FTA issues safety advisory for certain Proterra battery-electric buses

The Federal Transit Administration (FTA) has issued a safety advisory, alerting transit agencies that Phoenix Motorcars (Phoenix) is voluntarily recalling certain 2020-2021 800V Catalyst and 2020-2022 ZX5 battery-electric buses due to vehicle controller software inside certain vehicles that may allow the electrical circuit for the radiator fan to overheat, increasing the risk of a fire.

Background on the recall 

On Sept. 5, 2024, Phoenix informed the National Highway Traffic Safety Administration (NHTSA) of a safety defect affecting Proterra 800V Catalyst (model year 2019-2021) and Proterra ZX5 (model year 2020-2022) transit buses with a certain electrical and software architecture and of their intent to recall the affected vehicles (NHTSA Recall No.: 24V-655). The buses were produced between Dec. 14, 2019, through Nov. 8, 2021, and July 21, 2020, through Jan. 31, 2023, respectively.  

According to the information provided by Phoenix to NHTSA, the electrical connectors and terminals at one rooftop passthrough at issue in this recall are toward the maximum current limit that the terminals are rated for, with radiator fans running at 100 percent speed. Over time and with higher ambient temperatures, the circuit design can result in excessive heat at the terminals and, eventually, the melting of “wedge locks,” which could result in a thermal event, increasing the risk of a fire. 

In Safety Advisory 24-1, FTA notes that software updates were previously released as a technical service bulletin and Phoenix will redeploy the bulletin as the recall remedy. Per Phoenix, the update will limit the radiator fan speed to 80 percent, which reduces the load of each fan by approximately 46 percent and reduces the temperature at each terminal by approximately 71 percent above ambient without degrading performance of the cooling system. 

The safety advisory notes Phoenix, who completed the acquisition of Proterra Transit’s business line in January 2024 following Proterra filing for Chapter 11 bankruptcy on Aug. 7, 2023 in the District of Delaware, will provide the software remedy free of charge to owners of vehicles within the recall population. Version 4.10.2 or above contains the remedy and this version can be referenced on the vehicle's dashboard. 

Recommended action on Safety Advisory 24-1  

In Safety Advisory 24-1, FTA recommends bus transit agencies review their fleets to determine if they contain any of the affected transit buses. If so, FTA recommends that transit agencies download and update the vehicle software. FTA says bus transit agencies should continue to monitor their fleet for signs of overheating even after the update is complete to ensure the fire risk is mitigated. 

History of recalls 

Safety Advisory 24-1 isn’t the only time during the past 12 months that Proterra buses were involved in a recall. Proterra issued recall 23V-697 on Nov. 14, 2023, after the company decided that a defect that relates to motor vehicle safety exists in certain 2019-2022 40-foot and 35-foot Catalyst 800V Transit Buses with ProDrive 1.0 and certain 2019-2022 40-foot and 35-foot ZX5 800V Transit Buses with ProDrive 1.0.  

Vehicles within that recall population were equipped with a PowerPhase 250 HD traction inverter, manufactured by Danfoss Power Solutions, with a defective thermal paste that failed to adequately transfer heat away from the insulated-gate bipolar transistor. 

Proterra introduced the ZX5 electric bus with 738 kilowatt hours of energy in April 2022. The ZX5 Max delivers more than 300 miles of drive range on a single charge, depending on route conditions, configuration and operation.