U0408

Invalid Data Received From Throttle Actuator Control Module

U0408 is a generic OBD-II network diagnostic trouble code: Invalid Data Received From Throttle Actuator Control Module. It is logged by the engine control unit when the throttle monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
U0408
Group
Network
System
Throttle
Severity
Warning (MIL on, possible limp mode)
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What U0408 means

U0408 is defined as invalid data received from Gear Shift Control Module B, distinguishing it from U0404 which refers to the primary gear shift module. Module B typically refers to a secondary gear shift controller in vehicles that use redundant or dual-channel shift-by-wire architectures, or alternatively to a second node on the same physical selector assembly.

The fault logic is the same as U0404: the module is present on the bus and is actively transmitting, but the data content fails plausibility checks, checksum verification, or message counter validation in the receiving module. Transmission control systems that rely on dual confirmation of selector position to release safety interlocks are particularly sensitive to this fault.

Diagnosis follows the same path as U0404 but targets the B-designated module or connector. On many platforms the A and B designations represent separate connectors or sub-modules within the selector assembly rather than physically separate control units, so harness inspection of the full selector wiring loom is essential.

Common causes

Most-frequently reported root causes when U0408 is logged.

  • 1
    Failing secondary Gear Shift Control Module (Module B) with corrupted output data.
  • 2
    Damaged wiring or connector on the Module B circuit within the selector harness.
  • 3
    Incorrect part number or software version installed for the Module B position.
  • 4
    Internal sensor failure in the secondary shift position detector.
  • 5
    Intermittent short or open in the harness between the selector assembly and the TCM.
  • 6
    Low voltage supply to Module B causing erratic signal generation.
  • 7
    CAN bus interference or overload affecting Module B message integrity.

Symptoms drivers notice

Transmission locked in limp mode or restricted to a limited gear range.
Shift-by-wire selector may not respond normally or shows wrong position.
MIL and transmission warning lamp may both be active.
Park-lock interlock may prevent shift out of Park.
Multiple gear shift or transmission codes present alongside U0408.

How to diagnose U0408

A typical diagnostic flow when this code is present.

  1. 1
    Read all DTCs from TCM, ECM, and any body modules before clearing.
  2. 2
    Identify which physical component is designated Module B for this vehicle platform using OEM documentation.
  3. 3
    Inspect all wiring and connector terminals associated with the gear selector assembly.
  4. 4
    Monitor live data from the gear shift module to observe selector position values.
  5. 5
    Verify supply voltage and ground continuity at the Module B connector.
  6. 6
    Check software version and variant coding stored in Module B.
  7. 7
    Repair wiring faults or replace/reprogram Module B only after confirming a hardware or software root cause.

Related network codes

Frequently asked questions

What is the difference between U0404 and U0408?

U0404 refers to invalid data from the primary gear shift module (Module A), while U0408 refers to a secondary or B-designated gear shift module.

Do U0404 and U0408 often appear together?

Yes, if the selector assembly harness is damaged both A and B module faults can set simultaneously, which helps localize the problem to the shared wiring.

Can U0408 cause a no-start condition?

In shift-by-wire vehicles, yes: if the system cannot confirm Park position from Module B, the starter interlock may prevent cranking.

Is Module B always a separate physical part?

Not always. On some platforms A and B refer to separate connectors or sensor channels within a single selector unit rather than two distinct modules.

Disabling U0408 in software

RaceTune can permanently disable U0408 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.

Permanent
The monitor is disabled in the ECU itself — not just cleared. It cannot return.
Tailored to your file
Each patch is matched to your specific software version — never a one-size-fits-all file.
Reversible
The original file is always preserved. Reflash the stock to return the ECU to factory state.

Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.

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