Diagnosis & Repair
WARNING: This page is about a different car, the 2003 Lexus LS 430. However, it is still accessible from the selected car via links, so may be relevant.
Retrieve trouble codes. See RETRIEVING DIAGNOSTIC TROUBLE CODES under SELF-DIAGNOSTIC SYSTEM. If other codes are displayed, diagnose and repair those codes first and retest. If only DTC P0116 or P0125 are displayed, check thermostat. Replace as necessary. If thermostat is okay, replace ECT sensor and retest.