P1686

What Does Code P1686 Mean?

The diagnostic trouble code P1686 indicates a malfunction within the electrical circuit of the Metering Oil Pump (MOP) stepping motor. This code is generated by the Engine Control Module (ECM), also known as the Powertrain Control Module (PCM), when it detects an electrical anomaly in the MOP’s control circuit. The MOP is a critical component, particularly in rotary engines (e.g., Mazda RX-8), where it precisely injects small amounts of engine oil directly into the combustion chambers (rotor housings) to lubricate the apex seals and rotor-housing interface, as well as into the intake ports in some applications. The MOP utilizes a stepping motor for precise control over the oil flow rate, which varies based on engine RPM, load, and temperature. The ECM monitors the current draw, voltage, and resistance characteristics of the stepping motor circuit. When these electrical parameters deviate from the manufacturer’s specified range—indicating an open circuit, a short to ground, a short to voltage, or an excessively high or low resistance within the motor windings or associated wiring—the ECM interprets this as a circuit malfunction and sets the P1686 code. This failure mode prevents the ECM from accurately controlling the oil metering, which can have severe consequences for engine longevity and performance.

Common Symptoms

• Check Engine Light (CEL) Illumination: This is the most immediate and common symptom, as the ECM registers the fault.
• Rough Idling or Stalling: Improper lubrication can lead to increased friction, affecting engine smoothness.
• Reduced Engine Performance: Lack of proper lubrication can cause increased wear, reducing compression and overall power output.
• Excessive Exhaust Smoke: If the MOP is stuck open or delivering too much oil due it a control circuit malfunction, excessive oil combustion can occur. Conversely, if it’s not delivering enough, internal friction may lead to other issues that cause smoke (e.g., overheating).
• Increased Oil Consumption: If the pump is stuck in an open position or continuously active due to a circuit fault, more oil than necessary will be consumed.
• Engine Overheating: Inadequate lubrication significantly increases internal friction, leading to higher engine temperatures.
• Engine Damage (Long-Term): Prolonged operation with a malfunctioning MOP can lead to severe and irreparable damage to critical engine components, such as apex seals and rotor housings in rotary engines, due to insufficient lubrication.
• Limp-Home Mode Activation: The ECM may reduce engine power and limit RPM to prevent further damage.

What Causes the Code P1686?

• Faulty Metering Oil Pump (MOP) Stepping Motor: Internal electrical failure within the motor windings (open circuit, short circuit, excessive resistance) is a primary cause.
• Damaged or Corroded Wiring Harness: Open circuits, short circuits to ground or voltage, or high resistance in the wiring leading to the MOP connector from the ECM. This can be caused by chafing, heat damage, rodent damage, or environmental corrosion.
• Corroded or Loose Electrical Connector: Poor connection at the MOP sensor or ECM due to bent pins, corrosion, or improper seating can interrupt the circuit integrity.
• Contaminated Connector: Oil, dirt, or moisture ingress into the MOP connector can cause electrical shorts or high resistance.
• ECM/PCM Failure: While less common, a faulty ECM that cannot properly control or monitor the MOP circuit can set this code. This is typically ruled out after thoroughly checking other components.

How to Diagnose and Troubleshoot

Diagnosing P1686 requires a systematic approach using a digital multimeter (DMM) and an OBD-II scanner.

1. Retrieve and Document Freeze Frame Data: Connect an OBD-II scanner and retrieve all active and pending codes, along with freeze frame data. This data provides a snapshot of engine conditions (RPM, load, temperature, etc.) when the code was set, which can be useful for replication or identifying contributing factors. Clear the codes after documentation.
2. Visual Inspection: Perform a thorough visual inspection of the MOP, its electrical connector, and the entire wiring harness leading to the ECM. Look for signs of:
   • Chafed, cut, or burnt wiring.
   • Corroded or bent pins in the MOP connector.
   • Loose or disconnected connectors.
   • Signs of oil or fluid contamination in the connector or wiring.
   • Physical damage to the MOP unit itself.
3. Check MOP Stepping Motor Resistance:
   • Disconnect the MOP electrical connector.
   • Using a DMM set to ohms, measure the resistance across the MOP stepping motor terminals. Refer to the vehicle’s specific service manual for the correct pinout and expected resistance values. Stepping motors typically have multiple windings, and resistance should be consistent across specified pairs.
   • An infinite reading indicates an open circuit, and a reading close to zero ohms might indicate a short circuit within the motor. Values significantly outside the specified range indicate an internal motor fault.
4. Inspect Wiring Harness for Continuity and Shorts:
   • With the MOP connector disconnected and the ECM connector also disconnected (to prevent damage to the ECM), use a DMM to check the continuity of each wire between the MOP connector and the ECM connector. Resistance should be very low (typically less than 0.5 ohms).
   • Check each wire for shorts to ground by placing one DMM lead on a known good chassis ground and the other on each wire terminal at both the MOP and ECM connectors. There should be no continuity (infinite resistance).
   • Check for shorts to power by ensuring there is no voltage on the MOP circuit wires with the ignition key in the ON position (without the ECM connected).
5. Verify Power and Ground at MOP Connector:
   • With the ignition ON, but the MOP still disconnected, check for the presence of battery voltage and a good ground signal at the appropriate pins of the MOP connector using a DMM. Consult the service manual for the correct pin assignments.
   • A missing power or ground signal indicates a wiring issue or a faulty fuse/relay affecting the MOP circuit.
6. ECM Output Verification (Advanced): If all MOP and wiring checks pass, the issue might reside within the ECM. This typically requires specialized diagnostic equipment or substitution with a known good ECM, often a dealership-level diagnosis.
7. Road Test: After any repairs, clear the code and perform a road test under conditions similar to those recorded in the freeze frame data to confirm the repair and ensure the code does not return.

Recommended Repairs and Solutions

Based on diagnostic findings, the following repairs are commonly recommended:

• Replace the Metering Oil Pump (MOP) Assembly: If the MOP stepping motor’s internal resistance is found to be out of specification, or if there is physical damage to the unit, replacing the entire MOP assembly is the most common and effective solution. It is crucial to use an OEM (Original Equipment Manufacturer) or equivalent quality part due to the precision required for oil metering.
• Repair or Replace Wiring Harness: If damaged, chafed, or corroded wiring is identified, perform precise repairs using appropriate connectors, heat-shrink tubing, and wire of the correct gauge. In cases of extensive damage or multiple faults, replacing the relevant section of the engine wiring harness may be necessary. Ensure all connections are watertight and secure.
• Clean or Replace Electrical Connectors: If corrosion or contamination is found in the MOP connector, use electrical contact cleaner to meticulously clean the terminals. For severely corroded or damaged connectors, replacement of the connector housing and terminals is advisable. Ensure the connector seats properly and locks into place.
• Address Power/Ground Issues: If a lack of power or ground was identified at the MOP connector, trace the circuit back to the source, checking fuses, relays, and ground points for continuity and integrity. Repair any breaks or shorts found in the circuit.
• ECM Replacement: Only consider ECM replacement as a last resort, after all other components and wiring have been thoroughly tested and verified to be in good working order. ECM replacement typically requires programming to the specific vehicle, which should be performed by a qualified technician with specialized equipment.

Mechanic’s Tip: When working with MOP systems, especially in rotary engines, always ensure that the oil lines are properly purged of air after MOP replacement or service to prevent initial starvation. Consult the service manual for the specific bleeding procedure. Improper MOP operation can lead to catastrophic engine failure, so thorough diagnosis and correct repair are paramount.