P0128

3UZ-FE ENGINE CONTROL SYSTEM: SFI SYSTEM: P0128: Thermostat Malfunction
P0128 - Thermostat Malfunction

DESCRIPTION

This DTC is output when the Engine Coolant Temperature (ECT) does not reach 75°C (167°F) despite sufficient engine warm-up time.





MONITOR DESCRIPTION





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 coolant 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 engine cooling system or thermostat and set a DTC.

MONITOR STRATEGY





TYPICAL ENABLING CONDITIONS





TYPICAL MALFUNCTION THRESHOLDS





CONFIRMATION DRIVING PATTERN





- Stop the engine and allow it to soak.

- Connect the Techstream to the DLC3.

- Turn the ignition switch to ON and turn the Techstream on [A].

- Enter the following menu items: Powertrain / Engine / Data List / Coolant Temp and Intake Air.

- Check that "Coolant Temp" is 56°C (133°F) or less and "Intake Air" is between 0 and 35°C (0 and 95°F).

- Set the heater to MAX HOT with fresh air mode selected and turn the A/C off.

- Start the engine and drive the vehicle at 80 km/h (50 mph) for 15 minutes [B].

HINT
- Data can be captured relatively easily by using the snapshot function in the Data List.
- Enter the following menu: Data List / Function / Snap-shot configuration / Record time / 5 min.
- Data capture can be started by using the target point function. For example, setting the target point to an engine speed of 4000 rpm and then racing the engine, etc. to raise the engine speed to a speed of 4000 rpm or more will cause data capture to start.
- After "Coolant Temp" stabilizes, check that "Coolant Temp" is 75°C (167°F) or higher [C].

HINT
If "Coolant Temp" is below 75°C (167°F) while driving the vehicle at 80 km/h (50 mph), inspect the cooling system and thermostat.

WIRING DIAGRAM





INSEPCTION PROCEDURE

HINT
Read freeze frame data using the Techstream. Freeze frame data records the engine conditions when a malfunction is detected. When troubleshooting, freeze frame data can help determine if the vehicle was running or stopped, if the engine was warmed up or not, if the air-fuel ratio was lean or rich, and other data from the time the malfunction occurred.

PROCEDURE

1. CHECK OTHER DTC OUTPUT (IN ADDITION TO DTC P0128)

(a) Connect the Techstream to the DLC3.

(b) Enter the following menus: Powertrain / Engine / Trouble Codes.

(c) Read the DTCs.

Result:





HINT
If any other codes besides P0128 are output, perform the troubleshooting for those DTCs first.

B -- GO TO DTC CHART Diagnostic Trouble Code Descriptions
A -- Continue to next step.

2. CHECK COOLING SYSTEM

(a) Check for defects in the cooling system that might cause the system to be too cold, such as an abnormal radiator fan operation or a modified cooling system.

(b) Check the valve opening temperature of the thermostat.

Standard:

Valve opening temperature is 80 to 84°C (176 to 183°F)

HINT
Also check that the valve is completely closed below the opening temperature shown above.

NG -- REPAIR OR REPLACE COOLING SYSTEM
OK -- Continue to next step.

3. INSPECT THERMOSTAT

(a) Inspect the thermostat Testing and Inspection.

NG -- REPLACE THERMOSTAT

OK -- REPLACE ECM Service and Repair