2022 Toyota Prius and Prius Prime ECU Software May Cause Loss Of Power

On 2021-09-30 16:30:11

Toyota Motor Engineering & Manufacturing (Toyota) is recalling certain 2022 Prius and Prius Prime vehicles because the Engine Control Unit (ECU) software may falsely detect an error and shut down the hybrid system.


In July 2021, Toyota received a field report alleging a 2022 Toyota Prius unexpectedly stalled and the dashboard warning lights illuminated. The dealer inspected the vehicle and found a Diagnostic Trouble Code (DTC) indicating a hybrid vehicle ECU "Monitoring Processor Unexpected Operation." The ECU from this vehicle was recovered to the supplier for investigation, but the supplier could not identify any problems with it.


Toyota began reviewing the hybrid vehicle ECU logic. They found that a validation check occurs on the transmission shift position signal and that the ECU logic in the subject vehicles would set the DTC after this signal fails the validation check only one time. This review also identified that other ECUs (in vehicles other than the subject vehicles) had a different logic for this validation check (i.e., they require this signal to fail the validation check more than one time).


The subject vehicles are equipped with a hybrid vehicle ECU which, among other tasks, validates a signal reporting the transmission shift position by checking that the signal status remains consistent within an approximately 0.1 millisecond window during each check. Due to incorrect programming for this task, the hybrid vehicle ECU can incorrectly determine that this signal is erroneous if the shifter position status happens to change during this approximately 0.1 millisecond window. If this occurs, warning lights will illuminate and the hybrid system will shut down, resulting in loss of the motive power.


Based on additional investigation, it was also identified that the circumstances needed to create this condition could occur when shifting the vehicle from any position, which is an operation that could be performed by a vehicle operator while driving at higher speeds. Thus, Toyota determined that it is possible that this condition could lead to illuminated warning lights and a loss of motive power while driving at higher speeds.


Owners receiving recall notices will be asked to return to their dealers to have the ECU software updated in their Prius. Toyota's number for this recall is 21TB07/21TA07 and the NHTSA campaign number is 21V-734.