P0688

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: (1) 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. (2) 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. (3) 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 patterns: 2001-2005 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 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. 2011-2014 Chrysler 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. 2007-2014 GM trucks (Silverado, Sierra, Tahoe) throw P0688 from a solder failure on the under-dash BCM/relay assembly. 2004-2011 Mazda 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).