Error Code P0718 is defined as Input/Turbine Speed Sensor A Circuit Intermittent. This is a generic but uncommon trouble code, meaning it applies to all vehicles equipped with the OBD-II system, especially those made since 1996 up to present. This code appears in vehicles with transmission fluid temperature sensor, such as Ford, Honda, Mazda, Mercedes Benz, Volkswagen, etc. Specifications on the definition, troubleshooting, and repairs, of course, vary from one make and model to another.
Error Code P0718 is detected when the PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes) has detected an erratic voltage input signal coming from the circuit of the input (or turbine) speed sensor with designation A. Input, and turbine speed sensors are identical design-wise, and works the same. Manufacturers simply use variations in terminology to describe them.
An input/turbine speed sensor is usually electromagnetic; it’s a 3-wire sensor that works by monitoring the transmission input speed (recognized by the PCM as revolutions per minute or RPMs). It’s positioned near the back of the torque converter (at the transmission input shaft) and can be either threaded directly into the transmission case or mounted through a bolt/stud.
On the transmission main (or input) shaft, there are grooves specially designed (or a notched reluctor wheel). To complete the electromagnetic circuit, the input shaft (or reluctor wheel) passes in close proximity to the end of the sensor. As the grooves (or notches) pass by the sensor, the circuit is interrupted; forming an electronic pattern is formed. The PCM then receives this pattern formation as a waveform pattern that it further interprets as transmission input/turbine speed.
To determine the right input RPM speed, transmission input/turbine speed is compared with the engine load percentage, engine RPM speed, and transmission output speed. If the actual input/turbine speed fails to coincide with the desired input/turbine speed, then the Check Engine lamp will be activated, and an error code will be stored.
Error Code P071 means there’s an intermittent input circuit voltage for input/turbine speed sensor.
- Erratic speedometer (odometer) operation
- Inoperable speedometer (odometer)
- Transmission unable to shift properly
- Increase in fuel consumption
- Erratic shift or harsh points from the transmission
Potential causes for this code usually include:
- Faulty input sensor
- Burnt, damaged, or loose connectors and wiring
- Metal debris accumulated in magnetic sensor
- Defective valve body
- Defective or programming error in PCM
How to Check
To diagnose this code, you will need a diagnostic scanner, DVOM (digital volt/ohmmeter), a system wiring diagram, and a dependable vehicle information source. A portable oscilloscope can also help.
Start with a visual inspection of the wiring and connector system. Check for damages such as burnt, corroded, or disconnected wires and connectors. Repair or replace as necessary. Then, test the battery, including the cables and cable ends. Remove any sign of corrosion and secure the terminals (if applicable).
If both input and output sensor codes are present, then find the malfunctioning circuit using the scanner data stream. For a responsive data delivery, narrow the test to include only related data.
When removing the sensors, check for excessive metal debris on magnetic contact. Make sure to remove excessive debris before re-installing it again. When the sensors are removed, check the interruption grooves and reluctor wheel notches. Make sure to test individual sensors, use the DVOM and follow the specifications set by the manufacturer (you can find this in the service manual). Replace faulty sensors that don’t comply with the specifications.
Disconnect any related controllers before testing the resistance and continuity using the DVOM. Not doing this could cause damage to the controller. Test the system circuits for open or shorted wiring. Repair or replace any defective circuits. If all system circuits and sensors are working properly, then you can suspect a defect or programming error in the PCM.
How to Fix
As with most error code problems, repairs for this code depend entirely on the underlying issues. Some of the common fixes include:
- Replacement of the valve body
- Replacement of one or more shift solenoids
- Replacement of input/turbine speed sensor
- Repair or replacement of damaged, open, or shorted circuits or wirings
- Draining, flushing, and replacement of transmission fluid
- Replacement of engine coolant temperature sensor
Since this code comes with symptoms that affect the vehicle’s drivability and performance, it’s considered a serious problem. It may not keep the vehicle from being driven, but it can lead to a number of dangerous situations, such as unpredictable shifting, or stalling in traffic.
Excessive metallic debris (accumulated to the electromagnetic sensor) can cause erratic input/output speed sensor readings.
Sensor to reluctor clearance is extremely important, so make sure that mounting surfaces/threaded holes are clear of any debris or obstructions.
Be careful when removing the sensors, as transmission fluid may escape from the opening. Some input speed sensor wiring can also be damaged by transmission fluid leaks, so make sure to check it as well when replacing wiring and sensors.