The Cold Weather Diesel Reliability Act of 2025 mandates the Environmental Protection Agency (EPA) to revise regulations to enhance the reliability and safety of essential diesel vehicles in cold climates. Congress found that current emissions control systems, particularly those involving diesel exhaust fluid (DEF), can lead to automatic engine shutdowns or power reductions in extreme cold, posing significant safety risks and logistical challenges for critical transportation. The bill specifically requires the EPA to authorize manufacturers to temporarily suspend engine derate or shutdown functions triggered by emissions control system faults when ambient temperatures are at or below zero degrees Celsius. This suspension is allowed only when necessary to prevent occupational danger or loss of essential transportation functionality, with normal emissions control resuming above freezing. Additionally, the legislation mandates a year-round exemption from diesel exhaust fluid (DEF) system requirements for vehicles operating primarily north of 59 degrees north latitude or in conditions where DEF use is impractical due to prolonged freezing temperatures. These measures aim to ensure critical mobility and prevent equipment failure in remote, cold regions. The Act explicitly states that it does not waive compliance with any emissions standards under the Clean Air Act, except for these specific temporary cold-weather operational modes and the DEF system exemption.