U0400 code meaning
Invalid Data Received
What U0400 means
U0400 is the unspecific entry of the invalid-data block. It says that a module receives messages from another node on time but rejects their content, without naming the sender; the codes from U0401 upward name the partner. GM's 2011 Saab 9-5 manual handles U0400 to U05FF in one procedure: the receiver doubts the integrity of incoming data, the test runs between 9 and 16 V while the network works, no circuit check is involved, and if the code stands alone the named module is reprogrammed, then replaced. Harley-Davidson shows the usual trigger in bulletin M1720: on 2021-2024 Pan America models, updating only the ECM leaves a U040000 that cannot be cleared, so the BCM must be flashed first. Lost contact is the U01xx block, software conflicts are U03xx. Effects depend on the module that stores it.
U0400 at a glance
- Check engine lightUsually not
- Emissions testNot an emissions code
- Limp modeNot expected
- Repair difficultyWorkshop only
- Typical labor0.3 to 2 hours
- Clears after repairWhen cleared with a scanner
Symptoms of U0400
- Code cannot be cleared or returns at once
- Functions of the storing module limited or warning lamp on, depending on the vehicle
Common causes of U0400
- Most likelyModules on different software levels after a partial update
- PossibleReplaced module not programmed or configured
- Less likelyDisturbed bus signal from poor connections or low supply voltage
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
CAN bus wiringControl module
What each part of U0400 means
| U | Network system: communication between the control modules (CAN bus). |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 00 | The exact fault in that area. |
What to do next
- Note U0400 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 U0400
What does U0400 mean?
U0400 means "Invalid Data Received". U0400 is the unspecific entry of the invalid-data block. It is a generic code, so it means the same on every make.
Is it safe to drive with U0400?
U0400 does not normally turn on the check engine light by itself. Watch for the symptoms above and have it checked soon.
Will U0400 fail an emissions test?
U0400 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 U0400 to fix?
Difficulty: workshop only. A typical repair takes 0.3 to 2 hours of work. This is a job for a workshop.
Will U0400 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.