Mazda Rotary Engine Oil Metering Pump Fault
The P1685 code indicates a malfunction in the Metering Oil Pump (MOP) system, which is exclusive to Mazda's Wankel rotary engines (RX series). This pump injects a small amount of engine oil directly into the combustion chamber to lubricate the apex seals and rotor housings. The ECU monitors the pump's position and electrical operation. The code sets when the control module detects a voltage or position signal outside the expected range in the MOP circuit, compromising internal lubrication of the rotary engine.
01. Most Common Causes
- Faulty or stuck Metering Oil Pump (MOP)
- Damaged oil pump position sensor
- Short circuit or open circuit in the MOP wiring harness
- Corroded or loose electrical connections at the MOP connector
- Low engine oil level in the sump
- Faulty engine control module (ECU) – rare
02. Symptoms You Will Notice
- Check Engine Light illuminated
- Engine may run rough or lack power
- Increased engine noise from poor lubrication
- Possible engine overheating or seizure if ignored
03. Step-by-Step Diagnosis
Scan for additional codes and verify P1685 is active.
Check engine oil level and condition; top off if low.
Inspect MOP electrical connector for corrosion, damage, or looseness.
Test MOP wiring for shorts, opens, or high resistance using a multimeter.
Measure MOP position sensor signal voltage and compare to specs.
If wiring and sensor check out, test or replace the MOP itself.
As a last resort, test ECU or check for software updates.
1. Frequently Asked Questions
Is it safe to drive?▼
No, driving with P1685 is not recommended. The oil metering pump is critical for lubricating the rotary engine's apex seals. Continued operation can cause severe engine damage, including seizure, due to inadequate lubrication.
How much does it cost to fix?▼
Repair costs typically range from $200 to $800, depending on the root cause. A new MOP pump costs $150–$400, plus labor (1–3 hours at $100–$150/hr). Wiring repairs are cheaper, while ECU replacement can exceed $1,000.