SCP (J1850) Invalid or Missing Data for Primary Id
Your vehicle's main computer can't receive or understand critical data being sent over the SCP communication bus, like a phone unable to decode an incoming text message. This prevents the ECU from coordinating with other modules and can cause various system failures.
The ECU monitors incoming SCP (J1850) bus messages from other vehicle modules such as transmission control, ABS, and body computers. It expects valid data frames at regular intervals with correct checksums and formatting. When the ECU fails to receive expected primary ID messages or detects corrupted data, it triggers this fault.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| SCP Message Reception Timeout | Valid messages received within 100-500ms intervals | No message or timeout exceeding threshold |
| Data Checksum/Integrity | CRC checksum matches expected value | Checksum mismatch or invalid data format |
Code U1103 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, U1103 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.