If the dashboard warns of overheating, the needle has entered the red zone, or steam is coming from under the hood, stop in a safe place. If you see steam or an obvious coolant leak, switch off the engine once you have stopped and call for help. Do not open the hood while steam is escaping, and do not remove the cap of a hot expansion tank or radiator.
When there are no signs of overheating, start with your car's normal readings. Below are four steps, from the dashboard to a record for the service shop. Using OBD-II logs from a Toyota Highlander dated 7 November 2026 as an example, we will show how to select comparable conditions for the check.
How can you tell when engine temperature is unusual for your car?
1. Check the dashboard warnings
First, rely on the instrument panel. After stopping, write down whether there was a warning, where the needle moved, and what the display showed. If overheating occurs, end the trip and follow the vehicle manual. A coolant smell in the cabin is also worth noting. If the temperature is rising at the same time, a warning has appeared, or steam is visible, stop. What to do about a coolant smell.
2. Find the coolant temperature
If there are no dangerous signs, connect a compatible adapter while parked. In the Android version of LLCAR AI0.19, open the "Center" tab and find the "Coolant" reading. This is the engine coolant temperature. It is transmitted by the control unit via OBD-II if the car provides the parameter 01 05. Oil temperature, when available, comes as a separate parameter. If the coolant temperature has not appeared, check whether other fresh readings are arriving. An adapter paired with the phone may not yet be receiving responses from the engine control unit (ECU). An empty field says nothing about how the cooling system is working. How to tell an adapter connection from an ECU response.
3. Compare similar trips
Compare readings on the same car. Match the air temperature, trip duration, traffic or highway driving, and air conditioning use. From records already saved, note when the engine reached its usual reading and under what conditions the temperature changed. Analyze the data after stopping. If the engine takes noticeably longer to warm up on a similar route, or the temperature drops on the highway, save that episode. Show the record to the technician so they can decide where to start, for example by cross-checking the sensor reading or testing the thermostat. Even a P0128 code by itself does not confirm that the thermostat needs replacing. If fuel consumption changed along with the warm-up, see what else could have affected the comparison.
In the Toyota Highlander logs, coolant temperature helped us pick comparable stretches of warm idle for the airflow comparison. The records covered engine speeds from 650 to below 700 rpm, calculated loads from 30 to below 35%, and coolant temperatures from 80 to below 85 °C. The upper limits were not included in the selection. Check your car's documentation for its operating range. Comparable Highlander readings (in Russian).
4. Record one example for your technician
State the make, model, and engine, the time of the episode, and any additional signs, such as a smell or steam. For the readings, use this record template.
Coolant ___ °C, outside ___ °C, ___ minutes elapsed, mode ___, air conditioning ___, dashboard message ___.
If a trouble code or freeze frame already exists, save it before any attempts to clear it. How to save a trouble code from your phone. Check the coolant level only after the car has cooled down, following the manual. If there are signs of danger, stop first and get help. Collect readings only when it is safe.
Where to find the temperature rules in LLCAR
In the Android version of AI0.19, open "Control" → "Diagnostic rules". There you can view the conditions of the "Critical coolant overheating (≥ 105 °C)" rule and the separate rule "Engine takes too long to warm up or temperature drops on highway". 105 °C is one of the catalogue thresholds for the coolant reading. Determine the operating temperature and overheating actions from your vehicle manual. If the car warns of overheating, act immediately without waiting for a notification on your phone. What LLCAR shows (in Russian).
What an observed sign means
| Observation | What to do and what you can conclude |
|---|---|
| The dashboard warns of overheating, there is steam, or a coolant leak is visible | Stop safely and follow the manual. These signs demand attention regardless of the number on the phone. |
| On similar trips, the engine has started taking longer to warm up | Save the time and conditions. A single record is not enough to decide it is time to replace the thermostat. |
| The temperature differs only in traffic or under load | Note the mode and compare with previous readings under the same conditions. Do not continue the trip for the sake of comparison if signs of overheating appear. |
| The coolant temperature has not appeared on the phone | Check the connection to the car and whether the parameter is available. An empty field by itself says nothing about the state of the cooling system. |
Quick answers
Should all engines have the same operating temperature? No. Rely on the manual for your specific car and on its readings under similar conditions. The rule threshold in the app and the operating temperature in the documentation answer different questions.
Can you judge warm-up by oil temperature instead of coolant? Compare the same parameter in the same mode. Oil temperature and coolant temperature are different measurements. If you are checking a coolant overheating warning, record the coolant reading and the dashboard message.
The number dropped after stopping. Can I continue driving? After an overheating warning, follow the procedure in the vehicle manual. A lower number on the screen is not enough to resume driving. If the safe course of action is unclear, or there is steam or a leak, call for assistance.
Understand first, replace later. If there are no dangerous signs, save one episode together with its conditions before deciding what to have checked at the service shop.
Sources
Ford, Hyundai, and Toyota manuals, the Android AI0.19 rule catalogue, and OBD-II logs from a Toyota Highlander dated 7 November 2026.