P0708 OBD-II: Transmission Range Sensor "A" Circuit High

What Does P0708 Code Mean?

OBD-II Code P0708 is defined as a Transmission Range Sensor Circuit High Input

The purpose of the automatic transmission is to match the engine's optimum power and torque characteristics to the driver's desired rate of acceleration and speed by auto-selecting different gear ratios or 'speeds' to power the wheels. The Transmission Range Sensor (also referred to as the PRNDL input and/or neutral safety switch) tells the transmission control module (TCM) and the powertrain control module (PCM) that the transmission is in park, reverse, neutral, drive, low, 2nd, 3rd etc. It is attached directly to the manual valve of the transmission, so when the shift lever is moved from park to drive, the Transmission Range Sensor communicates this critical information to the above mentioned vehicle computer systems.

Symptom

• Check Engine Light will illuminate
• Vehicle may not shift properly
• Vehicle may not go into gear
• Decrease in fuel economy
• In unusual cases, there are no adverse conditions noticed by the driver
• In some cases, there may be performance problems

Common Problems That Trigger the P0708 Code

• Defective Transmission Range Sensor (PRNDL input)
• Defective Transmission Range Sensor (PRNDL input) wiring or connector
• Defective Manual Valve
• Defective Manual Valve linkage
• Dirty transmission fluid that restricts the hydraulic passages

Common Misdiagnoses

• Internal Transmission problem
• Driveline problem

Polluting Gases Expelled

• HCs (Hydrocarbons): Unburned droplets of raw fuel that smell, affect breathing, and contribute to smog
• CO (Carbon Monoxide): Partially burned fuel that is an odorless and deadly poisonous gas
• NOX (Oxides of Nitrogen): One of the two ingredients that, when exposed to sunlight, cause smog

P0708 Diagnostic Theory for Shops and Technicians

When the code P0708 is set in the Powertrain Computer, it means that the Powertrain Computer or PCM is not seeing a clear, rational change in the data and electrical signals sent from the Transmission Range Sensor, especially after the vehicle is in motion. It usually means that the range sensor is indicating a high voltage drop or high resistance when the sensor should be reading continuity.

When diagnosing a P0708 code, it is important to record the freeze frame information and then to duplicate the code setting conditions with a test drive at speeds around 15-35 mph. Start out from a dead stop and gently accelerate. Watch the scan data to see if the Range Sensor is reporting accurate information to the TCM and PCM. If the Range Sensor is indicating a high voltage drop or no information to the TCM/PCM, then you may have located a problem that could set the P0708 code. The next step is to start doing pinpoint tests i.e voltage drops and continuity tests of the Transmission Range Sensor. I generally back probe the Range Sensor, so I bypass the wiring harness. This accomplishes 2 things, I'm isolating the sensor from the wring harness and checking the quality of the connector and it's connection. And, ALWAYS check the condition of the fluid. Is it dirty, brown and burnt or is it that clear cranberry color?