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 86, Low 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 86 will set when:
- DTCs 21, 22, 24, 28, 71, 72 and 74 are not set.
- Not in Park/Neutral or Reverse.
- TP is greater than 25 percent.
- Engine speed is greater than 300 RPM.
- Vehicle speed is greater than 7 MPH.
- Trans. gear ratio is less than 1.06 in 1st or 2nd gear.
- All conditions are met for 2 seconds.
PCM will default to 2nd gear, line pressure set to maximum and inhibits TCC operation.
- An out-of-range transmission range pressure switch could falsely indicate actual transmission range.
- This test compares known ratio for a commanded gear to calculated ratio displayed on scan tester.
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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.
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