SCP (J1850) Invalid or Missing Data for Primary Id
Your vehicle's communication network isn't receiving proper data from a primary control module, like a missing radio signal between connected computers. This prevents the engine control unit from getting critical information needed to run the engine properly.
The ECU monitors the J1850 bus network for valid data packets from primary modules at regular intervals. When expected messages fail to arrive or contain invalid checksums within the timeout threshold, the fault is triggered. The ECU cannot function optimally without this inter-module communication.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| J1850 Bus Message Timeout | Valid data received every 10-100ms | No valid data or corrupted checksum for >500ms |
| Primary Module ID Response | Expected module ID present in message | Missing or unrecognized Primary ID in data packet |
Code U1191 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, U1191 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.