OBD-II Code · Computer
P0688
ECM/PCM Power Relay Sense Circuit Open
Severity
high
Left alone, this one tends to take other parts with it.
Can you drive it
Not for long
Driving on this one risks further damage. Get it to a shop rather than carrying on with it.
Indicative cost (USD)
$30–$300
Varies by vehicle, diagnosis, parts, and local labor. This is not a repair quote.
Sense line for PCM relay open.
How a shop works this
Work from evidence to part, not the other way round. A code names a circuit or a symptom; it does not name the part that failed.
Note every code that is set, not just this one.
Codes that appear together usually narrow the cause faster than any one of them read alone.
Read the freeze frame — the conditions the fault was recorded under.
Cold or warm, idle or load, road speed and engine speed. A fault that only appears cold is a different fault from one that only appears under load, even with the same code on the scanner.
Confirm the fault is current rather than history.
Clear the code and see whether it returns under those same conditions. A code that does not come back may have been logged during a repair, a flat battery, or a bad connection that has since been disturbed.
Then work the likely-causes list in the order shown.
That order is what a shop checks first for this code — wiring and connections before the component they run to, because a connector costs nothing to inspect and is a common answer.
If the code comes back after a repair, the underlying fault is still there. That is the point to bring in a scanner with live data and freeze-frame, or a shop that has one.
Common symptoms
- Intermittent stall
- No start
Likely causes
- WiringMost common
- Failed relay
Read the full diagnostic procedureP0688 sets when the PCM's sense circuit on the main power relay (the relay that feeds the PCM itself plus some accessory circuits) reads open — the P…
P0688 sets when the PCM's sense circuit on the main power relay (the relay that feeds the PCM itself plus some accessory circuits) reads open — the PCM expected 12V on the sense pin with the relay energized and saw nothing. This is the PCM watching its own life-support circuit, and on most platforms the code only sets after a successful start (the PCM had to be alive to log it), which tells you the relay is intermittent, not stuck-off.
Cheapest-first ladder
- Pull the main/ECM/PCM relay from the underhood fuse box and inspect the socket terminals — spread or oxidized terminals are the #1 cause, $5 in DeoxIT and a pin-tightener handles it.
- Bench-test the relay — a standard 30-70A automotive relay should read 60-80 ohms across the coil and click crisply when 12V is applied.
- Backprobe the sense wire (typically the 87 or 87a leg routed to a PCM pin) and key the ignition on — should read 12V relay-on, 0V relay-off. If the relay clicks but no 12V reaches the PCM pin, the sense wire is broken or the relay contacts are pitted. Target threshold: 12V sense voltage with relay energized, drops to 0V within 1-2 seconds of key-off (depending on PCM keep-alive logic). Expensive-misdiagnosis caveat: on aged Honda K-series and Chrysler 3.5/3.7 vehicles a main relay that has welded its contacts or that opens intermittently under heat soak looks identical to PCM failure on the scan tool — a $25 relay versus a $700 PCM. Always swap the relay before quoting the module.
Vehicle-specific patterns2001-2005 Honda Civic/Accord/CR-V with the K-series 4-cylinder is the textbook P0688 vehicle — the main relay (PGM-FI relay) mounted under the dash a…
- 2001-2005Honda Civic/Accord/CR-V with the K-series 4-cylinder is the textbook P0688 vehicle — the main relay (PGM-FI relay) mounted under the dash above the driver's kick panel has a known solder-joint fracture on the relay PCB; reflowing the solder fixes the no-start AND clears the code, $0 in parts.
- 2011-2014Chrysler 200/300/Town & Country/Dodge Charger/Jeep Grand Cherokee with the 3.6L Pentastar throws P0688 from TIPM (Totally Integrated Power Module) corrosion and internal relay failure — Chrysler issued multiple recalls and TSBs but plenty of out-of-coverage cases still walk into shops; the TIPM is $400-800 plus programming.
- 2007-2014GM trucks (Silverado, Sierra, Tahoe) throw P0688 from a solder failure on the under-dash BCM/relay assembly.
- 2004-2011Mazda RX-8 sets P0688 from the main relay above the fuse box near the firewall — relay socket terminals oxidize from engine-bay humidity.
Estimated repair$25 (Honda relay) to $900 (TIPM replacement + programming on a Pentastar).
Look up another code
P0688 — frequently asked
What does a P0688 code mean?
Sense line for PCM relay open.
What causes a P0688 code?
Common causes: Wiring; Failed relay.
What are the symptoms of a P0688 code?
Typical symptoms: Intermittent stall; No start.
Is it safe to drive with a P0688 code?
Not for long. Driving on this one risks further damage. Get it to a shop rather than carrying on with it. Estimated repair: $30-$300.
How do I diagnose a P0688 code?
Work from evidence to part, not the other way round. A code names a circuit or a symptom; it does not name the part that failed. 1. Note every code that is set, not just this one. 2. Read the freeze frame — the conditions the fault was recorded under. 3. Confirm the fault is current rather than history. 4. Then work the likely-causes list in the order shown.
More free tools