P0688🔧 Engine computer power supply (PCM power relay)Stop and fix promptly

P0688: ECM/PCM Power Relay Sense Circuit Open (Meaning, Causes, Fixes)

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

What P0688 means

The powertrain computer (PCM) gets its battery voltage through a power relay that switches on with a signal from the ignition switch. OBD-Codes.com describes it as typically a five-wire relay: constant battery voltage on one wire, ground on another, the ignition switch signal on a third, voltage out to the PCM on a fourth, and a fifth power relay sense circuit the PCM uses to monitor relay voltage. OBD-Codes.com says P0688 is stored when the PCM detects that this sense circuit is open, and the check engine light may come on. RepairPal, which calls the code 'ECM/PCM Power Relay Sense Circuit/Open', describes it more broadly: the PCM sets P0688 if it detects an incorrect voltage signal from the power relay. It is a generic code for 1996 and newer vehicles, and OBD-Codes.com notes exact repair steps vary by year, make, model and engine. The sources differ on symptoms and causes. OBD-Codes.com lists a delayed or no start, inoperative electrical accessories and drivability issues, with a bad relay, a blown fuse or fusible link, or an open or shorted circuit as causes; RepairPal lists 'engine may not start' and 'hard or no start in cold weather', with PCM failure, power relay failure and wiring as causes. P0685, P0686 and P0687 cover the relay's control circuit.

Severity: can I keep driving?

Stop and fix promptly

Get it fixed soon. OBD-Codes.com says P0688 should be treated as severe because it can cause a no-start and drivability problems, and RepairPal rates it Urgent, meaning immediate action is needed.

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. Open or shorted wiring between the power relay and the PCM (OBD-Codes.com); wiring issue (RepairPal)
  4. Faulty engine computer (PCM) (RepairPal); OBD-Codes.com also mentions a PCM programming error once wiring and relay check out

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. Read all stored codes and freeze frame data with a scan tool and write them down, then clear the codes and drive to see if P0688 comes back. OBD-Codes.com says if it doesn't return and there are no drivability symptoms, the car can be driven normally; if it is intermittent it may be harder to find.
  2. Check for technical service bulletins (TSBs) that match your vehicle, code and symptoms.
  3. If the code comes right back, inspect the wiring and connectors around the PCM power relay and the PCM, and repair any broken or unplugged harnesses.
  4. Test the fuses and relays that feed the PCM power relay. OBD-Codes.com says to test fuses and fusible links with the circuit loaded so you don't misdiagnose them.
  5. With a multimeter and a wiring diagram, check for battery voltage and ground at the relay, then relay output voltage at the relay and at the PCM connector. OBD-Codes.com says no output at the relay points to the relay, output at the relay but not at the PCM points to an open circuit, and output at the PCM points to the PCM or its programming.

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 or stalls
  • You don't have a multimeter or a wiring diagram for the PCM power circuit
  • The code is intermittent and won't come back on demand
  • 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 P0688?

It's risky. OBD-Codes.com says to treat it as severe because it can cause a no-start and drivability problems, and RepairPal rates it Urgent. OBD-Codes.com adds that if the code doesn't come back after clearing and there are no symptoms, the car can be driven normally.

What's the difference between P0685 and P0688?

Both involve the relay that powers the engine computer. P0685 to P0687 cover the relay's control circuit (open, low and high). P0688 is about the separate sense circuit the computer uses to monitor the relay's voltage, which OBD-Codes.com says reads open.

Will P0688 stop my car from starting?

It can. OBD-Codes.com lists a delayed or no start, and RepairPal lists 'engine may not start' and a hard or no start in cold weather.

How much does it cost to fix P0688?

Neither source gives a repair cost for P0688. It can be a relay, a fuse, wiring or the computer, so diagnose it before buying parts.

Sources

  1. OBD-Codes.com: P0688 ECM/PCM Power Relay Sense Circuit Open
  2. RepairPal: P0688 ECM/PCM Power Relay Sense Circuit/Open (rated Urgent)

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