i heard that if engine light is on, engine goes to a safer mode. and runs with lower capacity not to damege engine. is that true?

yes

is it? even for a st? cause my light has been on for um, like almost 6 months cause the ger valve.. you mean my engine could be runnign faster or somthing??

DTC P0115 Engine Coolant Temp. Circuit Malfunction
CIRCUIT DESCRIPTION
A thermistor built into the engine coolant temp. sensor changes the resistance value according to the engine
coolant temp.
The structure of the sensor and connection to the ECM is the same as in the intake air temp. circuit malfunction
shown on page DI–28.
If the ECM detects the DTC P0115, it operates fail safe function in which the engine coolant temperature
is assumed to be 80°C (176°F).

here is just one example. this is out of the toyota manual