P1605

P1605 on a Toyota is a Knock Control CPU internal fault — the ECM ran its own internal diagnostic on the knock-control sub-processor (or the dedicated knock-control IC on older two-chip ECMs) and detected an internal failure. This is one of the rare Toyota P1xxx codes where the ECM is actually the failed part on a non-trivial percentage of cases, but you still walk the cheaper inputs first. The cheapest-first ladder: scan live knock-sensor data and watch the piezoelectric voltage signal (0-5V AC peaks during combustion knock events) while gently revving — flat-line zero on both knock sensors usually means a broken sensor wire or a corroded sensor connector, not an ECM failure. Next, ohm-out each knock sensor at the connector (typical Toyota knock sensor reads 120-280 kilohms across the element on a flat-response sensor or shows a tuned-resonance frequency curve on the tuned types). Then inspect the ECM connectors C24/C25/C26 for green corrosion, especially the ECM ground points on the firewall and intake manifold. If knock sensors are healthy and the ECM connectors are clean, an ECM reflash via Toyota Techstream sometimes clears P1605 (firmware bugs on early 2GR-FE and 2UZ-FE platforms). Only after all of that should you condemn the ECM. Don't replace the ECM before checking for a single failed knock sensor pulling the bus voltage down — 1UZ-FE / 2UZ-FE / 3UZ-FE engines have two knock sensors wired in parallel, and one shorted sensor can manifest as a CPU-internal fault.

Vehicle-specific patterns: 1998-2007 Land Cruiser / Lexus LX470 (2UZ-FE) and Tundra/Sequoia 4.7L throw P1605 from the well-known knock-sensor sub-harness failure that lives under the intake manifold — the sensors themselves are buried in the valley and the sub-harness wire insulation breaks down from heat soak; Toyota sells a replacement sub-harness as a single part for around $80, but the labor to pull the intake is 4-6 hours. 2007-2017 Camry / Avalon / Highlander 3.5L (2GR-FE) throws P1605 from a corroded knock-sensor connector on the rear bank that floods with rainwater entering through the cowl drain. 2001-2010 Lexus IS300/GS300/LS430 throws P1605 from ECM solder-joint cracks at the knock-control IC pins — TechStream reflash sometimes masks it; a rebuilt ECM is the real fix. 2003-2009 4Runner / FJ Cruiser (1GR-FE) throws P1605 from the same sub-harness pattern as the 2UZ. Estimated repair: $80 (single sensor on accessible engines) to $1,400 (sub-harness with intake removal or rebuilt ECM).