Navigating the complexities of your vehicle’s onboard diagnostics can be daunting, especially when faced with a trouble code like P0708. As an automotive expert at autelfrance.com, I’m here to break down this code, specifically Eobd/obdii P0708, which signals a “Transmission Range Sensor ‘A’ Circuit High” issue. This article will provide a comprehensive understanding of this diagnostic trouble code, its symptoms, potential causes, and effective repair procedures to help you get back on the road with confidence.
Decoding the P0708 Code: Transmission Range Sensor “A” Circuit High
The eobd/obdii p0708 code is a generic powertrain code, meaning it’s applicable across a wide range of OBD-II equipped vehicles. It falls under the transmission sub-group and is classified as a type “B” DTC. This designation is important because it means your vehicle’s Powertrain Control Module (PCM) or Transmission Control Module (TCM) is quite discerning. The check engine light won’t illuminate until the fault condition is detected during two consecutive driving cycles (key on-off, then on-off again).
The heart of the issue lies with the Transmission Range Sensor (TRS), sometimes referred to as the inhibitor switch or Neutral Safety Switch. This sensor is crucial because it informs the PCM or TCM about the gear lever’s position – Park, Reverse, Neutral, Drive, etc. The P0708 code sets when the computer receives conflicting signals from the TRS, indicating two different gear positions simultaneously for more than 30 seconds. If this occurs twice in a row, the check engine light illuminates, and the transmission is often forced into a “fail-safe” or “limp-home” mode to protect itself from further damage.
Understanding the Transmission Range Sensor
The Transmission Range Sensor (TRS) is typically located on the transmission case, often where the shift linkage connects. It’s designed to accurately relay the driver’s gear selection to the vehicle’s computer. Different vehicles may utilize internal or external TRS designs.
Example of an external Transmission Range Sensor (TRS):
Image: An example of an external Transmission Range Sensor (TRS) commonly found in automatic transmissions.
Recognizing the Symptoms of a P0708 Code
When the eobd/obdii p0708 code is active, you’ll likely notice several symptoms, indicating that your vehicle isn’t operating as it should:
- Check Engine Light Illumination: This is the most obvious sign. The malfunction indicator lamp will light up on your dashboard.
- Reduced Engine Power: A significant symptom is a noticeable lack of power, especially when starting from a standstill. This occurs because the transmission might be stuck starting in a higher gear, like third gear, instead of first.
- Transmission Limp Mode: The transmission may enter a “limp-home” mode, limiting gear selection and overall performance to prevent further damage.
- Difficulty Shifting Gears: You might experience hard or erratic shifting between gears.
- Inability to Start in Park or Neutral: In some cases, the faulty sensor can prevent the engine from starting unless the gear selector is in a specific position.
- Reverse Lights Malfunction: The reverse lights might not activate when the vehicle is in reverse, or they might stay on constantly.
Severity of the P0708 Code
Ignoring the eobd/obdii p0708 code is not advisable. Continuing to drive with this issue can lead to severe transmission damage. Operating in limp mode for extended periods can cause undue stress and wear on transmission components. It’s crucial to address this problem promptly to prevent potentially expensive internal transmission repairs.
Common Causes of the P0708 Trouble Code
Several factors can trigger the eobd/obdii p0708 code. Pinpointing the exact cause is essential for effective repair. Here are the most common culprits:
- Faulty Transmission Range Sensor (TRS): This is the most frequent cause. The sensor itself may be malfunctioning due to internal wear, electrical issues, or damage.
- Misadjusted Transmission Shift Cable/Linkage: If the shift cable or linkage is not properly adjusted, it can cause the TRS to send incorrect signals to the PCM/TCM, leading to the P0708 code.
- Damaged Wiring or Connectors: Wiring harnesses or connectors leading to the TRS can become damaged, corroded, or disconnected. This can disrupt the signal flow and trigger the code. Look for chafed wires, broken connectors, or corrosion.
- Misadjusted Range Sensor: Even a new sensor needs to be correctly adjusted upon installation. Incorrect adjustment can lead to inaccurate readings and the P0708 code.
- PCM or TCM Failure (Rare): While less common, a faulty PCM or TCM can, in rare instances, be the cause. However, it’s crucial to rule out other possibilities first, as PCM/TCM failures related to range sensor circuits are infrequent.
Diagnosing and Repairing the P0708 Code: A Step-by-Step Approach
Diagnosing the eobd/obdii p0708 code requires a systematic approach. Here’s a breakdown of diagnostic and repair procedures:
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Initial Scan and Code Verification: Use an OBD-II scanner to confirm the presence of the P0708 code and check for any other related trouble codes. Clear the codes and test drive the vehicle to see if the code returns.
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Inspect the Transmission Range Sensor:
- Visual Inspection: Visually inspect the TRS, its wiring, and connectors for any signs of damage, corrosion, or loose connections.
- Sensor Adjustment Check: Verify the TRS adjustment according to the vehicle manufacturer’s specifications. Misadjustment is a common issue, especially after transmission work.
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Wiring and Circuit Testing:
- Voltage and Ground Checks: Use a digital multimeter (DMM) to check for proper voltage and ground at the TRS connector. Refer to the vehicle’s wiring diagram for pin assignments.
- Signal Circuit Testing: With the key in the “ON” position and the engine off (KOEO), back-probe the TRS signal circuits at the sensor connector. Have an assistant slowly shift through each gear position. Each signal circuit should show voltage in only one gear position. Voltage present in multiple gear positions indicates a likely faulty TRS.
- Continuity Testing: Check the continuity of the wiring between the TRS connector and the PCM/TCM. Look for open circuits or shorts to ground or voltage.
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Shift Linkage/Cable Inspection: Carefully inspect the transmission shift cable or linkage for proper adjustment, damage, or binding. Ensure smooth and full movement of the linkage.
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Sensor Replacement and Re-test: If the TRS is suspected to be faulty, replace it with a new sensor. Ensure the new sensor is properly adjusted according to the manufacturer’s instructions. After replacement, clear the DTCs and test drive the vehicle to verify the repair and ensure the code does not return.
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PCM/TCM Diagnosis (Last Resort): Only after thoroughly checking the sensor, wiring, and linkage should a PCM/TCM fault be considered. Before replacing the PCM/TCM, carefully inspect the TRS circuit for any shorts that might have damaged the module. Replacing a PCM/TCM is often costly and should be a last resort after all other possibilities are exhausted.
Important Note on PCM/TCM Failure: While PCM/TCM failure is rare in P0708 cases, it’s crucial to address any underlying issues that may have caused the module damage, such as shorts in the TRS circuit, before installing a new PCM/TCM.
Related OBD-II Codes:
Understanding related codes can provide further context. Codes commonly associated with P0708 and Transmission Range Sensor issues include: P0705, P0706, P0707, and P0709. Exploring these codes can offer additional insights into potential transmission-related problems.
Seeking Further Assistance
If you’re still encountering difficulties with the eobd/obdii p0708 code or require more in-depth troubleshooting guidance, consider seeking expert advice. Online automotive forums dedicated to car repair can be valuable resources.
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Disclaimer: This information is for informational purposes only and is not intended as professional repair advice. We are not responsible for any actions taken on your vehicle based on this information. All content is copyright protected.
