Diagnosis & Repair

WARNING: This page is about a different car, the 2003 Toyota Prius. However, it is still accessible from the selected car via links, so may be relevant.

NOTE: If problem exists with by-pass valve for HCAC system, it is possible that DTC P1431 will be set before DTC P1436 and DTC P1437. Using Toyota hand-held tester or scan tool, read FREEZE FRAME data. Freeze frame data records engine conditions when malfunction is detected.

Check for any other DTCs. If only DTC P1431 exists, go to next step. If other DTCs exists, diagnose and repair those DTCs first and then retest.
Check operation of vacuum sensor. See VACUUM SENSOR FOR HYDROCARBON ADSORBER CATALYST SYSTEM under ENGINE SENSORS & SWITCHES in SYSTEM & COMPONENT TESTING - 4-CYLINDER article. If vacuum sensor operates properly, check for restricted vacuum line to vacuum sensor and repair as necessary. If vacuum sensor does not operate properly, replace vacuum sensor.