Bosch-Common Rail System – ECT sensor

The ECT sensor is located in the small coolant circuit of the engine and measures the coolant temperature. This sensor is a temperature-sensitive resistance element with an NTC (Negative Temperature Coefficient). The voltage value supplied by the ECT sensor is assigned to a corresponding temperature value by the PCM.

The ECT is used for the following calculations:

• Idle speed
• Injection timing
• Injected fuel quantity
• EGR quantity
• Glow plug control
• Actuation of the temperature gauge and glow-plug warning indicator
• Fan control

Effects of faults

When a sensor malfunctions or overheating of the engine occurs, the “engine overheating” fail-safe mode is enabled.

In this mode, engine power is reduced by injecting less fuel. If the engine temperature still continues to increase, the engine power output is decreased further still, depending on the vehicle version. In fail-safe mode the cooling fans run at maximum power.


As described previously, the engine coolant temperature is used in a variety of calculations and thus has an important effect on exhaust emissions. In addition, the engine coolant temperature is required
to define the warm-up cycle. Therefore, this is a MIL active component. The monitoring system continuously checks if the values output by the ECT sensor are within limits. The PCM interprets deviations from limit values as an open control loop or a short circuit (to ground and to battery). The ECT sensor is checked for plausibility by the fact that a specific calibrated temperature increase has to occur within a set period of time after the engine starts. The plausibility check is only performed if the limit check was completed without any faults.

Plausibility check

T Engine coolant temperature
T1 Assumed engine coolant temperature
T2 Minimum temperature
T3 Minimum temperature not reached
t Time
1 Timer
2 Implausible temperature increase
3 Expected minimum temperature increase
4 Plausible temperature increase
5 Timer cancellation

Performing the plausibility check:
• After the engine has been started, the PCM assumes an engine coolant temperature value.
• If the engine speed and the injected quantity exceed a calibrated value due to the temperature value assumption, a timer is started in the PCM.
• During timing, the PCM checks whether a sufficient temperature increase and a calibrated minimum temperature are reached.
• If this is not reached after timeout, an implausible value is assumed and a DTC is stored.
• If, however, a sufficient temperature increase and a calibrated minimum temperature are reached during timing, the plausibility check is deemed to have been successful and is stopped.

In the event of a fault, the engine management system reverts to a substitute value and the engine runs at reduced power output. In this case, the cooling fans are switched to run at maximum power.

Possible diagnostic trouble codes: P0115, P0116, P0117, P0118.

07. August 2018 by Ron
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