Error Code P077A is defined as Output Speed Sensor Circuit – Loss Of Direction Signal. The Error Code P077A is a generic trouble code; it applies to all vehicles equipped with the OBD-II, especially those made since 1996 up to present. It commonly applies but not limited to, vehicles from Chevrolet, Dodge, Ford, Honda, and Toyota.

When the P077A appears in the vehicle, this means the PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes), has determined a loss of directional signal from the output speed sensor.

Output speed sensors are usually made of electromagnetic variety. The transmission output shaft is affixed with a type of toothed reluctor ring or gear that the output speed sensor uses. As the out shaft spins, the reluctor ring spins as well. The reluctor ring’s raised teeth complete the output speed sensor circuit when it passes close to the proximity to the stationary electromagnetic sensor. Interruptions in the sensor circuit are created as the reluctor passes by the electromagnetic tip of the sensor. The PCM (and other controllers) receives these combinations of interruptions and circuit completions and reads in a waveform pattern, as a representation of the transmission output speed.

The sensor either threads straight into the transmission housing, or is held in a place with a bolt. To prevent fluid leaking from the sensor opening, an O-ring seal is used.

To determine whether the transmission is shifting properly and running efficiently, the PCM then compares transmission input speed with the output speed.

If the Error Code P077A is stored, this means the PCM has determined an input voltage signal from the output speed sensor which indicates that the reluctor ring is stuck. When the signal of the output speed sensor is unable to fluctuate, the PCM automatically assumes the reluctor ring has unexpectedly stopped moving.

Aside from the output speed sensor data, the PCM gets the vehicle speed input data and wheel speed input data. The PCM compares these signals to determine if the reluctor ring is moving effectively (based on the output speed sensor signal). If the output speed sensor signal is unable to move, this could be caused by either an electric or a mechanical problem.

Common Symptoms

Other transmission and ABS error codes may be stored, along with this error code. Here are some common symptoms of this code.

  • Intermittent operation of the speedometer/odometer
  • Delayed or slippage of the transmission
  • Abnormal shift patterns in the transmission
  • Activation or deactivation of the traction control system (if applicable)

Possible Causes

Common causes of this error code include:

  • Accumulation of metal debris on the output speed sensor
  • Defective output speed sensor
  • Damaged or worn reluctor ring
  • Open or shorted circuits or connectors (especially those close to the output speed sensor)
  • Failed mechanical transmission

How to Check

Diagnosis for this code starts with a visual inspection of the connectors and wiring systems. The output speed sensor must be removed and cleaned off of excessive metallic debris caused by the magnetic tip. Be careful when removing the sensor, as hot transmission fluid from the sensor opening may leak. Open or shorted circuits and connectors must be repaired.

After removing the sensor, check the reluctor ring. If the reluctor ring looks cracked, damaged, worn (missing teeth), then this may be the cause of the problem.

Then, if the problem shows other symptoms related to the transmission system, check the automatic transmission fluid. The fluid must look clean and not smell burnt. If the transmission fluid is low (one-quart), then refill it with the right fluid. Make sure you check for leaks. The transmission must have the right amount of transmission fluid and in good mechanical condition before you proceed on the next steps of the diagnosis.

Next, you will need a diagnostic scanner with an integrated oscilloscope, a DVOM (digital volt/ohmmeter), and a source of information about the error code.

Connect the scanner to the vehicle diagnostic port to retrieve all stored trouble codes, including the freeze-frame data. Before clearing the codes, take note of this information as you may need them throughout the diagnosis.

Look for the applicable TSB (Technical Service Bulletin) using vehicle information source. Find the TSB that matches the symptoms and codes stored in your vehicle. This will likely lead to a quick and accurate diagnosis.

Use the scanner data stream to check the output speed while test driving the vehicle. Narrow the stream to display only the pertinent fields to improve the speed and accuracy of data delivery. Inconsistent and erratic signals from the input and output speed sensors may be caused by wiring issues, problems in the electrical connector, or sensor problems.

Then, disconnect the output speed sensor. Do a resistance test using the DVOM. Check with the vehicle information source, it should show wiring diagrams, connector views, connector pin-out charts, and the recommendations and specifications from the manufacturer. If the output speed sensor doesn’t coincide with the specifications, then it must be defective.

Use the oscilloscope to get the live data from the output speed sensor. Check both the output speed sensor signal wire and ground wire. You may have to jack or lift the vehicle to perform this test. After lifting the drive wheels off the ground and the vehicle is secure, run the drivetrain up while looking at the waveform pattern from the oscilloscope. Look for any inconsistencies and glitches in the waveform pattern made by the output speed sensor signal.

Disconnect the connectors from the related controllers when checking for the circuit resistance and continuity tests with the DVOM. Make sure you don’t forget to do so, as it may result in damages in the controller.

How to Fix

  • Repair open or shorted connectors and wirings
  • Replacement of the reluctor ring
  • Cleaning the speed sensor off of metal debris
  • Refilling of transmission fluid, make sure there are no leaks
  • Repair or replace damaged wiring in the input and output speed sensors
  • Replace electrical connectors
  • Replacement of speed sensors
  • Repair of mechanical transmission failure

The condition that leads to this error code could lead to serious transmission failure. Thus, it is important to address this problem as soon as possible.