Chassis, generic

C0521 Multi-axis Acceleration Sensor Module B Range/Performance

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

Same fault type as C051C, but for a second inertial module B: its rates or accelerations lie outside what driving physics permits. No manufacturer explains C0521 publicly, and which unit is B on a given car is not documented either. A second unit is common on vehicles with automated driving, where Bosch offers IMUs up to two-fold ASIL-D and stresses redundancy. A GM patent explains what corrupts such readings: six bias terms and three mounting angles, which plant calibration only partly removes and which change over time; GM estimates them from satellite navigation data. Assistance features that rely on module B, and possibly stability control, are suspended while the code is present.

Likely causes

Module B loose, tilted or bracket bent after a repair or impact
Bias of the accelerometers or gyros drifted out of tolerance
Defective sensing element inside module B

Symptoms

  • Driver assistance or ESC warning message
  • Adaptive cruise or lane functions unavailable
  • Module B reading not zero at standstill (scan tool)

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€80–1,200
Time0.5–2.5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

multi axis accel module b

Related codes

C051CC0522C0523C0524C0525

References: patents.google.com, bosch-mobility.com, patents.google.com.