SCP (J1850) Invalid or Missing Data for Vehicle Speed
Your vehicle's computer isn't receiving proper speed data from the J1850 communication network, like a GPS losing signal and not knowing how fast you're going. This prevents the engine and transmission from adjusting properly for current driving conditions.
The ECU monitors the J1850 serial data bus for valid vehicle speed signals from the speedometer or ABS module. It expects periodic speed updates within specific timing intervals and data formats. If the signal is missing, corrupted, or delayed beyond thresholds, the fault is triggered.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| J1850 Bus Speed Signal | Valid data every 10-100ms | No signal or invalid data for >500ms |
| Speed Data Value | 0-255 km/h range with valid checksum | Out of range or corrupted checksum |
Code U1040 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, U1040 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.