P0685 — ECM/PCM Power Relay Control Circuit Open

OBD2 code P0685 flags a problem with the sensor and actuator circuits (ECM/PCM power relay control circuit open). The car is usually drivable while you plan the repair. Common culprits: faulty sensor or actuator of the affected circuit, control module driver fault, blown fuse feeding the circuit.

powertrain (engine/transmission)

Is it safe to drive?

Short-term driving is OK In most cases the car remains drivable, possibly with reduced performance or a disabled auxiliary system. If symptoms include stalling, no-start or brake/steering warnings, treat it as urgent.

How to diagnose with an OBD scanner

Read the code and freeze frame Connect an OBD2 scanner, confirm P0685 is stored, and save the freeze frame data — it shows engine conditions when the fault was detected.
Check for companion codes and TSBs Related codes often reveal the real root cause. Also check manufacturer technical service bulletins (TSB) for your make and model — many P0xxx issues have factory fixes or software updates. Search by VIN on the manufacturer site or nhtsa.gov.
Inspect wiring and connectors Visually inspect the wiring harness and connectors related to the ECM/PCM power relay control for damage, corrosion, chafing or poor contact.
Subsystem check Do a continuity test end-to-end on the ECM/PCM power relay control circuit and wiggle the harness while measuring — intermittent opens show up as fluctuating readings.
Subsystem check Measure reference voltage, ground and signal continuity at the component connector.
Subsystem check Wiggle-test the harness while watching the signal in live data — dropouts reveal broken wires.
Verify the repair Clear the code, complete a full drive cycle including the conditions from the freeze frame, and confirm P0685 does not return.

Approximate repair cost

Repair cost depends on which component actually failed. Typical scenarios for P0685 (parts and labor, USD, independent shop rates): Repair wiring or connectors: $80–$250; Control module repair: $300–$1,000; Replace the faulty sensor or actuator: $150–$500. Overall, fixing P0685 usually costs between $80 and $1,000. An accurate diagnosis before buying parts saves money — the most expensive scenario is rarely the first one to check.

Common questions

What are the symptoms of P0685?

Drivers most often notice: possible limp mode depending on the affected component, Check Engine Light on, increased fuel consumption in some cases, the affected system operates intermittently or not at all.

How much does it cost to fix P0685?

Repair wiring or connectors — $80–$250; Control module repair — $300–$1,000; Replace the faulty sensor or actuator — $150–$500. Exact cost depends on the root cause, your vehicle and local labor rates — a proper diagnosis is the cheapest first step.

What is the most common cause of P0685?

The most common causes are: faulty sensor or actuator of the affected circuit, control module driver fault, blown fuse feeding the circuit.

What does OBD2 code P0685 mean?

How do I diagnose P0685 myself?

Start with a scan: confirm P0685, save the freeze frame and note any companion codes. Then inspect the wiring and connectors related to the ECM/PCM power relay control, and verify the suspect component against live data before buying parts.

When your engine control module decides something is wrong with the way it receives power, it logs OBD2 code P0685. This specific fault points to an open circuit in the ECM or PCM power relay control circuit — essentially, the electrical pathway that feeds voltage to the control module has been interrupted. "Open" is the automotive term for a break in the wire; no current can pass through a gap, and the module notices immediately when that expected voltage disappears.

The powertrain control module is the brain of your engine and transmission. It doesn't just sit there passively — it constantly watches voltages, ground connections, and signal lines throughout the vehicle. When P0685 triggers, the module has confirmed that the relay control circuit is not completing its path under normal operating conditions. It takes more than a single bad reading to set this code; the fault must repeat under specific parameters the manufacturer defines. That's why the freeze frame data stored alongside P0685 is so valuable — it records engine load, RPM, coolant temperature, and vehicle speed at the exact moment the fault occurred, giving you precise conditions to reproduce during diagnosis.

The underlying problem is almost always electrical. A broken wire, a connector that's come unplugged, heavy corrosion eating away at terminals, a blown fuse, or a relay that has failed internally — any of these will create the open circuit condition. The good news is that "open" faults are often among the easiest to find with basic multimeter testing, and the cheapest fixes are usually the first place to look.

Common symptoms of P0685

Check Engine Light illuminates on the dashboard
Vehicle may enter limp mode, limiting power and performance
Affected systems run intermittently or stop functioning entirely
Fuel economy may worsen noticeably
In rare cases, the engine may hesitate during acceleration

What causes P0685?

Damaged or broken wire in the ECM/PCM power relay control harness
Connector is unplugged, has a backed-out terminal, or shows signs of corrosion
Blown fuse protecting the power relay control circuit
Internally failed ECM/PCM power relay
Poor or corroded ground connection at the control module
Wiring that has become open or shorted somewhere in the circuit
Faulty control module driver that cannot properly switch the relay
Failed sensor or actuator on the affected circuit
Loose, corroded, or physically damaged connector

How to diagnose P0685

Connect an OBD2 scanner and verify that P0685 is present. Save the freeze frame data before clearing anything — this snapshot shows you the exact engine conditions when the module detected the fault.
Pull all stored and pending codes. Related codes often point directly to the failing component. Check manufacturer TSBs for your specific make and model; many P0xxx codes have published technical service bulletins with known fixes or software updates.
Perform a thorough visual inspection of the wiring harness and connectors tied to the ECM/PCM power relay. Look for chafed insulation, broken strands, corrosion buildup, and any signs of heat damage.
Use a multimeter to check continuity end-to-end on the power relay control circuit. Wiggle the harness gently while measuring — a fluctuating reading indicates an intermittent open that would not show up otherwise.
Measure reference voltage, ground integrity, and signal continuity at the component connectors. Compare your readings to the manufacturer's specifications.
Clear the code, then drive the vehicle through a complete drive cycle including the conditions recorded in the freeze frame. If P0685 returns, you have confirmed the fault is still present.

Repair options and costs

Start with the least expensive possibilities and work toward the costly ones:

Wiring or connector repair: $80–$250. Often just a matter of cleaning corrosion, reforming a bent terminal, or soldering a broken wire.
Control module repair: $300–$1,000. In-house module repair shops can sometimes fix internal relay driver failures for less than full replacement.
Sensor or actuator replacement: $150–$500. Labor varies widely depending on location — rear oxygen sensors on some trucks take twenty minutes, while an intake manifold sensor on a transverse V6 may require hours.

Total out-of-pocket for P0685 typically falls between $80 and $1,000. An accurate diagnosis before purchasing any parts will keep costs on the low end — the worst-case scenario is rarely where you should start.

Can you drive with P0685?

Most vehicles remain drivable when P0685 is active, though you may notice reduced performance or a disabled auxiliary system. If the car stalls, fails to start, or displays brake or steering warnings, treat the situation as urgent and have it towed rather than risking a breakdown on the road.