptom byte alongside the code, and record every other restraints code stored with it. The pattern matters more than any single entry.
- 4.With the system safely disabled following the manufacturer's procedure, locate the sensor. Position varies considerably by make — door, B-pillar, C-pillar or rocker are all used.
- 5.Check the sensor's mounting before its wiring. A satellite sensor reads structural deceleration and must be solidly fastened to structure; a loose bolt or a corroded mounting face produces faults that look electrical.
- 6.Inspect the connector for moisture, corrosion and backed-out terminals, and check the harness where it flexes.
- 7.Verify supply and ground at the sensor against specification before condemning the sensor itself.
- 8.After any repair, clear the codes and confirm the warning light extinguishes and stays out through a drive cycle.
Repair cost
$100 – $700
If the cause is a configuration mismatch, the fix is scan tool time to code the module correctly and typically runs $100 to $250 with no parts at all. A connector or harness repair is $120 to $400. A satellite sensor replacement with the required system checks is usually $250 to $600 depending on how much trim has to come out to reach it. SRS diagnostic time alone is commonly $100 to $200. This is professional work in every case because of the pyrotechnic devices involved.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with airbag / srs crash sensor replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
Leave this one to a qualified shop. It typically involves emissions-critical components, refrigerant handling, or other work that requires manufacturer-grade tooling, training, or certification. DIY attempts often produce a more expensive problem than the original code.