One problem related to battery propulsion system has been reported for the 2004 Toyota Prius. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2004 Toyota Prius based on all problems reported for the 2004 Prius.
Total breakdown of consumer's wife 2004 Prius on September 2, 2005 in san jose, California. The vehicle suddenly lost power while merging into the interstate. A loss of gas engine power with failure of the internal combustion/engine to start, resulted in only electric motor propulsion and breakdown on the highway as the battery quickly went to single pink bar level within a 1/2 mile. Multiple mil lamps lit up. Began to merge onto the highway from a stop speed of 30 or 40 mph and slowly accelerated into heavy traffic. Had to pull off to the breakdown lane of the highway with all of its debris , etc. The dealer completed the repairs at san jose's capitol Toyota on 9/2/05. The dealer fixed the car. But the consumer contacted a Toyota pries advisor who stated if there was a service bulletin for this characteristic that would prevent the roadway failure then why was it not executed during prior services on 3/9/05, 5/26/05 and 6/09/05. This indicated that the car was reliable after the repairs. The consumer, an accomplished engineer, said Toyota knew about software bugs. A software bug did not impair operation until the right combination of environmental factors was reached. All pries had this potential bug that contaminated the electronic control module (echo).