FixVault
Home › Lexus › 2000 › LS 400 › Repair and Diagnosis › Engine Performance › System › Engine Controls - Self-Diagnostics › Diagnostic Tests › DTC P1520: Stoplight Switch Signal Malfunction › Diagnosis & Repair

Diagnosis & Repair

  1. Check stoplights. If stoplights do not function properly, repair as necessary and retest system. See appropriate wiring diagram in EXTERIOR LIGHTS article in ACCESSORIES & EQUIPMENT. If stoplights function properly, go to next step.
  2. If using Lexus scan tool, go to next step. If not using Lexus scan tool, go to step  4.
  3. Connect Lexus scan tool to Data Link Connector (DLC) No. 3. See Figure. Turn ignition on. Using scan tool, read STP signal while depressing and releasing brake pedal. STP signal should be on when brake pedal is depressed. STP signal should be off when brake pedal is released. If STP signal does not operate as specified, go to step  5. If STP signal operates as specified, check wire harness and connectors. Problem is intermittent.
  4. Access ECM behind glove box. Turn ignition on. Backprobing, measure voltage between ground and terminal No. 6 (Blue/Black wire) at ECM E11 connector. See Figure. With brake pedal depressed, voltage should be 7.5-14.0 volts. With brake pedal released, voltage should be less than 1.5 volts. If voltage is not as specified, go to next step. If voltage is as specified, check wire harness and connectors. Problem is intermittent.
  5. Check for short circuit in wiring harness between ECM and stoplight switch. See appropriate wiring diagram in WIRING DIAGRAMS article. Repair as necessary and retest system. If wiring harness is okay, replace ECM. Retest system.
About FixVault