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OBD-II Diagnostic Trouble Code

P0685 — ECM Power Relay Control Circuit/Open

P0685 means the ECM has detected an open in the control circuit it uses to energize its own main power relay. The ECM monitors the relay driver output and saw a voltage or current indicating the circuit is open rather than switching the relay normally. Many heavy diesel ECMs are fed directly from the batteries without a main relay, so this code is seen mostly on gas medium-duty trucks and on systems that use an ECM power relay.

Severity 4/5 Truck-relevant
Stop within 50 mi
Severity
Stop within 50 mi ECM Power Relay Control Circuit/Open
Code
P0685
Category
Powertrain
Subsystem
Network
Truck-relevant
Yes
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What this means for truckers

P0685 (ECM Power Relay Control Circuit) — the relay that holds power to the ECM during shutdown has a fault. Symptoms: no-start, intermittent stalls, slow scan tool communication. Replace ECM power relay first ($25-80) before suspecting deeper issues. This is a 5-minute fix on most trucks.

Before you replace parts — try this first

P0685 on the Network system is most often corroded, loose or burned terminal at the ecm power relay socket or fuse/relay box — not a dead part. On commercial diesels especially, work these in order before spending money on parts:

  1. Confirm the symptoms. Match what your truck is doing to the Symptoms section below. If the code is set but the symptoms are not there, clear the code and re-test after a drive cycle — it may not return.
  2. Clean and dry the sensor connector. Water, road salt, or oil seeping into a connector is the #1 fake sensor failure. Disconnect, look for green corrosion or moisture, spray with electrical contact cleaner, dry, reseat. Many faults clear here for $0.
  3. Inspect the wiring harness. Trace the wire from the sensor toward the ECM — look for chafing against the engine block, melted insulation near the turbo or exhaust, or pinch points where a clamp has rubbed through.
  4. Test the sensor before swapping it. Most Common Causes (below) list the expected voltage or resistance range — a $20 multimeter saves a $300 sensor swap that does not fix the problem.
  5. Only then, replace the part. Use the OEM or a tier-1 aftermarket. Cheap sensors throw the same code in two weeks.
Symptoms

Crank-no-start or no-start with the ECM failing to power up fully, intermittent stalling or shutdown, loss of communication with the ECM on the diagnostic link, and the check-engine light. Some trucks run normally with only a stored code if the fault is intermittent.

Common causes
  • CommonCorroded, loose or burned terminal at the ECM power relay socket or fuse/relay box ($80–$300)
  • CommonFailed ECM power relay (open coil) ($40–$180)
  • OccasionalChafed or broken wire between the ECM relay driver pin and the relay coil ($150–$600)
  • OccasionalBlown fuse feeding the relay coil or poor ground at the relay ($30–$200)
  • RareInternal ECM relay driver failure ($1200–$3500)
How to fix
Check fuses feeding the ECM relay and swap the relay with a known-good identical relay
When to use: First step on any no-start or no-communication complaint with this code
Inspect the relay socket and fuse box terminals for corrosion, heat damage and loose pin fit; repair terminals
When to use: When the relay tests good or the fault is intermittent with vibration
Back-probe the relay coil circuit and check continuity from the ECM driver pin to the relay, plus coil resistance and coil feed/ground
When to use: When the relay and socket are good but the code returns
Verify ECM powers and grounds, then consider ECM repair or replacement and reprogramming
When to use: Only after wiring and relay are proven good
Affects most

No specific vehicle pattern found in our complaint database. This code can appear on any vehicle that supports OBD-II diagnostics.

Related

Frequently Asked Questions

Can I drive with code P0685?
Drive with caution. P0685 is severity 4 — stop within ~50 miles. Most commercial diesel codes at this severity (turbo, fuel rail, transmission solenoid) can damage adjacent expensive components if left unrepaired. Get to a service location before highway-speed loads.
How much does it cost to fix P0685 on a commercial truck?
Total repair cost ranges $30–$3500 depending on the root cause. The cheapest fix is usually the first one listed below — Corroded, loose or burned terminal at the ECM power relay socket or fuse/relay box — which is also the most common. On commercial trucks the cost is usually higher than passenger vehicles due to part availability and shop labor rates ($120–185/hour at most truck shops, vs $90–130 at general auto shops). DIY-capable fixes (sensors, hoses, filters) are at the low end; major component replacements (turbo actuator, high-pressure fuel pump, transmission solenoid pack) are at the high end. Always diagnose with a scan tool BEFORE replacing parts — guessing wrong on a $1,500 turbo actuator when the actual issue was a $30 boost hose is a common expensive mistake.
Will P0685 fail a DOT inspection?
P0685 is generally not a direct DOT inspection fail item by itself. Inspectors focus on safety-critical systems (brakes, steering, tires, lights, hours-of-service). However, the check engine light alone can trigger additional inspection scrutiny. Best practice: fix the code and clear it before a planned inspection.
What's the difference between P0685 and related codes?
P0685 is closely related to: P0670 (Glow Plug Control Module Circuit), P0671 (Cylinder 1 Glow Plug Circuit), P0672 (Cylinder 2 Glow Plug Circuit), P0673 (Cylinder 3 Glow Plug Circuit). These codes share the same subsystem (network) and often appear together. If you see this code, scan for the full code list — fixing the root cause usually clears multiple companion codes at once. If only P0685 is set without the companions, the issue is more isolated.
How do I prevent P0685 from coming back after a repair?
For P0685 (ECM Power Relay Control Circuit/Open), prevention centers on the network system. Pay attention to ground connections at every PM — bad grounds are 70% of communication codes. Inspect harness routing for chafe points. Don't disconnect modules with the key on.

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