RX-8 OMP Diagnostics & Testing
Diagnosing the Series 1 RX-8 oil metering pump: symptoms, codes P1686 to P1688, Mazda's diagnostic order, the electrical and nozzle tests with figures, and a result sheet.
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Work from the cheap checks to the expensive ones, as Mazda’s flow does: connectors, power and wiring, then the switch, and the pump last. An OMP code usually means a connector, wiring fault or the position switch, not a worn-out pump. Read the codes and freeze-frame data and follow the diagnostic order before buying anything.
An engine run with a failed OMP and no premix can destroy its seals quickly. If in doubt, add premix to the fuel while you diagnose.
Symptoms
| What you see | What it can mean |
|---|---|
| Check-engine light with P1686, P1687 or P1688 | The PCM’s switch check failed (see below). |
| Engine restricted, won’t rev freely | Fail-safe may be active: stepper fixed at step 7, injection and ignition fixed. Owners typically report being held to about 3,000-3,500 rpm. |
| Code only when hot, or after the car has sat | Often a corroded or stiff pump or connector (owners found coolant or water above the pump), or a marginal switch. |
| Oil level stops falling | A warning, not a bonus: delivery may have stopped. The PCM has no flow sensing, so no code doesn’t mean oil is flowing. |
| Oil weeping at the pump or pipe unions | Seal or washer problem. Check the pump-to-cover face and the pipe connections. |
| Blue smoke | Oil is burning. Mazda’s smoke flow checks OMP control first, then compression, oil condition and pressure, then pipe and nozzle air or clogging. |
Fault codes
Mazda’s 2003 RX-8 manual has three OMP codes. All are continuous monitors: the MIL comes on if the fault is seen in the first drive cycle, a pending code is set, freeze-frame data is recorded and the code is stored.
| Code | Mazda’s title | What the PCM saw |
|---|---|---|
| P1686 | Metering oil pump control circuit, low flow side problem | Stepper driven beyond the standard step, but the switch input stayed off. |
| P1687 | Metering oil pump control circuit, high flow side problem | Stepper driven less than the standard step, but the switch input was on. |
| P1688 | Metering oil pump control circuit, initial check problem | Switch input was on during the stepper’s initial check. |
All three share the same possible causes: the pump or its switch, a connector or terminal, an open or short to ground in the pump supply or coil wiring, or the PCM. P1686 adds an open in the switch wiring; P1687 and P1688 add a short to ground on it.
Some online lists call P1684 a “position sensor” fault and P1685 to P1688 stepper-circuit faults, one per winding. Mazda’s 2003 RX-8 table doesn’t say that: it lists only P1686 to P1688, defined by switch state against stepper position, and each code’s checks cover all four coil lines together. P1685 appears once, in a caution on the switch-inspection page, with no page of its own. Owners report P1684 and P1685 on Series 2 cars as pump oil-pressure codes, a different system.
What goes wrong in practice
Mazda’s flow lists every possible cause. These owner reports (RX8Club and similar, mostly 2005-2022) suggest an order of likelihood. They are community experience, not Mazda data.
| Cause | What owners found |
|---|---|
| Water or coolant at the pump or connector | A weeping hose or pipe above the pump drips onto it. Corroded, stiff internals and hot-start-only codes that cleared once cleaned and dried. Intermittent P1688 thread. |
| Position switch moved on the pump | One owner turned the switch to the end of its travel for more oil (he estimated about 30°; others in the thread said 10-19°) and got an immediate code and limp mode. Backing it off 5-10° and resetting the ECU cleared it. See MOP adjustments and the servicing page. |
| Worn or intermittent switch | A resistive track that wears or drops out when hot. One switch read 1,132 Ω against the 400 Ω limit and a used pump stopped the code, though the owner also suspected a harness short; last report “fixed for 5 miles so far” (thread). |
| Stuck mechanism | A stiff stepper freed with lubricant; the code cleared. |
| Harness and connector | A bare wire in the loom, salt or water in the 3-pin connector, no loom over it. |
| The pump itself | Replaced successfully in several cases, including units whose coils and switch read in spec. Used pumps sometimes fail again or arrive mis-set. |
| The PCM | Last in Mazda’s flow; no confirmed case in the owner reports. |
One poster, “for training”, shorted two stepper coil wires and got all three codes at once (thread). That fits Mazda’s logic: the codes describe the switch check, not which winding failed.
Diagnostic order
Mazda’s flow for P1686, P1687 and P1688, condensed. Go in order and stop when you find the fault.
- Record the freeze-frame data and check for related service information.
- Inspect the pump connector for damaged, pulled-out or corroded pins.
- Pump supply: ignition on, engine off, voltage at pump terminals C and D (harness side) to body ground should be B+.
- Shorts to ground: ignition off, no continuity from pump terminals A, E, B and F (harness side) to body ground.
- Inspect the pump; replace it if faulty.
- Inspect the PCM connector.
- Opens in the coil wiring: ignition off, continuity between pump terminals A, E, B, F and PCM terminals 2V, 2W, 2Y, 2AB.
- Inspect the switch connector.
- Switch wiring. P1686: check continuity from switch terminal A to PCM 2N and C to PCM 1U (an open is the fault). P1687 and P1688: check switch terminal A isn’t shorted to ground.
- Inspect the switch. If faulty, Mazda’s remedy is a new pump (the switch is part of it).
- Reconnect, clear the code, start the engine and see if it returns. Only then replace the PCM. Owners say a real fault returns within seconds to a minute (one reported about 15 seconds); that is experience, not Mazda. A code that returns only occasionally points to a marginal connector, switch or water, so go back to steps 2-10.
- Do the after-repair procedure and check for other codes.
Pump connections
| Connector | Terminal | Goes to |
|---|---|---|
| 6-pin (stepper) | C and D | Supply from the main relay (B+ with ignition on) |
| 6-pin | E | PCM 2W (coil No.1) |
| 6-pin | F | PCM 2AB (coil No.2) |
| 6-pin | A | PCM 2V (coil No.3) |
| 6-pin | B | PCM 2Y (coil No.4) |
| 3-pin (switch) | A | PCM 2N (switch signal) |
| 3-pin | C | PCM 1U (switch ground) |
Electrical checks
All at 20 °C. Disconnect the connector before measuring resistance.
| Check | How | Specification |
|---|---|---|
| Supply voltage | 6-pin connector, C and D, ignition on | Battery voltage |
| Stepper winding resistance | 6-pin connector: D-B, D-F, C-A, C-E | 30.6-35.6 Ω each. Out of range: replace the pump. |
| Position switch resistance | Idle the engine, stop it, disconnect the 3-pin connector, measure A-C | 400 Ω or less. Out of range: replace the pump. At rest the switch should conduct, not read open. |
With the pump out you can also check each winding terminal against the pump body (no continuity expected). That check isn’t on Mazda’s inspection page.
Switch check at the PCM
Mazda checks the switch circuit at PCM terminal 2N using the PCM’s learning function:
- Disconnect the battery negative to clear the OMP learning value, then reconnect.
- Measure at PCM terminal 2N: ignition ON 0 V; just after engine start B+; idling after start 0 V.
- If the readings are wrong but the wiring is good, replace the pump.
Mazda says this can’t be done correctly if the stepper is faulty, so first check P1685 to P1688 aren’t stored. It also gives wiring checks from switch terminal A to PCM 2N and C to PCM 1U (no open) and from A to ground and to the power supply (no short).
Live data
With a scan tool that shows the PIDs (Mazda’s procedure uses WDS), watch MOP_POS (the PCM’s step count) and MOP_SW (the switch). Mazda’s operation check moves the motor with MOP_POS and expects MOP_SW to change at 53 steps or over. If it changes, Mazda sends you to the oil pipe (leaks or cracks between pump and nozzle), the nozzle (clogging) and the hose between the intake manifold and nozzle (clogging, leaks, poor connection). If it doesn’t, check the motor and switch and replace the pump if faulty.
If your scan tool can only read MOP_POS, that still helps: the target step should climb with engine speed. In two Forscan logs of my car it read 7 throughout (see engine running data), which is Mazda’s fail-safe step.
Oil nozzle checks
Each rotor has two nozzles. Mazda’s test:
- Remove the nozzle.
- Apply 66.5 kPa (0.68 kgf/cm², 9.65 psi) or more of vacuum to port A with a vacuum pump.
- It must hold. If it drops 2.66 kPa (0.027 kgf/cm², 0.39 psi) or more in one minute, replace the nozzle.
Mazda’s other two checks are on the running engine and the parts: look for oil leaks at the pump-to-front-cover face and where the oil pipe connects (engine idling), and check the pipes, nozzles and the pump’s connector bolts for clogging. Don’t clear a blockage by drilling: the orifice size is part of the metering. More on the nozzles page.
Result sheet
| Check | Expected | Measured | Pass? |
|---|---|---|---|
| Codes and freeze frame | none stored | ||
| Pump supply, terminals C and D (ignition on) | B+ | ||
| Windings D-B, D-F, C-A, C-E | 30.6-35.6 Ω each | ||
| Windings to ground (A, E, B, F) | no continuity | ||
| Switch A-C (engine idled then stopped) | 400 Ω or less | ||
| Switch wiring A to 2N, C to 1U | continuity | ||
| 2N after clearing learn value | 0 V / B+ / 0 V | ||
| MOP_POS climbs with rpm | progressive | ||
| Nozzles hold vacuum | drop under 2.66 kPa per minute | ||
| Pipes, connector bolts | not clogged | ||
| Leaks at pump and unions | none |
If everything passes but a code returns after clearing, Mazda’s last step is the PCM.
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