P0690

P0690 sets when the PCM's sense circuit on the main power relay reads HIGH — the PCM saw voltage on the sense pin when it expected the relay to be off, OR the voltage exceeded the upper operating threshold (typically over 16V) when energized. This is the PCM detecting either a stuck-closed relay (contacts welded) or a back-feed from another circuit on the relay's load side. Cheapest-first ladder: (1) Pull the relay and bench-check for welded contacts — they should open with no continuity when the coil is de-energized; any continuity across 30-87 with coil off means the contacts have welded, replace the relay. (2) Inspect the load-side circuit for back-feed — aftermarket installs (dash cams, lift-pumps, light bars) wired to a constant-12V source can back-feed through a shared splice and confuse the sense circuit; pull aftermarket fuses one at a time and re-test. (3) Verify alternator output is in spec (13.5-14.7V running) — an over-volting charging system can push the sense voltage past the upper threshold and look like a sense fault when the actual problem is regulation. Target threshold: 12V sense relay-on (max 16V), 0V sense relay-off. Expensive-misdiagnosis caveat: a stuck-closed main relay can keep the PCM powered after key-off and slowly drain the battery overnight, which gets misdiagnosed as parasitic draw (alarm system, dome light, etc.) when the actual cause is a $25 relay. On Chrysler TIPM-equipped vehicles, a P0690 with a sealed TIPM means the only repair is the full $400-800 module plus programming.

Vehicle-specific patterns: 2001-2005 Honda Civic/Accord/CR-V K-series throws P0690 less often than P0688/P0689 but when it does, it's almost always a welded main-relay contact set after a high-current event (e.g., a jump-start with reversed cables). 2011-2014 Chrysler 3.6L Pentastar TIPM internal relays weld closed at a known failure rate; the parasitic-drain symptom is the giveaway. 2007-2014 GM trucks with under-dash relay blocks throw P0690 from heat-aged contacts that weld during a brief overcurrent. 2004-2011 Mazda RX-8 throws P0690 from main-relay welding after running with chronically low compression and a dragging starter (the starter pulls high amps and the relay carries some of the inrush). 2003-2009 Toyota 4Runner/Tacoma 1GR-FE V6 sets P0690 from corroded EFI main-relay sockets that put intermittent high voltage on the sense pin. Estimated repair: $25 (relay) to $900 (TIPM).