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Home >> Toyota >> 2021 >> Highlander LE, 4WD >> Repair and Diagnosis >> External Pages >> Different variant/trim >> Section 52 (Engine Control System (A25A-FXS) - Diagnostic Codes (P0128-P0171)) >> SFI System >> DTC P0128-00: Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature); DTC P0128-07: Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature) Mechanical Failure [11/2023 - ] >> Description

DTC P0128-00: Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature); DTC P0128-07: Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature) Mechanical Failure [11/2023 - ]: Description

WARNING: This page is about a different variant/trim than selected.

The ECM uses the engine coolant temperature sensor, installed to the water outlet, and the No. 2 engine coolant temperature sensor, installed to the radiator pipe assembly, to monitor the operation of the thermostat.

DTC No. Detection Item DTC Detection Condition Trouble Area MIL Memory Note
P0128-00 Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature) All of the following conditions are met for 5 seconds or more (2 trip detection logic):
  • The engine is started cold and then warmed up.
  • When the estimated engine coolant temperature is 75°C (167°F) or higher, the actual engine coolant temperature is less than 75°C (167°F).
  • Water inlet with thermostat sub-assembly (thermostat)
  • Cooling system
  • Engine coolant temperature sensor
  • No. 2 engine coolant temperature sensor
  • ECM
 Comes on DTC stored
  • SAE Code: P0128
  • DTC for Mexico Models: Does not apply
P0128-07 Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature) Mechanical Failure All of the following conditions are met for 30 seconds or more (2 trip detection logic):
  1. Water inlet housing with water pump sub-assembly is operating,
  2. The engine coolant temperature is less than 75°C (167°F).
  3. The difference between the rate of change of the engine coolant temperature sensor and No. 2 engine coolant temperature sensor is within the threshold.
  4. The difference between the temperature detected by the engine coolant temperature sensor and that detected by the No. 2 engine coolant temperature sensor is within the threshold.
  • Water inlet with thermostat sub-assembly (thermostat)
  • Cooling system
  • Engine coolant temperature sensor
  • No. 2 engine coolant temperature sensor
  • ECM
 Comes on DTC stored
  • SAE Code: P0128
  • DTC for Mexico Models: Does not apply
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