Airbags

B0040 — Left-Side Airbag Circuit Fault (SRS)

Severity: high Drive with care, to a shop

What B0040 means

The SRS module constantly tests the resistance of each airbag igniter circuit. B0040 is set when the left-side circuit (driver, left side or third-row left airbag, depending on the vehicle) reads below the expected range, or when the module sees an internal fault in that control channel. The module then disables that airbag and turns on the SRS light.

Symptoms

  • SRS/airbag warning light on
  • Left-side airbag disabled by the module
  • Possible airbag fault message on the display
  • No engine performance change
  • Passenger airbag off light may stay on or flash

Possible causes

  1. Short or low resistance in the left airbag circuit
  2. Damaged or corroded wiring and connectors in the circuit
  3. Faulty airbag module or igniter
  4. Faulty impact sensor
  5. Internal SRS module or communication fault
  6. Connector under the seat unplugged or damaged after seat work

How to diagnose

  1. Scan all modules with an SRS-capable tool and save the freeze frame data
  2. Use the wiring diagram to identify which airbag the circuit serves
  3. Inspect connectors under the seats and near the steering column for damage, moisture and corrosion
  4. Ask whether the seat, trim, steering wheel or clock spring was recently serviced
  5. Check the clock spring if it is the driver airbag circuit
  6. Measure circuit resistance only as the service manual specifies; never probe an igniter directly
  7. Check SRS module power and ground
  8. Clear the code after the repair and confirm it does not return

Common fixes

  • Repair or replace damaged wiring and connectors
  • Replace a faulty clock spring
  • Replace a faulty airbag module or sensor
  • Replace or reflash the SRS module with the required coding

Questions and answers

Can I drive with B0040?

Yes, the car will run, but the left airbag may not deploy in a crash. Have it checked as soon as you can.

Can I test the airbag circuit myself?

Working on SRS circuits is risky because an airbag can deploy accidentally. Disconnect the battery, wait as the manual says, and never apply power to an igniter. If unsure, see a shop.

How do I clear B0040?

Fix the cause first, then clear the code with an SRS-capable scan tool. Without a repair it will come back.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.