P0686🔧 Engine computer power supply (PCM power relay)Stop and fix promptly

P0686: ECM/PCM Power Relay Control Circuit Low (Meaning, Causes, Fixes)

Reviewed 2026-10-25 · Based on 2 cited source(s)

What P0686 means

Most OBD-II vehicles feed battery voltage to the powertrain computer (PCM) through a relay, though OBD-Codes.com notes a few makers still use only a fused circuit. When the ignition is switched on, a small coil in the relay is energized (often with a light click), its contacts close, and battery voltage flows out to the PCM and, on many designs, to a controller area network (CAN) signal terminal. OBD-Codes.com says P0686 is stored when a low or no battery voltage condition is detected, by the PCM or another controller, in the circuit that powers the PCM or the circuit other controllers use to monitor PCM supply voltage. RepairPal puts it more broadly: the PCM sets P0686 if it detects an incorrect voltage signal from the power relay. It is a generic code for 1996 and newer vehicles, but OBD-Codes.com notes the cause can differ a little between brands. The sources differ in emphasis. OBD-Codes.com lists the ignition switch, fuses and fusible links, and battery cable ends along with the relay and wiring, while RepairPal lists only PCM failure, power relay failure and wiring. RepairPal also lists a hard or no start in cold weather, which OBD-Codes.com does not mention. P0685 covers an open in the same relay control circuit.

Severity: can I keep driving?

Stop and fix promptly

Get it fixed right away. RepairPal rates P0686 Urgent, meaning immediate action is needed, and says the engine may not start, especially in cold weather. OBD-Codes.com says a no-start usually comes with this code, so ignoring it is not likely to be an option. If the engine still starts and runs, OBD-Codes.com points to the engine computer itself or a programming error, so have it diagnosed before you rely on the car.

Common symptoms

Likely causes

Roughly most to least common, based on the sources below. Your vehicle may differ.

  1. Bad PCM power relay (OBD-Codes.com, RepairPal)
  2. Blown fuse or fusible link (OBD-Codes.com)
  3. Corroded or damaged wiring or connectors, especially near the PCM relay (OBD-Codes.com); wiring issue (RepairPal)
  4. Faulty ignition switch, or a loose or disconnected terminal at the ignition switch (OBD-Codes.com)
  5. Loose or corroded battery cable ends (OBD-Codes.com)
  6. Faulty engine computer (PCM) or a programming error (OBD-Codes.com, RepairPal)

Diagnostic steps you can do

⚠ Before you start

Work on a cool engine, parked on level ground with the parking brake set. Never work under a car supported only by a jack.

  1. Check for technical service bulletins (TSBs) for your vehicle first; OBD-Codes.com notes your problem may be a known issue with a known fix.
  2. Look over the relay box, wiring and connectors. OBD-Codes.com says to watch for relays that have crept out of their sockets or have corroded blades or terminals. Check that the battery and battery cable ends are tight and clean.
  3. Turn the key on and feel or listen for the PCM relay to click. OBD-Codes.com says swapping in an identical relay (matching part numbers) is a handy test, and to check alternator output too.
  4. With a multimeter and a wiring diagram, OBD-Codes.com describes checking the relay for constant battery voltage (none points to a fuse or fusible link), ignition-on voltage (none points to a fuse or the ignition switch), a good ground, and then output voltage to the PCM and CAN. No output with good inputs points to the relay; output at the relay but none at the PCM points to wiring. Unplug the computers before measuring resistance.

Typical fixes

Repair costs vary widely by vehicle, region, parts brand and shop, so we don't quote prices. Get a written estimate after diagnosis.

🛠 When to call a pro

  • The car won't start and you can't find a bad relay, fuse or battery connection
  • You don't have a multimeter or a wiring diagram for the PCM power circuit
  • A fuse keeps blowing after you replace it
  • Tests point to the engine computer, which may need programming to your vehicle

Where to buy

Plain search links. Always match parts to your exact year, make, model and engine.

OBD2 scanners on Amazon Parts at Advance Auto Parts catalog at RockAuto

FAQs

Can I drive with P0686?

Usually you can't, because OBD-Codes.com says a no-start usually comes with it. RepairPal rates it Urgent. If the car does start and run, get it diagnosed soon; OBD-Codes.com says that points to the engine computer or its programming.

What's the difference between P0685 and P0686?

Both involve the relay that powers the engine computer. P0685 is the circuit open code, and P0686 is the circuit low code, set when low or no voltage is detected. OBD-Codes.com and RepairPal list very similar causes for both.

Is P0686 just a bad relay?

Often, but not always. Both sources list the power relay and wiring, and OBD-Codes.com adds blown fuses or fusible links, the ignition switch and loose or corroded battery cables. Swapping in an identical relay is a quick test before buying parts.

How much does it cost to fix P0686?

Neither source gives a repair cost for P0686. It can be a relay, a fuse, a cable end or wiring, so diagnose it before buying parts.

Sources

  1. OBD-Codes.com: P0686 ECM/PCM Power Relay Control Circuit Low
  2. RepairPal: P0686 ECM/PCM Power Relay Control Circuit Low (rated Urgent)

Last reviewed 2026-10-25. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.