P0337

P0337 means the CKP signal voltage is stuck below the ECM's low-threshold limit — typically below 0.5V on a Hall-effect signal that should be swinging 0-5V, or a VR signal that is producing less than 200 mV peak-to-peak at cranking. The signal is present electrically but too weak to count as valid pulses. Cheapest-first ladder: back-probe the signal wire at the ECM connector (not the sensor) and scope it during cranking — if the signal is healthy at the sensor but weak at the PCM, you have wire resistance somewhere in between. Measure signal-wire resistance end-to-end (should be under 5 ohms for a 6-foot run); anything over 10 ohms means corroded pins or a damaged conductor. Check for poor sensor ground — back-probe the sensor ground pin and measure to battery negative with key on (should be under 50 mV; over 200 mV means the ground path is dirty). Inspect the sensor tip for oil contamination or accumulated debris that increases the effective air gap and drops signal amplitude. The expensive misdiagnosis is replacing the sensor and PCM when the actual fault is a green crusty C1 main engine harness connector behind the alternator.

Vehicle-specific patterns: BMW N54 (2007-2010 335i, 535i, 1M, Z4) is the textbook P0337/P0335 hot-start failure — the CKP sensor fails only after heat-soak, will start fine cold, then crank-no-start after a 10-minute restaurant stop, then start fine again after cooling. Replace the sensor and the connector pigtail together because the heat also degrades the connector seal. Nissan VQ35DE (Altima, Maxima, Pathfinder 2002-2008) loses CKP signal amplitude as the sensor magnetics weaken from exhaust-heat aging — bench-test will show a sensor that ohms in-spec cold but produces 60% of normal output hot. GM 3.6L LY7/LLT throws P0337 when the CKP pigtail melts against the exhaust manifold heat shield on high-mileage units. Honda K24 (2003-2011 Accord, Element, CR-V) Hall-effect CKP loses amplitude as the internal IC degrades from underhood heat cycling. Estimated repair: $140 to $480.