OBD-II Diagnostic Trouble Code
P0687 — ECM Power Relay Control Circuit High
P0687 means the ECM power relay control circuit voltage is higher than expected. The ECM saw the relay driver circuit high when it should have been low, usually from a short to battery voltage in the control wire or a faulty relay.
Severity 4/5
Truck-relevant
Stop within 50 mi
Severity
Stop within 50 mi
ECM Power Relay Control Circuit High
Code
P0687
Category
Powertrain
Subsystem
Network
Truck-relevant
Yes
What this means for truckers
P0687 (ECM Power Relay Control Circuit High) — relay control shorted to power. Relay may stay energized after key off, draining battery.
Before you replace parts — try this first
P0687 on the Network system is most often relay control wire shorted to a battery-voltage circuit in the harness — not a dead part. On commercial diesels especially, work these in order before spending money on parts:
- 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.
- 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.
- 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.
- 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.
- Only then, replace the part. Use the OEM or a tier-1 aftermarket. Cheap sensors throw the same code in two weeks.
Symptoms
The ECM may stay powered after key-off causing battery drain, or the relay may not release; some trucks have a no-start or intermittent shutdown. The check-engine light is on and the code may set at key-off or key-on.
Common causes
- CommonRelay control wire shorted to a battery-voltage circuit in the harness ($150–$600)
- CommonFailed ECM power relay (internal short or stuck contacts) ($40–$180)
- OccasionalIncorrect or aftermarket relay installed in the socket ($20–$100)
- RareECM relay driver failure ($1200–$3500)
How to fix
Verify the correct relay part is installed and swap with a known-good relay
When to use: First check, especially after recent electrical work
With relay and ECM disconnected, check the control circuit for voltage present (short to power)
When to use: When the relay swap does not clear the code
Inspect harness near battery feeds and aftermarket accessory wiring for damage or splices
When to use: When a short to power is found or the truck has added accessories
Replace or repair the ECM after wiring is verified
When to use: Only when the circuit is clean and the code persists
Affects most
No specific vehicle pattern found in our complaint database. This code can appear on any vehicle that supports OBD-II diagnostics.
Related
- → Browse all DTC trouble codes
- → Decode a VIN to check codes for your truck
- → Owner-operator maintenance guides
- → P0670 — Glow Plug Control Module Circuit
- → P0671 — Cylinder 1 Glow Plug Circuit
- → P0672 — Cylinder 2 Glow Plug Circuit
- → P0673 — Cylinder 3 Glow Plug Circuit
- → P0674 — Cylinder 4 Glow Plug Circuit
Frequently Asked Questions
Can I drive with code P0687?
Drive with caution. P0687 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 P0687 on a commercial truck?
Total repair cost ranges $20–$3500 depending on the root cause. The cheapest fix is usually the first one listed below — Relay control wire shorted to a battery-voltage circuit in the harness — 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 P0687 fail a DOT inspection?
P0687 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 P0687 and related codes?
P0687 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 P0687 is set without the companions, the issue is more isolated.
How do I prevent P0687 from coming back after a repair?
For P0687 (ECM Power Relay Control Circuit High), 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.