P0685

ECM/PCM Power Relay Control Circuit Open

P0685 is a generic OBD-II powertrain diagnostic trouble code: ECM/PCM Power Relay Control Circuit Open. It is logged by the engine control unit when the glow monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
P0685
Group
Powertrain
System
Glow
Severity
Critical (limp mode / no-start)
Need P0685 disabled?
RaceTune permanently disables any OBD-II trouble code on supported ECUs — for motorsport, off-road, and export use.
→

What P0685 means

P0685 indicates that the PCM has detected an open or abnormal condition in the circuit it uses to control the main ECM/PCM power relay coil. In most vehicles, the PCM does not receive battery power directly at all times; instead it commands a relay coil through an internal driver, and the relay contacts then supply battery voltage to the PCM's main power pins. This self-enabling architecture means the PCM must first receive an initial key-on feed through a separate ignition path, then energise the relay to sustain its own power supply. P0685 is set when the PCM's relay coil driver senses that the commanded output is not producing the expected coil current — indicating an open circuit, a missing ground, or a relay that is not responding.

Real-world failure modes are predictable: a relay with degraded coil resistance or burnt contacts fails to close reliably; a corroded main relay connector introduces enough resistance to drop the supply voltage below threshold; or the wiring between the PCM relay driver output and the relay coil develops a break. The result is often an intermittent long-crank condition — the engine cranks normally but the PCM drops out before injection and ignition pulses are delivered — or a vehicle that cranks but will not sustain a start. The ECM may also fail to power down correctly after key-off, draining the battery overnight.

Because the main relay also powers injectors and other critical loads on many platforms, a fully failed relay produces a no-start with no injector pulse rather than simply a reduced-power condition. Relay and connector inspection should always precede PCM diagnosis, as the relay itself is inexpensive and corrosion at the relay base is the single most common physical fault.

Common causes

Most-frequently reported root causes when P0685 is logged.

  • 1
    Faulty ECM/PCM main power relay with a degraded or open coil winding that does not energise when commanded.
  • 2
    Burnt, corroded, or loose contacts in the relay socket or the main relay connector reducing the voltage available to the coil circuit.
  • 3
    Open or chafed wire in the relay coil control circuit between the PCM relay driver output and the relay coil terminal.
  • 4
    Missing or high-resistance chassis ground for the relay coil return path preventing the coil from completing its circuit.
  • 5
    Blown fuse in the relay coil supply line removing voltage from the coil side of the relay.
  • 6
    Corroded or loose battery cables causing intermittent voltage drops that prevent the relay from latching reliably.
  • 7
    Rare internal PCM relay driver failure where the PCM's own output transistor cannot pull the coil line to ground.

Symptoms drivers notice

No-start condition — engine cranks normally but does not fire because the PCM loses power before delivering injector or ignition pulses.
Long, difficult, or intermittent crank before the engine starts, particularly after the vehicle has sat or during cold weather.
Check Engine Light illuminated; the code may be stored even though the engine eventually starts.
ECM fails to power down after key-off, causing parasitic battery drain and a flat battery overnight.
Audible relay clicking or chattering from the fuse box area as the relay coil intermittently engages and drops out.

How to diagnose P0685

A typical diagnostic flow when this code is present.

  1. 1
    Connect a scan tool, retrieve all DTCs and freeze frame data, and note any companion codes such as P0689 that would indicate a feedback-side fault as well.
  2. 2
    Visually inspect the ECM/PCM main power relay and its socket for corrosion, melted plastic, burnt terminals, and loose fitment; replace the relay if any damage is visible.
  3. 3
    Swap the ECM/PCM relay with a known-good relay of the same part number from another fuse box position and retest — this is the fastest no-cost diagnostic step.
  4. 4
    With the relay removed, use a multimeter to measure coil resistance between the relay coil terminals; compare to specification (typically 70–120 Ω) and replace if out of range.
  5. 5
    Check continuity from the PCM relay driver output pin (per wiring diagram) through the harness to the relay coil terminal; repair any open or high-resistance segment found.
  6. 6
    Verify relay coil ground integrity by measuring resistance from the relay coil return terminal to chassis ground; should be near 0 Ω.
  7. 7
    If relay and all external wiring check out, suspect an internal PCM relay driver fault and consult manufacturer-specific procedures before authorising PCM replacement.

Related powertrain codes

Frequently asked questions

Can a bad relay cause a no-start even if the relay clicks?

Yes. An audible click means the coil energised, but it does not confirm the relay contacts are passing current. Burnt or pitted contacts can click yet fail to deliver battery voltage to the PCM's main supply pins. Test contact output voltage directly with a multimeter.

Why does P0685 cause the battery to go flat overnight?

The PCM uses the same relay to shut itself down after key-off. If the relay driver circuit is faulted, the PCM may not be able to de-energise the relay correctly, leaving the module and its associated loads powered continuously and draining the battery.

Is P0685 the same as P0689?

They are closely related but distinct. P0685 is a fault in the coil control circuit — the PCM cannot properly drive the relay ON. P0689 is a fault on the relay output feedback (sense) line — the relay may be operating, but the PCM's feedback pin is not confirming the expected supply voltage. Both codes can appear together when the relay or its connector is heavily corroded.

Should I replace the PCM if P0685 persists after relay replacement?

Only after exhausting all wiring checks. Inspect and clean every connector in the relay coil circuit, verify both supply and ground continuity with a multimeter, and confirm the fuse is healthy. PCM internal driver failure is genuinely uncommon; wiring and connectors account for the majority of persistent cases.

Disabling P0685 in software

RaceTune can permanently disable P0685 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.

Permanent
The monitor is disabled in the ECU itself — not just cleared. It cannot return.
Tailored to your file
Each patch is matched to your specific software version — never a one-size-fits-all file.
Reversible
The original file is always preserved. Reflash the stock to return the ECU to factory state.

Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.

Got P0685 in your scan?

Upload your ECU file — we'll identify the exact software version and confirm whether a disable is available for your car.

Upload your file