System: Network (U-Code)
SAE Standard (Generic)
Family: Lost Comm Chassis ABS Module
Code U0121: Lost Communication With Anti-Lock Brake System (ABS) Control Module A
Diagnostic & Technical Explanation
U0121 indicates loss of contact with a chassis-dynamics module -- ABS/stability control, steering angle sensor, ride-height/suspension control, power steering, or a differential/brake-blending actuator -- all mounted low near the subframe, wheel wells, or under-floor tunnel where connectors take the worst exposure to road salt, water spray, and vibration of any module class. Unlike cabin electronics, these connectors are often sealed but still fail at the seal itself after years of thermal cycling, so a corroded or water-intruded low-mount connector is the dominant cause ahead of any harness break.
First-listed procedure
Rank #1
Inspect the low-mount connector for water intrusion and corrosion
Easy DIY
This is the first procedure in the authored
consideration order, not a measured cause likelihood. Execute it only when its prerequisites
and diagnostic findings support it. Its cost and labor estimates (where known), the step-by-step procedure and the
remaining 3 procedure(s) are in the licensed dataset. Ordering context.
Full family: Lost Comm Chassis ABS Module
What this page does not show
This page publishes the fault mechanism and the code index. The licensed dataset adds, for
every one of the 15,886 codes, the full authored repair set (56,729 procedures in total)
with the fields deliberately withheld here:
- Procedures 2..n after the first-listed procedure
- Aftermarket parts cost ranges in USD (
est_parts_cost_min/max_usd)
- Professional labor-hour estimates (
est_labor_hours)
- Step-by-step diagnostic and repair instructions
- Replacement-part mappings and catalog lookups
- 6,637 manufacturer-specific OEM codes across 32 makes ($149 tier)