P2404

P2404 means the ECM is seeing the LDP pressure-sense signal in range electrically (it's not pegged high or low) but the value is not behaving the way the ECM expects during a commanded pump cycle — typically the signal isn't moving when the pump activates, or it's drifting in ways that don't correlate with the test sequence. A healthy sense signal reads 2.3-2.7V at atmospheric rest, climbs to roughly 3.5-4.0V when the pump pressurizes the system to about +5 inH2O during the leak test, and decays back to atmospheric over the test window (typically 30-180 seconds); the ECM flags P2404 when the signal stays flat during commanded pump operation or doesn't decay at the expected rate. Cheapest-first ladder: data-log the sense voltage during a bidirectional pump activation (flat line = blocked sense port, no pump action, or clogged canister vent), perform a smoke-test of the EVAP system to verify the pump can actually build pressure (a sealed cap missing means the pump never reaches threshold), and inspect the canister vent valve for stuck-open condition (vent stuck open = pump can't build pressure regardless of how good the sensor is). Expensive misdiagnosis caveat: VW/Audi/Chrysler LDP is mounted in a notorious water-collection point above the rear axle — corrosion at the connector is the #1 root cause; replace the pigtail before replacing the pump, and rule out a stuck-open canister vent solenoid before condemning the sense element.

Vehicle-specific patterns: 1998-2009 VW/Audi (Jetta, Passat, Golf, A4, A6) throws P2404 from a clogged sense port inside the rear-axle LDP — water or canister carbon migrates into the sense passage and the sensor stops responding to pressure changes. 1998-2008 Chrysler/Jeep/Dodge (Sebring, Wrangler, Grand Cherokee) NVLD/LDP combination units throw P2404 when the internal diaphragm tears and the sensor sees no delta during a pump cycle. 1996-2005 Mercedes-Benz (W202, W203, W211) throws P2404 from a stuck canister vent valve that never closes for the leak test, leaving the system unable to build any pressure for the sensor to read. Common LDP repair pattern: replace pump + harness pigtail together; standalone pump replacement leaves the same root cause. Estimated repair: $220 to $800.