Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
DTC 85, Undefined Ratio Error (4L80-E): Notes
PCM calculates ratio based on transmission input speed and output speed sensor readings. PCM compares known transmission ratio to calculated ratio, for particular gear range selected. DTC 85 will set when:
- DTCs 21, 22, 24, 28, 71, 72 and 87 are not set.
- TP is greater than 25 percent.
- Not in Park/Neutral or 4th gear.
- Engine speed is greater than 300 RPM.
- Vehicle speed is greater than 7 MPH.
- All conditions are met for 2 seconds.
PCM will default to line pressure set to maximum and inhibits TCC operation.
- An out-of-range transmission range pressure switch could falsely indicate the actual transmission range.
- This test checks calculated ratio to determine if ratio is within parameters.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.