U0421 code meaning
Invalid Data Received From Suspension Control Module A
What U0421 means
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.
U0421 at a glance
- Check engine lightUsually not
- Emissions testNot an emissions code
- Limp modePossible
- Repair difficultyWorkshop only
- Typical labor0.3 to 2.5 hours
- Clears after repairWhen cleared with a scanner
Symptoms of U0421
- Service air suspension or leveling warning
- Ride height no longer adjusts
- Stability or brake system message alongside
Common causes of U0421
- Most likelySuspension module software hang, cleared by a power reset
- PossibleLow supply voltage or weak battery during start-up
- Less likelyInternal fault of the suspension control module
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Suspension control module a
What each part of U0421 means
| U | Network system: communication between the control modules (CAN bus). |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 21 | The exact fault in that area. |
What to do next
- Note U0421 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about U0421
What does U0421 mean?
U0421 means "Invalid Data Received From Suspension Control Module A". Suspension control module A runs air springs, ride height or adjustable dampers and broadcasts level and status data. It is a generic code, so it means the same on every make.
Is it safe to drive with U0421?
Drive gently and only to a workshop. With U0421 the car can go into limp mode (reduced power) to protect itself.
Will U0421 fail an emissions test?
U0421 is not an emissions code, so on its own it usually does not fail an emissions test. Other stored codes or a lit check engine light still can.
How hard is U0421 to fix?
Difficulty: workshop only. A typical repair takes 0.3 to 2.5 hours of work. This is a job for a workshop.
Will U0421 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.