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, tighten to the workshop spec (around 20 N·m for many Toyota sensors—check the manual).
6. Refill and bleed the cooling system, verify for leaks, then clear any codes and recheck live data.

There’s no fixed replacement interval for these sensors, but it’s smart to check their readings whenever servicing the cooling system, after thermostat changes, or if the Hiace is showing poor economy or starting habits. Use quality coolant and keep connectors clean to help the sensors live a long, drama-free life.

FAQs

Where is the coolant temperature sensor on a 1999 Toyota Hiace?
On most petrol models, the ECT sensor threads into the coolant outlet housing near the thermostat at the front or side of the cylinder head. There’s often a separate single-wire sender nearby for the dash gauge. Diesel placements are similar—look around the thermostat housing or top water neck.

What are the signs a temperature sensor is failing?
Hard cold starting, high idle when warm, black exhaust smoke on petrol models, poor fuel economy, radiator fans running constantly, or a dead/pegged temp gauge. The check engine light may appear with codes P0115–P0119. Live data that doesn’t match ambient temp on a cold start is a giveaway.

Do diesel Hiace models use different temperature sensors?
They still use a coolant temp sensor, but the ECU/glow controller logic is different. The sensor feeds glow timing, cold idle-up and protection. Some diesels also use an intake air sensor in the manifold or air cleaner. Replacement steps are similar, always follow the diesel-specific specs in the Toyota manual.