P0689

P0689 sets when the PCM's sense circuit on the main power relay reads low — the PCM expected 12V on the sense pin with the relay commanded on and saw something below the threshold (typically under 3-4V). This is closely related to P0688 (open) but indicates the wire is shorted to ground OR the relay contacts have high resistance and only partial voltage is making it through. Cheapest-first ladder: (1) Pull and bench-test the relay — coil should be 60-80 ohms, contacts should pass a continuity check with no measurable resistance when energized; any contact resistance above 0.2 ohms means pitted contacts and the relay needs to be replaced. (2) Voltage-drop test from the relay output terminal to the PCM sense pin with the relay energized — should be under 0.1V drop; anything higher is wire corrosion, often at a body connector or splice pack. (3) Inspect the sense wire for a chafe-to-ground point — these failures cluster at firewall pass-throughs and along the inner fender harness routing. Target threshold: 12V sense relay-on, 0V relay-off, with under 0.1V drop end-to-end on the sense circuit. Expensive-misdiagnosis caveat: the same Honda K-series and Chrysler Pentastar relay-PCM ambiguity applies here — swap the $25 relay and re-run the scan before quoting a $700 PCM, and on the Chrysler platforms specifically, check the TIPM internal relay before condemning anything because the TIPM is a sealed assembly that can fail the sense circuit through internal corrosion regardless of what the bench-test on the external relay shows.

Vehicle-specific patterns: 2001-2005 Honda K-series (Civic, Accord, CR-V, RSX) throws P0689 alongside P0688 when the main-relay solder joints crack and intermittently short to chassis as the PCB flexes — reflow fixes both. 2011-2014 Chrysler 3.5/3.7/3.6 Pentastar lineup (Town & Country, Grand Caravan, Charger, Wrangler) throws P0689 from TIPM water intrusion via the cowl drain — corrosion bridges the relay output to ground inside the module; the TIPM is the only fix. 2007-2014 GM full-size trucks throw P0689 from under-dash relay solder failures combined with chafe at the steering column harness. 2004-2011 Mazda RX-8 throws P0689 when the relay socket terminals corrode enough to drop sense voltage below threshold without fully opening. 2003-2008 Subaru Forester/Outback with the EJ25 throws P0689 from a known fuse-box water intrusion via the cowl drain (similar mechanism to the Chrysler TIPM). Estimated repair: $25 (relay) to $900 (TIPM + programming).