P1778 AG4 Torque Modulation Module Failure
P1778 is a Nissan/Infiniti-specific code indicating a fault in the torque modulation module of the AG4 4-speed automatic transmission. This code points to an anomaly in the torque converter control system or a communication issue between the Transmission Control Module (TCM) and Engine Control Module (ECM) regarding torque management. Proper torque modulation is critical for torque converter clutch control, smooth gear shifts, and drivetrain protection.
01. Most Common Causes
- Faulty torque converter clutch (TCC) solenoid
- Malfunctioning transmission input/output speed sensor
- Defective Transmission Control Module (TCM)
- Damaged wiring or connectors between TCM and ECM
- Internal torque converter wear or mechanical failure
- Outdated or corrupted TCM software
- Faulty accelerator pedal position sensor (APS) or electronic throttle
02. Symptoms You Will Notice
- Check Engine Light illuminated
- Harsh or delayed shifts
- Torque converter clutch not engaging or slipping
- Reduced fuel economy
- Transmission overheating
- Vehicle may enter limp mode
03. Step-by-Step Diagnosis
Scan for codes and verify P1778 is present
Inspect transmission fluid level and condition
Test TCC solenoid resistance and operation
Check input/output speed sensor signals
Examine wiring and connectors for damage or corrosion
Update TCM software if available
Perform torque converter stall test to assess mechanical condition
1. Frequently Asked Questions
Is it safe to drive?▼
Driving with P1778 is not recommended as it can cause harsh shifting, transmission overheating, and potential internal damage. Have the vehicle diagnosed and repaired promptly.
How much does it cost to fix?▼
Repair costs vary widely depending on the root cause, ranging from $150 for a solenoid replacement to $2,500+ for a torque converter or TCM replacement.