Parts for your 1999 Toyota Hiace-Temperature sensors

1999 Toyota Hiace temperature sensors: what they do and how to look after them

Temperature sensors are absolutely fitted to the 1999 Toyota Hiace. Technical sources including the Toyota Hiace Repair Manual (1998–2004, Engine and EM sections), the Toyota Electrical Wiring Diagram (EWD) for Hiace of the same era, and workshop guides like Haynes confirm the use of an Engine Coolant Temperature (ECT) sensor for the ECU, a separate sender for the dash gauge, and an Intake Air Temperature (IAT) sensor (on many petrol models it’s integrated into the airflow meter). Petrol engines such as the 2RZ-E/3RZ-FE rely on these for fuelling and timing, while diesel variants (5L and 1KZ-TE in some markets) use coolant temperature input for glow control, idle-up, and protection strategies.

On this Hiace, temperature sensors tell the ECU how warm the engine and intake air are, so it can adjust fuel mixture, ignition timing and idle speed, switch the radiator fans, and even trim A/C operation under load. A crook ECT can cause hard cold starts, rich running, elevated fuel use, erratic idle, or the thermo fans stuck on. It’ll often log OBD fault codes like P0115–P0119. A lazy IAT can cause hesitation and soggy throttle response. For diesels, incorrect coolant temp input can stretch glow times, knock economy around, and light the warning lamp.

For servicing, focus on good basics: clean electrical connectors, sound earths, and a healthy cooling system. Scan live data on a cold start—the ECT and IAT should read close to ambient under the bonnet, then the ECT should climb smoothly as it warms. If a scan tool isn’t handy, an ohmmeter check against the manual’s resistance chart works well. Replace sensors that read out of spec, have green crusty terminals, or trigger intermittent codes.

1. Let the engine cool completely and relieve any coolant pressure.
2. Disconnect the negative battery terminal.
3. Drain a litre or two of coolant from the radiator tap to drop the level below the sensor.
4. Unplug the connector and remove the ECT sensor with a deep socket (often 19 mm).
5. Install the new sensor with its sealing washer