Subaru IMPREZA GD 2007 Series Manual page 1845

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Diagnostic Trouble Code (DTC) Detecting Criteria
GENERAL DESCRIPTION
Y: DTC P0128 COOLANT THERMOSTAT (ENGINE COOLANT TEMPERATURE
BELOW THERMOSTAT REGULATING TEMPERATURE)
1. OUTLINE OF DIAGNOSIS
Detect malfunctions of the thermostat function.
Judge as NG when the engine coolant temperature is lower than the estimated engine coolant temperature
and the difference between them is large. Judge as OK when the engine coolant temperature becomes to
75°C (167°F), and the difference is small, before judging NG.
2. COMPONENT DESCRIPTION
(9)
(1) Valve
(2) Spring
(3) Stopper
3. ENABLE CONDITION
Secondary Parameters
None
4. GENERAL DRIVING CYCLE
Always perform the diagnosis continuously.
(8)
(2)
(7)
(5)
(6)
(4)
(3)
(4) Piston
(5) Guide
(6) Rubber seal
Enable Conditions
GD(H4DOTC)-54
(2)
(1)
EN-01692
(7) Stop ring
(8) Wax element
(9) Jiggle valve