The transmission control module has detected that the actual gear ratio in 4th gear does not match the expected value. The automatic transmission fails to achieve or maintain the correct ratio during 4th gear engagement.
Common causes
Faulty or stuck shift solenoid
Insufficient hydraulic pressure due to pump wear or internal leaks
Worn or burnt 4th gear clutch packs or bands
Clogged transmission filter or low fluid level
Faulty transmission input or output speed sensor
Contaminated or obstructed valve body
Symptoms
Check engine or transmission warning light on
Slipping or loss of drive in 4th gear
Transmission skipping 4th gear or entering limp mode
Transmission overheating
Abnormal noise during gear engagement
Severity
High: continued operation with an incorrect gear ratio causes accelerated clutch wear and risk of severe transmission damage.
What to do
Check automatic transmission fluid (ATF) level and condition
Read all related fault codes and review transmission live data
Test shift solenoids with a multimeter and scan tool
Measure transmission hydraulic pressure with a gauge
Inspect the valve body and replace defective solenoids