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U0421 Invalid Data Received From Suspension Control Module A

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

Suspension control module A runs air springs, ride height or adjustable dampers and broadcasts level and status data. U0421 tells that a receiving module gets these frames but finds them corrupt or implausible. GM spells out one receiver: in the 2025 tables the electronic brake module watches the vehicle level control frames for a frozen rolling counter or a wrong protection value, sets the fault at 8 errors in a window of 16 and does the same with 3 of 16 for the top speed limit sent by the automatic level control; all are Type C, so no MIL. FCA documents U0421 on 2019-2020 Ram 2500/3500 with air suspension: before an air suspension module is replaced, pull its 20 A fuse F50, clear U0132, U0421 and U0422 and road test, and replace only if they return. Ride height may stay fixed. U0132 is lost contact.

Likely causes

Suspension module software hang, cleared by a power reset
Low supply voltage or weak battery during start-up
Internal fault of the suspension control module

Symptoms

  • Service air suspension or leveling warning
  • Ride height no longer adjusts
  • Stability or brake system message alongside

Repair estimate

DifficultyShop_onlyWorkshop job
Parts and labour€50–900
Time0.3–2.5 h

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

Parts involved

suspension control module a

Related codes

U0132U0321U043AU0422

References: gsi.ext.gm.com, static.nhtsa.gov.