Low, degraded or contaminated transmission fluid causing poor clutch pressure; Worn or burnt clutch packs or bands used in reverse; Faulty shift or pressure control solenoid, or sticking valve body valve; Faulty input or output speed sensor, or damaged sensor wiring and connectors; Internal hydraulic leak, such as worn seals or a leaking valve body gasket, reducing line pressure; Outdated or faulty transmission control unit calibration where the manufacturer has a known fix for this fault; Internal mechanical damage to the gear set or torque converter.

P077F

Reverse 2 Incorrect Ratio.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
7: AreaTransmission
7F: FaultThe specific fault in that area

What it means

P077F means the transmission control unit has detected an incorrect gear ratio while the transmission is in the second reverse gear or reverse range configuration. It calculates the ratio from input (turbine) speed and output speed and found it did not match the expected value, which usually points to slip or a hydraulic or sensor fault. Not every transmission has a Reverse 2, so the exact gear or clutch combination depends on the model and gearbox type.

Symptoms

Slipping or flaring when reversing
Delayed or harsh engagement of reverse
Engine revs rise but the car barely moves backwards
Gearbox warning light or limp mode
Burning smell from the transmission or dark fluid
Other gear ratio or speed sensor codes stored alongside

Possible causes

Low, degraded or contaminated transmission fluid causing poor clutch pressurelikely
Worn or burnt clutch packs or bands used in reverselikely
Faulty shift or pressure control solenoid, or sticking valve body valvelikely
Faulty input or output speed sensor, or damaged sensor wiring and connectorspossible
Internal hydraulic leak, such as worn seals or a leaking valve body gasket, reducing line pressurepossible
Outdated or faulty transmission control unit calibration where the manufacturer has a known fix for this faultrare
Internal mechanical damage to the gear set or torque converterrare

How to diagnose

Read all stored codes and freeze frame data, noting gear selected, speeds, fluid temperature and whether other ratio or solenoid codes are present
Check the transmission fluid level, condition and smell, following the manufacturer's procedure for the model, as some transmissions are sealed
Inspect wiring, connectors and the sensor area on the transmission for damage, oil ingress or corrosion
Compare live input and output speed sensor data against vehicle speed and engine speed for plausibility
Road test or stall test in reverse as the manufacturer allows, watching live ratio and slip values
Check solenoid commanded versus actual pressure and test solenoid resistance against specification
Check for any manufacturer service information or calibration updates relating to this code for the specific model
Carry out a pressure test or drop the pan to inspect for debris and clutch material if the fault persists

How to fix

Correct the fluid level or replace degraded fluid and filter using the specified type
Repair or replace damaged wiring, connectors or faulty speed sensors
Replace the faulty solenoid or repair the valve body as required
Overhaul or replace the transmission with worn clutches, seals or internal components
Apply a manufacturer-issued control unit calibration update only if one is documented for this fault

Can I keep driving?

Medium severity

The car can usually be driven carefully for a short time, but avoid reversing under load and avoid towing. Continued slip can burn the clutches and cause expensive damage, so have it checked soon.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.