What it means
While in 8th gear, the measured input/output ratio does not match the calibrated value — slipping clutch, wrong gear actually engaged, or speed sensor data wrong.
At a glance
- Usually lights the check-engine light
How it works
The transmission control module (TCM) compares the transmission input speed with the output speed to check whether 8th gear is holding its expected ratio. This code sets when that ratio is outside the calibrated range, which can result from slipping inside the transmission or inaccurate speed-sensor data.
Common causes
Most likely first.
- Internal clutch or friction element slipping
- Low or degraded transmission fluid
- Hydraulic pressure loss or valve body fault
- Incorrect input or output speed sensor signal
- Internal transmission wear or damage
Symptoms
- Check engine or transmission warning light on
- Harsh, delayed, or missing shifts
- Possible limp mode (locked in one gear)
- Slipping or shuddering during gear changes
How to diagnose it
- Record freeze-frame data and review commanded gear, input speed, output speed, and related transmission codes to see when the 8th-gear ratio error occurred.
- Check the transmission fluid level and condition using the vehicle maker’s specified procedure, and look for leaks that could reduce hydraulic pressure.
- Inspect the input and output speed sensor connectors and wiring for looseness, corrosion, damage, or fluid intrusion.
- Compare live input- and output-speed readings during a safe road test to check whether either signal drops out or becomes implausible as 8th gear is commanded.
- If the speed signals are credible, have the transmission’s hydraulic pressure and 8th-gear friction elements assessed using manufacturer procedures.
Common fixes
- Repair a transmission fluid leak and correct the fluid level with the specified fluid.
- Replace degraded or incorrect transmission fluid if the service condition and manufacturer procedure support it.
- Repair damaged wiring or connections at the input or output speed sensor.
- Replace a confirmed faulty input or output speed sensor.
- Repair the valve body or internal friction elements if testing confirms pressure loss or 8th-gear slip.
Can I keep driving?
If the transmission slips, shudders, shifts harshly, or enters limp mode, avoid continued driving and arrange prompt repair; further slipping can worsen internal damage. If it shifts normally, limit driving to necessary short trips and avoid heavy loads or high-speed travel until diagnosed.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.