Possible causes

– Faulty Engine Control Module (ECM)
– ECM harness is open or shorted
– ECM circuit poor electrical connection
– Shorted sensor on 5 volt circuit

When is the code detected?

The Engine Control Module (ECM) has detected the sensor 5 volt reference circuit voltage is higher or lower than expected

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)

P0641 SATURN Description

The control module has 2 internal 5-volt reference buses called “5-volt reference 1″ and”5-volt reference 2”. Each reference bus provided 5-volt reference circuit for more than one sensor. Therefore, a fault condition on one 5-volt reference circuit will affect the other 5-volt reference circuits connected to the reference bus. The control module monitors the voltage on the 5-volt reference buses.