Unexpected Mechanical Gear Disengagement


The PCM has received an input signal from the transmission output speed sensor that is either irrational, erratic, or in some way incorrect. Without a usable output speed signal the transmission cannot provide a proper shift strategy and the control module may assume (in some instances) that unexpected mechanical gear disengagement has occurred.

Code Set Parameters

In the event of unexpected mechanical gear disengagement, the output shaft speed sensor may appear (to the PCM) to be functioning erratically. While code setting parameters vary between manufacturers, most PCMs are programmed to expect output speed to increase gradually until a shift point is reached. If output speed fails to increase gradually or if the PCM receives an erratic signal (or no signal at all) a code will be stored and a malfunction indicator lamp may be illuminated. Some models may require multiple failure cycles in order for a malfunction indicator lamp to be illuminated.


The most noticeable symptoms may include a harsh shift, failure to shift, a decrease in fuel economy, the speedometer may fluctuate wildly or fail to operate, the engine may stall when coming to a stop, engine misfire, sag, stall, or hesitation, a stored code, and possibly an illuminated service engine soon lamp. An input or output speed sensor code often accompanies this code, as well. In some cases there may be no adverse symptoms noticed at all.

Common Causes

The most common causes of this code being stored are due to a defective output speed sensor or input speed sensor. Of course mechanical failures are also a possibility. Other common causes include shorted, open, corroded, or damaged wiring circuits or connectors, worn or faulty double clutch (and/or cover) components, worn, broken, or damaged gears and/or shafts, dirty or contaminated transmission fluid that restricts flow, a defective valve body, or a faulty transmission controller or PCM.

Common Misdiagnosis

Mistakes range from attempting to diagnose this type of code as an engine/turbine speed malfunction when it is in fact an internal mechanical transmission problem. Technicians also report that unnecessary vehicle speed sensor replacement is the result of one particularly common misdiagnosis.


  • A scanner (or code reader), a digital volt/ohmmeter, and possibly an oscilloscope will be helpful in diagnosing this code. Most OBD-II equipped automobiles utilize an output speed sensor to provide the PCM with a transmission output speed signal
  • If the output signal to the PCM becomes erratic in comparison to the input speed sensor signal, the PCM may interpret it as unexpected mechanical gear disengagement
  • Of course, the possibility of actual unexpected mechanical gear disengagement (due to gear or shaft failure) is also a distinct possibility. To perform a thorough diagnosis, any transmission leaks should be repaired and the transmission should be filled with the proper amount and type of fluid (if the fluid smells really "burnt" and you are experienced enough to detect it, you may go ahead and condemn the transmission)
  • The transmission should then be operated normally and rechecked
  • However, leaks that originate from the front seal (behind the torque converter) will likely require removal of the transmission, at which time you may want to consider a transmission rebuild if the mileage dictates. If the transmission appears to be in good working order, and doesn't appear to be overheating or low on fluid, suspect a faulty input or output speed sensor
  • On most applications the sensor either threads directly into the transmission housing or is mounted to the exterior of the transmission housing using a bolt
  • The sensor is typically of the electro magnetic design and uses a reluctor ring (or a specialized set of splines) on a shaft as reference points
  • As the output shaft is turned (by the engine) the electro magnetic sensor uses the spaces between the splines to provide voltage signal interruptions in the circuit
  • These interruptions are received by the PCM as square waveform patterns of varying degrees of voltage and translated into transmission output speed
  • The PCM then compares transmission output speed data with engine input speed data to determine shift patterns, as well as certain engine drivability functions. The typical output speed sensor uses a three-wire connector but check the vehicle manufacturer's wiring diagram to confirm the specific design of the vehicle in question
  • The first wire is a reference voltage signal (usually 5-volts), the second wire will normally be a ground wire, and the third wire will be a signal wire
  • As the reluctor ring passes by the electro magnetic sensor, the 5-volt reference signal is completed with the protruding metal surfaces
  • The recessed surfaces of the reluctor provide voltage interruptions and these interruptions are input to the PCM via the signal wire
  • Begin your diagnosis with a visual inspection of system wiring, connectors, and components
  • Repair or replace any open, shorted, damaged, or corroded items as required and retest the system to make sure that repairs were successful
  • Pay particular attention to wiring and connectors that have been contaminated due to engine oil or transmission fluid leaks, burned on hot exhaust pipes, or damaged due to road debris. If the system wiring, connectors, and components appear to be in normal working order, connect the scanner to the diagnostic connector and record all stored trouble codes and freeze frame data
  • This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored
  • After the codes are cleared, operate the vehicle to see if the code returns
  • If the code fails to immediately return, you may have an intermittent condition
  • Intermittent conditions can prove to be quite a challenge to diagnose and in extreme cases may have to be allowed to worsen before a correct diagnosis can be made
  • If the code immediately returns, check for reference voltage and ground signals at the output speed sensor
  • If either the reference voltage or ground circuits are open, use the digital volt/ohmmeter to check for continuity (disconnect all related control modules from the circuit before checking circuit resistance or controller damage may occur) and resistance in both circuits
  • Repair or replace system circuits/connectors as required and retest the system to ensure that repairs were successful. Using the manufacturer's wiring diagram for the input/output speed sensor, test all related circuits and the sensor for resistance and continuity and compare your findings with manufacturer's specifications
  • Repair or replace system circuitry, connectors, and/or components that fail to coincide with manufacturer's specs
  • Always retest the system to ensure a successful repair. If all system circuits are intact, connect the oscilloscope and observe live transmission output speed sensor data
  • Watch for glitches or "soft spots" in the wave form pattern and repair or replace system circuitry, connectors, or components as required. If all circuits coincide with manufacturer's specifications, and the waveform pattern is within acceptable limits, suspect a defective PCM
  • Remember that PCM failure is rare and replacement will require reprogramming.