Procedure (ordered) with embedded theory and how the repair fixes the fault.

1) Safety & tools (quick)
- Tools: suitable spanners/sockets, multimeter, scan tool (live data), thread sealant or new crush washer, coolant catch container, funnel, shop rags, torque wrench, gloves, eye protection.
- Theory note: relieve pressure and cool engine to avoid scalding and coolant loss; contamination and air in the cooling system will affect sensor readings.

2) Locate the coolant temperature sensor
- On this engine the sensor is mounted in the cylinder head/thermostat housing area (consult the GH1H workshop manual for exact position).
- Theory: sensor must sit where coolant represents actual engine jacket temperature so ECU/gauge get correct thermal input.

3) Read fault codes and observe live data (before removing anything)
- Use a scan tool to read DTCs and monitor engine coolant temperature (ECT) value while cold and warming up.
- Theory: the ECU uses the sensor voltage/resistance to compute temperature. Symptoms (hard starting when hot/cold, poor idle, wrong gauge/fan operation) correspond to incorrect ECT data. Capturing live data shows whether the ECU sees implausible jumps, stuck value, or open/short.

4) Visual and harness inspection
- Visually check connector, wiring insulation, pins for corrosion or damage. Wiggle the harness while watching live data for intermittent changes.
- Theory: the sensor is just a thermistor; wiring faults produce the same symptoms as a bad sensor. Repairing wiring instead of replacing the sensor may be the correct fix.

5) Electrical bench check at the connector (engine cold, ignition ON where required)
- Back-probe the harness connector: check reference voltage from ECU (often 5V supply or pull-up) and measure signal voltage to ECU. With ignition on you should see a stable reference; unplug the sensor and measure voltage on the signal pin (it will typically sit near reference when open).
- With sensor connected, measure voltage change as engine warms (via scan tool or meter). Alternatively remove sensor and measure its resistance on the bench at a known temperature.
- Theory: most coolant temperature sensors are NTC thermistors — high resistance when cold, low when hot. The ECU reads voltage across the thermistor. Open-circuit -> high voltage/erratic reading; short -> low voltage. Comparing measured resistance/voltage to specification identifies if the sensor itself is out of spec or if wiring/ECU is at fault.

6) Remove sensor with minimal coolant loss
- Cool engine, catch coolant, or drain to below sensor level. Disconnect connector, unscrew sensor, inspect sealing washer and thread.
- Theory: removing the sensor can let coolant out and introduce air. Air pockets at the sensor location produce false low-temperature readings (air insulates), so proper drainage/bleeding is necessary before concluding sensor is bad.

7) Test the removed sensor
- Measure resistance at ambient (and after warming with hot water) and compare to the GH1H spec chart in the manual (or compare cold vs hot behaviour: resistance should fall as temperature rises).
- Theory: a failed thermistor will show open circuit, short, or resistance values not matching temperature. Mechanical corrosion or internal break causes wrong readings; replacement is justified when out-of-spec.

8) Fit the replacement sensor
- Clean thread area, fit new crush washer or thread sealant as specified, screw in and torque to the manual specification.
- Reconnect the harness firmly.
- Theory: correct sealing and torque ensure no coolant leak and good thermal contact. Thermal conduction between coolant and the sensor tip is required for accurate sensing.

9) Refill and bleed the cooling system
- Refill to correct level and remove trapped air per Hino bleed procedure (engine run/thermostat cycles, open bleed nipples if fitted).
- Theory: air pockets around the sensor or in cooling passages cause slow/false warm-up readings. Proper bleeding restores correct thermal contact and ensures the ECU sees real coolant temperature.

10) Verify operation and clear codes
- Clear DTCs, then with scan tool monitor ECT: cold reading should be plausible at ambient; temperature should rise continuously as engine warms; fans should engage at correct setpoints; gauge should read correctly; no return of DTCs.
- Road test or load test to verify steady behavior and no overheating, leaks, or intermittent faults.
- Theory: replacing a faulty sensor restores correct resistance-to-temperature relationship so the ECU receives correct data. That correct input allows the ECU to adjust fuel enrichment, idle, fan control, injection timing and display functions properly — removing the symptoms caused by wrong ECT data.

How the repair fixes specific faults (concise)
- Open circuit in sensor or connector: ECU sees extremely high voltage or no change → wrong/flagged temperature and limp behavior. Replacing sensor or repairing connector restores expected resistance/voltage so ECU calculates real temp.
- Shorted sensor: ECU sees low voltage → reports high temperature or disabled cold enrichment; replacement removes the short.
- Intermittent wiring/connector: causes flickering/erratic temp reading → fixing connector or replacing sensor stops the intermittent signal.
- Air pocket after removal or poor thermal contact: sensor reads lower than real temperature → bleeding and correct installation restore thermal coupling so readings reflect actual coolant temp.
- Wrong part (different resistance curve): even a working sensor with wrong calibration gives incorrect temperature to ECU — using correct OEM spec sensor restores correct transfer function.

Notes and cautions (brief)
- Always compare resistance/voltage to the GH1H workshop manual spec tables rather than relying on generic numbers.
- Tighten to specified torque and use correct washer/seal to avoid leaks and sensor damage.
- If after replacement the ECU readings remain implausible, re-check wiring and ECU reference voltage; an ECU fault is rare but possible.

End.
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