P1685

What Does Code P1685 Mean?

DTC P1685 signifies a “Metering Oil Pump Stepping Motor Circuit Malfunction.” This code is set when the Engine Control Module (ECM), also known as the Powertrain Control Module (PCM), detects an electrical fault within the control circuit of the metering oil pump’s stepping motor. The metering oil pump (MOP) is a critical component, primarily found in engines requiring precise, variable oil injection, such as rotary engines (e.g., Mazda Renesis) or advanced two-stroke engines. Its function is to deliver a controlled amount of engine oil directly to specific lubrication points, such as rotor housings, apex seals, or the intake manifold, independent of the main engine oiling system. The MOP utilizes a stepping motor for extremely precise control over the oil delivery rate, adjusting it based on engine RPM, load, and temperature. The ECM continuously monitors the stepping motor’s control circuit for proper voltage, current, and resistance characteristics. If the ECM detects an open circuit, a short circuit to ground or voltage, or an out-of-range resistance value within this circuit, indicating that the stepping motor is not responding as commanded or is electrically compromised, it will illuminate the Check Engine Light (CEL) and store code P1685.

Common Symptoms

• Check Engine Light (CEL) illumination: This is almost always the first and most direct symptom.
• Reduced engine performance or power: If the MOP is unable to deliver adequate lubrication, the engine may enter a limp-home mode or exhibit decreased power output to prevent damage.
• Excessive exhaust smoke: A malfunction leading to over-injection of oil can result in visible blue smoke from the exhaust, indicating oil combustion.
• Rough idle or misfires: Inconsistent oil delivery can disrupt combustion, leading to an erratic idle or misfires.
• Engine knocking or seizing: This is a severe, long-term consequence of insufficient lubrication due to a non-functional MOP, which can lead to catastrophic engine failure.
• Increased oil consumption: If the MOP is stuck in an overly-open position, leading to excessive oil injection.

What Causes the Code P1685?

• Faulty Metering Oil Pump (MOP) stepping motor: Internal electrical failure within the stepping motor itself, such as an open coil, shorted winding, or mechanical binding preventing rotation.
• Wiring harness issues: Open circuits, short circuits to ground, short circuits to voltage, or excessive resistance within the MOP stepping motor’s power supply, ground, or control signal wires.
• Corroded or loose electrical connectors: Poor electrical contact at the MOP connector or the ECM connector can disrupt circuit integrity.
• Failed Engine Control Module (ECM/PCM): While less common, a faulty internal driver circuit within the ECM responsible for controlling the MOP stepping motor can trigger this code.
• Contamination or mechanical binding within the MOP assembly: Sludge or debris obstructing the pump’s internal mechanism can cause the stepping motor to overwork, draw excessive current, or prevent it from reaching commanded positions, leading to an electrical fault detection.

How to Diagnose and Troubleshoot

Diagnosing P1685 requires careful electrical circuit testing and component verification:

1. Visual Inspection: Begin with a thorough visual inspection of the MOP assembly and its associated wiring harness. Look for obvious signs of damage, fraying, chafing, or exposed wires. Check the MOP electrical connector for corrosion, bent pins, or a loose connection. Trace the wiring back towards the ECM, inspecting for any potential pinch points or rodent damage.
2. Scan Tool Data Analysis: Connect an OBD-II scan tool and check for any other related diagnostic trouble codes. Monitor live data parameters related to the MOP, if available (e.g., commanded MOP position/duty cycle, actual MOP feedback if a position sensor is present). Some advanced scanners may offer a bidirectional control test to command the MOP stepping motor, allowing you to observe its physical operation and listen for clicking noises, indicating movement.
3. Electrical Circuit Testing (with DMM):
   • Disconnect the MOP connector: With the ignition OFF, disconnect the electrical connector from the metering oil pump assembly.
   • MOP Stepping Motor Resistance Check: Using a Digital Multimeter (DMM), measure the resistance across the various pins of the MOP stepping motor. Refer to the vehicle-specific service manual for the exact pinout and specified resistance values (stepping motors typically have multiple coils, e.g., A/B phases). An open circuit (infinite resistance) or a very low resistance (indicating a short) compared to specifications points to an internal motor fault.
   • Wiring Harness Continuity and Resistance Check: Backprobe the ECM connector and test for continuity between the appropriate ECM MOP driver pins and the corresponding pins at the disconnected MOP harness connector. Check for excessive resistance (>0.5 ohms). Also, check for shorts to ground and shorts to voltage on all MOP harness wires with the ignition ON (power wires should show battery voltage, ground wires should show continuity to ground, signal wires should not show constant voltage or ground when disconnected from the MOP).
   • Power and Ground Supply Check: With the ignition ON, use the DMM to verify proper power supply (typically battery voltage, ~12V) and a good ground at the MOP harness connector.
4. ECM Output Verification (Advanced): If all wiring and the MOP motor test good, use an oscilloscope to monitor the square wave control signals from the ECM to the MOP stepping motor while the engine is running or during an actuator test. This can verify if the ECM is sending the correct control pulses.

Recommended Repairs and Solutions

Once the diagnostic steps have identified the root cause, the appropriate repair can be made:

• Repair or Replace Wiring/Connectors: If the visual inspection or DMM tests indicate a damaged wiring harness, corroded connector pins, or an open/short in the circuit, repair the affected section of the harness or replace the connector. Always use OEM-quality repair techniques (e.g., solder and heat shrink for splices).
• Replace the Metering Oil Pump (MOP) Assembly: This is often the most common resolution if the MOP stepping motor itself fails the resistance tests or is confirmed to be mechanically bound. In most applications, the stepping motor is an integral part of the MOP assembly and is not serviced separately. Ensure the replacement part is an OEM equivalent or meets OE specifications for proper function and longevity.
• Clean MOP and Associated Lines: If the MOP’s internal components are suspected of being fouled with sludge or debris, and it’s economically viable, attempt to clean the pump and its oil lines. However, internal binding often necessitates replacement.
• ECM Replacement/Reprogramming: If all wiring, connectors, and the MOP assembly are confirmed to be in good working order, and the oscilloscope confirms a lack of proper control signals from the ECM, then an ECM failure is indicated. This requires specialized tools for replacement and programming to the vehicle.
• Post-Repair Verification: After completing any repairs, clear the DTCs from the ECM using a scan tool. Perform a comprehensive road test under varying engine loads and RPMs. Re-scan the vehicle to ensure that P1685 does not return and check live data for proper MOP operation. Ensure engine oil levels are maintained correctly, especially after MOP replacement.