OBD-II Diagnostic Trouble Code
P0686 — ECM Power Relay Control Circuit Low
P0686 indicates the ECM power relay control circuit voltage is lower than expected. The ECM saw the relay driver circuit pulled low when it should be high, typically from a short to ground, a shorted relay coil or excessive resistance in the circuit.
Severity 4/5
Truck-relevant
Stop within 50 mi
Severity
Stop within 50 mi
ECM Power Relay Control Circuit Low
Code
P0686
Category
Powertrain
Subsystem
Network
Truck-relevant
Yes
What this means for truckers
P0686 (ECM Power Relay Control Circuit Low) — ECM power relay control shorted to ground. Relay can't be commanded on. May cause no-start.
Before you replace parts — try this first
P0686 on the Network system is most often relay control wire chafed and shorted to ground against frame or cab — 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
No-start or hard start, ECM losing power or resetting while driving, intermittent stall, possible loss of the diagnostic link, and the check-engine light. A short to ground can also blow the related fuse repeatedly.
Common causes
- CommonRelay control wire chafed and shorted to ground against frame or cab ($150–$600)
- CommonFailed ECM power relay with shorted coil ($40–$180)
- OccasionalCorrosion or water intrusion in the relay/fuse box causing leakage to ground ($100–$450)
- OccasionalWeak batteries or poor main power feed pulling system voltage low ($100–$700)
- RareECM relay driver fault ($1200–$3500)
How to fix
Load test batteries and check cable connections and charging voltage
When to use: First, especially if other low-voltage codes are present
Measure relay coil resistance and replace a relay that is shorted or out of spec
When to use: When batteries are good and the code sets with key on
Disconnect the relay and ECM and check the control wire for a short to ground; repair chafed harness
When to use: When fuses blow or resistance to ground is found
Clean and seal a corroded relay/fuse center
When to use: When corrosion or water is visible in the power distribution box
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 P0686?
Drive with caution. P0686 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 P0686 on a commercial truck?
Total repair cost ranges $40–$3500 depending on the root cause. The cheapest fix is usually the first one listed below — Relay control wire chafed and shorted to ground against frame or cab — 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 P0686 fail a DOT inspection?
P0686 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 P0686 and related codes?
P0686 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 P0686 is set without the companions, the issue is more isolated.
How do I prevent P0686 from coming back after a repair?
For P0686 (ECM Power Relay Control Circuit Low), 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.