What does it mean?
This diagnostic trouble code (DTC) is a generic powertrain code, meaning it applies to all vehicles manufactured from 1996 onwards (Ford, Honda, Mazda, Mercedes, VW, etc.). Although generic, specific repair steps may vary by make/model.
If a P0704 code has been stored in your OBD-II vehicle, it simply means the Powertrain Control Module (PCM) has detected a malfunction in the clutch switch input circuit. This code applies exclusively to vehicles equipped with manual transmissions.
Certain manual transmission functions are electronically monitored by the PCM. The gear lever position and clutch pedal position are among these functions. In some models, turbine input and output speed are also monitored to determine the degree of clutch slippage.
The clutch is a mechanical coupling that connects the engine to the transmission. In most cases, it is actuated using a rod (with a pedal at the end) that pushes the piston of a hydraulic clutch master cylinder mounted on the firewall. When the clutch master cylinder is pressed, hydraulic fluid is pushed into the slave cylinder (mounted on the transmission). The slave cylinder actuates the clutch pressure plate, allowing the engine to be engaged and disengaged from the transmission as needed. Some models use a cable clutch, but this type of system is becoming less common. Pressing the pedal with the left foot disengages the transmission from the engine. Releasing the pedal allows the clutch to engage the flywheel, propelling the vehicle in the desired direction.
The main function of the clutch switch is to act as a safety device so the engine does not start with the transmission unintentionally engaged. The clutch switch is primarily designed to interrupt the starter signal (from the ignition switch) so the starter is only activated if the clutch pedal is pressed. The PCM and other controllers also use the clutch switch input for various engine control calculations, automated braking functions, as well as for implementing hill hold and start-stop functions.
The P0704 code relates to the clutch switch input circuit. Consult your vehicle’s service manual or All Data (DIY) for component locations and other specific information regarding this particular circuit for your vehicle.
Symptoms and Severity
Various driving, safety, and traction control functions may be interrupted when a P0704 code is stored. This code should be treated as urgent because of this.
Symptoms of a P0704 code may include:
- Intermittent or failed engine starting
- Reduced fuel efficiency
- Excessive engine idle speed
- The traction control system may be disabled
- Safety functions may be disabled on some models
Possible causes for this code being set are:
- Faulty clutch switch
- Worn clutch pedal lever or clutch pedal pivot ring
- Shorted or open wiring and/or connectors in the clutch switch circuit
- Blown fuse or burned fuse link
- PCM programming error or faulty PCM
Diagnostic and Repair Procedures
A scan tool, a digital volt/ohmmeter, and a service manual (or All Data DIY) for your vehicle are all the tools that will be needed to diagnose a P0704 code.
A visual inspection of the clutch switch circuit wiring is a good place to start your diagnosis. Test all system fuses and replace blown fuses as needed. Test the battery load, check battery cables and battery ground cables at this point. Also check the alternator output.
Locate the diagnostic connector, connect the scan tool, and retrieve all stored codes and freeze frame data. Note this information, as it may be helpful for further diagnosis. Clear the codes and test drive the vehicle to see if the code resets immediately.
If it does: Use the DVOM to check battery voltage at the clutch switch input circuit. Some vehicles are equipped with multiple clutch switches to accommodate multiple functions. Consult All Data DIY to determine your clutch switch configuration. If the input circuit has battery voltage, press the clutch pedal and check for battery voltage on the output circuit. If there is no voltage on the output circuit, suspect a faulty or misadjusted clutch switch. Also ensure the clutch pivot lever and pedal arm are mechanically sound. Check for play in the clutch pedal pivot ring.
If there is voltage on both sides of the clutch switch (when the pedal is pressed), check the clutch switch input circuit at the PCM. This could be a battery voltage signal or a reference voltage signal; consult your vehicle manufacturer’s specifications. If there is an input signal to the PCM, suspect a faulty PCM or a PCM programming error.
If there is no clutch switch input signal at the PCM connector, disconnect all associated controllers and use the DVOM to test the resistance of all circuits in the system. Repair or replace any shorted or open circuits (between the clutch switch and the PCM) as necessary.
Additional Diagnostic Notes:
Check system fuses with the clutch pedal pressed. Fuses that may appear normal during initial testing may fail when the circuit is placed under load.
Often, a worn clutch pivot arm or clutch pedal pivot ring can be misdiagnosed as a faulty clutch switch.