AG4 Transmission Torque Modulation Fault
The transmission can't properly control engine torque during shifts, like a driver struggling to smoothly modulate the gas pedal while changing gears. This fault means the ECU detected a mismatch between requested and actual torque delivery to the transmission.
The ECM monitors torque converter clutch solenoid commands and actual transmission response via pressure sensors and speed sensors. It compares the requested torque reduction (to smooth shifts) against actual engine output. If the difference exceeds threshold for too long, the fault sets.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Torque Modulation Response Time | 50-200 milliseconds | >300 milliseconds or no response |
| Pressure Sensor Agreement | ±10 PSI variance | >15 PSI variance between expected and actual |
Code P1778 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, P1778 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.