P1166

P1166 on Honda is an A/F Sensor Heater Power Circuit fault on Bank 1 Sensor 1, distinct from P1163 (sensor performance) in that P1166 specifically targets the +B power feed to the heater coil rather than the heater's low-side driver or the sensing element itself. The Honda wideband heater draws roughly 1.0 to 2.5 A at operating temperature, with cold inrush briefly higher; the PCM watches heater current and supply voltage and sets P1166 when supply voltage at the sensor's +B pin falls outside spec while the heater is commanded on. Cheapest ladder: check the PGM-FI main relay (a known Honda weak point that ages out of older Civics and Accords with intermittent symptoms before throwing a hard code), inspect the under-hood fuse box for the AFR heater fuse (commonly 15A), and back-probe the sensor's +B pin key-on, engine-running for battery voltage within 0.3V of the battery itself. Pull the sensor connector and inspect for green oxidation, melted pins, or pin-back. Measure heater resistance cold across the heater pins (Honda wideband typical 2 to 4 ohms; out-of-spec means kill the sensor). The expensive misdiagnosis is throwing a $200 to $300 OE Denso AFR sensor at the car when the actual fault is a $40 main relay or a corroded fuse-box terminal that drops voltage under load.

Vehicle-specific patterns: Honda Civic D17A and R18 (2001-2011) sees P1166 frequently from PGM-FI main relay degradation. Honda Accord K24A4/A8 (2003-2007) sets P1166 from a corroded under-hood fuse box where the AFR heater feed terminal greens over from road-salt intrusion. Honda Odyssey J35 V6 (2005-2010) sees P1166 commonly from sensor pigtail damage where the harness routes near the exhaust manifold heat shield. Honda CR-V K24Z (2007-2011) typically sets P1166 from a failing heater element inside the sensor itself by 130k to 160k miles. Estimated repair: $30 to $440.