P0686 Code: ECM/PCM Power Relay Control Circuit Low
The main relay control line is being pulled to ground – usually a shorted relay coil or a chafed wire.
At a glance
Severity: Moderate
- Meaning
- ECM/PCM Power Relay Control Circuit Low
- System
- ECU & Outputs · Powertrain
- Safe to drive?
- With caution
- Typical repair cost
- $25–$1,800
- DIY difficulty
- Moderate
The ECM switches its own main power relay through a driver output and constantly checks the voltage on that line. P0686 means the voltage was lower than it should be for the commanded state, which in practice means the line, or the relay coil hanging on it, has a path to ground. The most common source is the relay itself: a coil with shorted windings or a failed suppression diode draws far more current than normal. A control wire chafed through near the battery tray or relay box, or eaten by battery acid, does the same. Depending on whether the ECM switches the relay's ground side or supply side, the effect is a relay that won't pull in, an intermittent stall, or a relay that stays on longer than it should. Repeated shorts can also damage the ECM's driver, so find the short before fitting yet another relay.
Can I keep driving? If the engine runs normally, you can drive to a shop, but avoid long trips because the relay may drop out. If a new relay fails again within days, stop replacing relays and have the wiring checked – the short can damage the ECM.
How to read P0686
- P Powertrain
- 0 Generic (SAE)
- 6 Computer & outputs
- 86 Specific fault
Symptoms
- Intermittent stalling or a no-start that comes and goes
- Main relay feels hot to the touch or its case is discolored
- Replacement relays fail again after a short time
- Blown fuse in the relay or ECM feed circuit
- Check engine light, sometimes with P0685 stored alongside
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Relay coil with shorted windings or a failed suppression diode
- Control wire chafed to ground in the engine bay harness
- Battery acid or water corrosion near the battery tray and relay box
- Incorrect relay fitted, with a different coil or diode layout
- ECM relay driver damaged by an earlier short
- Aftermarket wiring tapped into the ECM power circuit
Diagnosis step by step
Note when the code sets
The freeze frame tells you whether P0686 set at key-on, while driving or during after-run. A code that sets at key-on together with a no-start points to a hard short; one that sets while driving on rough roads suggests a chafing wire.
Tools: OBD-II scanner
Examine and measure the relay
Pull the relay and look for a scorched case or melted legs. Measure the coil: typical values are around 50–120 ohms, and anything well below that means shorted windings. If the relay has a diode, check it in both directions with the meter's diode test – conduction both ways means it has failed.
Tools: Multimeter
Check the control wire to ground
With the relay out and the ECM connector disconnected, measure between the relay socket's control terminal and a clean ground. The reading must be open. Any low resistance means the wire has a ground path; wiggle the harness to catch an intermittent one.
Tools: Multimeter
Inspect around the battery tray
Harnesses routed past the battery and relay box can be eaten by acid or leaking electrolyte, leaving green, crumbly copper under intact-looking insulation. Peel back the tape where the harness passes the battery and check for swelling or discoloration.
Tools: Flashlight, Gloves
Confirm the correct relay part
Compare the part number with the fuse box label or parts catalog. Relays with an internal diode are polarity-sensitive, and a generic relay with a lower coil resistance can overload the ECM driver even though it clicks normally.
Tools: Flashlight
Test the ECM driver with a test light
With the wiring proven good and a correct relay on hand, connect a test light between battery positive and the control terminal of the empty socket. It should light when the ECM commands the relay on. No light with good wiring means the driver was damaged and the ECM needs repair.
Tools: Multimeter
Where the parts are
- 1Battery
- 2Alternator
- 3Engine computer (ECM)
- 4CAN bus
- 5Transmission
- 6Wheel speed sensor
- 7High-voltage battery
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Replace relay with the correct part | Yes | $25–$120 |
| Repair control wire shorted to ground | No | $120–$380 |
| Repair acid-damaged harness near the battery | No | $150–$500 |
| Repair ECM relay driver (mail-in service) | No | $250–$600 |
| Replace ECM with programming | No | $700–$1,800 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Emissions / smog test impact
Will fail
P0686 keeps the check engine light on, so the car fails an OBD emissions test. After repairing the short and clearing the code, allow a few drive cycles for the readiness monitors to set.
Frequently asked questions
Why does my new main relay keep failing?
Because something is overloading it or its control circuit. A wire with a partial short to ground, a relay of the wrong type or a damaged ECM driver will keep killing relays. Measure the circuit before buying another one.
Can a shorted relay damage the ECM?
Yes. The ECM's driver is designed for the small current of a healthy relay coil. A shorted coil or a wire to ground forces it to carry far more, and repeated events can burn out the driver so the ECM can no longer switch its own power.
How is P0686 different from P0685?
P0685 means the control circuit is open – nothing is connected. P0686 means the line is being pulled toward ground, so the search is for a short, an overloaded coil or a damaged driver rather than a break.
Is it okay to use a generic relay from the parts store?
Only if its coil resistance, pin layout and diode or resistor arrangement match the original exactly. Generic relays that fit the socket but differ inside are a known cause of this code.
Sources
- SAE J2012 – Diagnostic Trouble Code Definitions
- ISO 15031-6 – Road vehicles, diagnostic trouble code definitions (EOBD)
- Manufacturer public service information (cross-checked)
- Methodology: How We Build Code Pages · Data Sources We Use · Editorial Policy