Understanding OBD2 Code P704: Transmission Range Sensor Circuit Malfunction

The Obd2 Code P704 indicates a problem with your vehicle’s Transmission Range Sensor (TRS) circuit, a critical component for automatic transmission vehicles. This article dives deep into the P704 code, explaining its causes, symptoms, diagnostic procedures, and potential solutions, empowering you to address this issue effectively.

What is OBD2 Code P704?

The P704 code, “Transmission Range Sensor/Switch Input Circuit Malfunction,” signals a problem within the electrical circuit that connects the TRS to the Powertrain Control Module (PCM) or Transmission Control Module (TCM). The TRS, also known as the neutral safety switch or gear selector switch, tells the PCM or TCM the position of your gear shifter (Park, Reverse, Neutral, Drive, etc.). This information is crucial for various functions, including starting the vehicle, controlling shift timing, and activating the backup lights. A malfunction in the TRS circuit can disrupt these functions and lead to drivability issues.

Symptoms of a P704 Code

Experiencing a P704 code can manifest in several ways, some more noticeable than others. Common symptoms include:

Check Engine Light illuminated: This is the most common and often the first indicator of a problem.
Intermittent starting problems: The vehicle may refuse to start in Park or Neutral, requiring you to try different gear positions.
Erratic shifting: The transmission may shift harshly, slip between gears, or delay shifting.
Improper gear indicator readings: The gear indicator on the dashboard may display the wrong gear or flash intermittently.
Backup lights malfunction: The backup lights may not turn on when shifting into Reverse.
Limp mode: In some cases, the vehicle may enter limp mode, limiting speed and performance to prevent further damage.

OBD2 Code P704: Common Symptoms

Causes of a P704 Code

Several factors can contribute to a P704 code, ranging from simple electrical issues to more complex mechanical problems:

Faulty TRS: The most common culprit is a malfunctioning TRS due to wear and tear, internal damage, or corrosion.
Damaged wiring or connector: Broken, corroded, or loose wires or connectors in the TRS circuit can disrupt the signal transmission.
Short circuit or open circuit: A short circuit or open circuit in the wiring harness can prevent the TRS from communicating with the PCM/TCM.
Low transmission fluid: In some cases, low transmission fluid levels can indirectly affect the TRS operation.
Faulty PCM/TCM: While less common, a faulty PCM/TCM can also cause communication problems with the TRS.

Diagnosing OBD2 Code P704

Proper diagnosis is crucial for effectively resolving the P704 issue. Here’s a step-by-step guide:

Retrieve the code: Use an OBD2 scanner to read the stored trouble codes.
Inspect the TRS connector: Check for any loose, damaged, or corroded connections. Clean and tighten or replace as needed.
Check the wiring harness: Visually inspect the wiring harness for any signs of damage, chafing, or shorts.
Test the TRS with a multimeter: Following manufacturer specifications, test the TRS for resistance and voltage to ensure it’s functioning correctly.
Check transmission fluid level: Ensure the transmission fluid is at the proper level.
Test the PCM/TCM: If all other components check out, a faulty PCM/TCM might be the cause. This requires advanced diagnostic tools.

OBD2 Code P704: Diagnostic Steps

How to Fix OBD2 Code P704

Based on the diagnosis, the solution may involve:

Replacing the TRS: This is the most common fix for a P704 code.
Repairing or replacing wiring or connectors: Fixing damaged wires or connectors restores proper signal transmission.
Addressing transmission fluid issues: Adding or changing transmission fluid can resolve issues related to low fluid levels.
Replacing or reprogramming the PCM/TCM: This is a more complex procedure and should be performed by a qualified technician.

Conclusion: Addressing P704 Effectively

Understanding the OBD2 code P704 is crucial for maintaining your vehicle’s performance and safety. By addressing the underlying causes of the TRS circuit malfunction, you can prevent further complications and ensure smooth operation. Don’t ignore the P704 code; taking prompt action will keep your vehicle running smoothly.

FAQ

Can I drive with a P704 code? While you might be able to drive, it’s not recommended. It could lead to further damage or safety concerns.
How much does it cost to replace a TRS? The cost varies depending on the vehicle make and model but generally ranges from $100 to $300, including parts and labor.
Is the TRS the same as the neutral safety switch? Yes, the TRS is often referred to as the neutral safety switch or gear selector switch.
Can I replace the TRS myself? It’s possible, but it requires some mechanical aptitude and the right tools.
What other codes are related to the P704? Other related codes might include P0705, P0706, and P0707, indicating issues with other parts of the transmission range sensor circuit.
How do I prevent future P704 codes? Regular vehicle maintenance, including checking transmission fluid levels and inspecting wiring, can help prevent future problems.
What if the P704 code returns after replacing the TRS? If the code returns, it suggests a more complex issue, such as a wiring problem or a faulty PCM/TCM. Further diagnosis is required.

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