EN

Severity: High Code type: Generic (all makes)

P0685 Code: ECM/PCM Power Relay Control Circuit/Open

The engine computer can't switch its own main power relay – open coil, blown fuse or broken control wire.

At a glance

Severity: 4/5 – High4/5

Severity: High

Meaning
ECM/PCM Power Relay Control Circuit/Open
System
ECU & Outputs · Powertrain
Safe to drive?
With caution
Typical repair cost
$10–$1,800
DIY difficulty
Moderate

On many vehicles the engine computer doesn't get all of its operating power straight from the ignition switch. When the key is turned on, a wake-up signal starts the ECM, which then energizes a main power relay; that relay feeds battery voltage to the ECM's main supply pins and often to injectors, ignition coils, oxygen sensor heaters and other engine loads. The same relay keeps the ECM alive for a short after-run once the key is off, so it can store adaptations and run the cooling fan. P0685 means the ECM's driver found the relay control circuit open – the coil isn't connected. Typical results are a crank-no-start or a sudden stall, but on some designs the code is logged during after-run and the engine keeps running normally.

Can I keep driving? If the engine runs, drive it straight to a shop and avoid long trips; a relay that has already dropped out once can cut the engine without warning, taking power steering assist with it on some cars. If it won't start, it needs to be towed or diagnosed where it sits.

How to read P0685

How to read P0685P10263854
  1. P Powertrain
  2. 0 Generic (SAE)
  3. 6 Computer & outputs
  4. 85 Specific fault

Symptoms

  • Crank-no-start with no fuel pump prime and no injector pulse
  • Engine cuts out while driving and restarts only after a few minutes
  • Scan tool can't communicate with the ECM when the fault is present
  • No click from the main relay when the key is switched on
  • Several circuit codes for injectors, coils or O2 heaters stored at the same time

Possible causes

Ranked by how often each cause is behind this code (editorial estimate).

  1. Main relay coil burned open or relay failed internally 34%
  2. Blown fuse supplying the relay coil or the ECM's permanent feed 20%
  3. Corroded relay socket or fuse box, often from water ingress 18%
  4. Broken or disconnected control wire between ECM and relay 14%
  5. Missing ignition or wake-up signal to the ECM 8%
  6. Open relay driver inside the ECM 6%

Diagnosis step by step

Diagnostic flow: P06851Check whether the ECM is awake2Listen and feel for the relay3Check every fuse in the circuit4Bench-test the relay5Verify wake-up signal and controlpath6Inspect the relay box for water

  1. Check whether the ECM is awake

    Try to connect a scan tool with the key on. No communication with the engine module, while other modules respond, fits a main relay that isn't feeding the ECM. If the ECM talks, note all codes and freeze frame before going further.

    Tools: OBD-II scanner

  2. Listen and feel for the relay

    Locate the main relay with the owner's manual or fuse box lid (often labeled ECM, PCM, EFI MAIN or main relay). Put a finger on it while a helper turns the key on – a healthy relay clicks. Many also click again several seconds to a few minutes after key-off when the after-run ends.

    Tools: Flashlight

  3. Check every fuse in the circuit

    Test the relay's coil feed fuse, the fuse on its contact side and the ECM's constant battery feed with a meter across both fuse test points, not just by looking. A blown fuse here is quick to replace, but ask why it blew before driving off.

    Tools: Multimeter

  4. Bench-test the relay

    Measure coil resistance (roughly 50–120 ohms), then power the coil with 12 V and check that the switched contacts close with less than about 0.2 V drop under load. A relay that clicks but has burned contacts can still cause intermittent cut-outs.

    Tools: Multimeter, Pliers

  5. Verify wake-up signal and control path

    With the key on, confirm the ECM receives its ignition or wake-up input at the pin shown in the wiring diagram. Then check continuity from the relay's control terminal to the ECM pin; an open here, or no wake-up at all, explains why the relay is never commanded.

    Tools: Multimeter

  6. Inspect the relay box for water

    Relay carriers and electronics boxes under the cowl or near the battery take on water when drains clog. Look for rust on terminals, water marks and corroded relay legs, then clean, dry and fix the source of the leak.

    Tools: Flashlight, Screwdriver

Where the parts are

Ignition: coils and spark plugs per cylinder – P06851234567
Ignition: coils and spark plugs per cylinder
  1. 1Ignition coil
  2. 2Spark plug
  3. 3Fuel injector
  4. 4Camshaft sensor
  5. 5Crankshaft sensor
  6. 6Engine computer (ECM)
  7. 7Intake manifold

Repair options & costs

RepairDIY?Estimated cost
Replace main power relay Yes $25–$120
Replace blown fuse and trace the cause Yes $10–$150
Repair corroded relay socket or fuse box terminals No $100–$350
Repair open control or wake-up wire No $120–$400
Replace ECM with programming No $700–$1,800

Rough US estimates for parts and labor; shop rates vary by region. Not a quote.

Notes by brand

Volkswagen

On many VW group models the engine control power supply relay sits in a relay carrier under the dash or in the electronics box below the windshield cowl. Water from clogged cowl drains corroding that area is a well-known cause of main relay faults and no-starts.

Field data

Ford 2 Escape Hev, Escape Hybrid
Honda 2 Accord

Source: NHTSA Complaints API (US government open data), analyzed by DTC mention.

Emissions / smog test impact

Will fail

P0685 lights the check engine light, so the vehicle fails an OBD smog check. Because the relay often also powers the ECM, a repaired car may need several drive cycles before all readiness monitors show complete.

Frequently asked questions

Why does a relay problem stop the whole engine?

On most vehicles this relay supplies the ECM itself plus the injectors, coils and sensor heaters. Without it the engine computer either doesn't run at all or can't fire the injectors and plugs, so the engine won't start or dies.

Can I swap in a relay from another socket to test it?

Often yes, if an identical relay – same part number – is used elsewhere in the same box, for example for the horn. If the engine starts with the swapped relay, you've found the fault; don't borrow one that looks similar but has a different part number.

The engine runs fine – why is P0685 stored?

Some ECMs test the relay circuit during the after-run period after you switch the key off. An intermittent open at that moment sets the code without any symptom while driving, but it's a warning that the relay or its wiring is failing.

Could a weak battery cause P0685?

Low voltage during cranking can make a marginal relay drop out or prevent the ECM from waking properly, so it can contribute. Test the battery first, but a code that returns with a healthy battery points to the relay circuit itself.

Sources