Meaning

The TCM has detected that the transmission ratio measured in first gear does not match the expected value. The module calculates the ratio by comparing input shaft speed to output shaft speed during first gear engagement. The detected deviation exceeds the tolerance threshold defined by the manufacturer.

Common causes

• Worn or slipping clutch pack or band for first gear
• Stuck or failed shift solenoid for first gear
• Low ATF level or degraded/contaminated fluid
• Valve body fault causing insufficient hydraulic pressure in the first gear circuit
• Faulty input or output speed sensor
• Mechanical internal damage to the automatic transmission

Symptoms

• MIL illuminated
• Transmission locked in limp-home mode
• Slipping or harsh engagement of first gear
• Engine RPM rising without proportional vehicle movement
• Vibration or shudder when pulling away from standstill
• Reduced engine power output

Severity

What to do

1. Check ATF level and condition; replace if degraded or contaminated
2. Read all stored DTCs and monitor live input/output shaft speed data with a diagnostic scanner
3. Test resistance and operation of the first-gear shift solenoid
4. Inspect the valve body for corrosion, blockages, or hydraulic piston wear
5. If hydraulic and electrical components test within specification, perform an internal transmission teardown to assess first-gear clutch pack wear

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