Toyota Celica 2005 Repair Manual page 606

DTC P0128
HINT:
This is the procedure of "thermostat" malfunction detection.
CIRCUIT DESCRIPTION
If the engine coolant temperature (ECT) does not reach 75°C (167°F) despite sufficient warm − up time has
elapsed.
DTC No.
Conditions (a), (b) and (c) are met:
(a) Cold start
P0128
(b) Engine is warmed up
(c) THW < 75°C (167°F)
MONITOR DESCRIPTION
Threshold Threshold
(75_C (167_F)) (75_C (167_F))
The ECM estimates the coolant temperature based on starting temperature, engine loads, and engine
speeds. The ECM then compares the estimated temperature with the actual ECT. When the estimated cool-
ant temperature reaches 75_C (167_F), the ECM checks the actual ECT. If the actual ECT is less than 75_C
(167_F), the ECM will interpret this as a fault in the thermostat or engine cooling system and set a DTC.
2005 CELICA (RM1131U)
DIAGNOSTICS
Coolant Thermostat (Coolant Temperature
Below Thermostat Regulating Temperature)
DTC Detecting Condition
Estimated ECT Estimated ECT
ECT ECT
Time Time
− ENGINE (2ZZ−GE)
S Thermostat
S Cooling system
S Engine coolant temperature sensor
S ECM
5 sec. 5 sec
Indicated coolant temp. reading Indicated coolant temp. reading
DTC set (after 2 driving cycle) DTC set (after 2 driving cycle)
605
Trouble Area
DI−407
DIC79−02
A20488