## 1 Introduction  
The reliability of a Heating, Ventilation, and Air Conditioning (HVAC) system is defined as the probability that the HVAC system's components will perform the required function for a given time when used under specified working conditions [1]. The malfunction of an HVAC system can be due to various factors such as wear and tear, poor maintenance, and design flaws. HVAC system reliability is essential for minimizing the unexpected failures, the cost of the failures, and life cycle management. Reliability analysis helps to identify critical failure modes, components, and subsystems that are useful for improving the system design. Warranty data analysis using the field failure data collected at a service station helps to predict an HVAC system's early failures and associated failure cost. It can also be used to optimize the cost of failure, decide maintenance strategies, optimize spare provisions, shorten repair times, and set organization expectations [2, 3]. The manufacturing defects and potential failure causes can be identified and eliminated in the early design and development phase. A reliability study also plays a significant role in improving the reputation of the vehicle manufacturer. HVAC systems are commonly used in passenger vehicles to regulate the temperature and airflow within the driver and passenger compartments. It typically consists of a compressor, evaporator, condenser, refrigerant, ductwork, vents, and controls. The control unit allow the driver and passengers to adjust the temperature and airflow as per their convenience. The quality of the air within the compartment/cabin can be maintained by filtering out dust, pollen, and other pollutants [4]. Few HVAC systems also incorporate features like ionizers and ultraviolet lights to improve air quality further [5]. Basically, the HVAC system is a crucial system of modern passenger vehicles, as it assists in ensuring passenger comfort and safety while driving. For example, fogged-up windows due to malfunction of an HVAC system can affect the alertness of the driver and increase driver fatigue. A good quality HVAC system contributes to customer satisfaction and can play a crucial role in selling the vehicle. Reliability studies also help in ensuring compliance with countries' regulations and standards. Energy consumption can also be minimized by maintaining the desired reliability level of HVAC system. Energy-efficient HVAC systems can reduce fuel intake and greenhouse gas releases, aligning with environmental regulations and the United Nation's sustainability development goals. Tools such as a Reliability Block Diagram (RBD), Failure Mode and Effects Analysis (FMEA), and Fault Tree Analysis (FTA) are widely used to identify the potential failure modes of an HVAC system along with their causes and effects on the system and environment [2; 6]. The Pareto analysis allows one to prioritize design modification areas and mitigate potential failures [7]. The reliability assessment requires data and reliability metric information such as failure rates, Mean-Time-To-Failure (MTTF), mode, median, standard deviation, skewness, and kurtosis. The reliability studies are carried out using the data collected from the field, the literature, reliability testing, and expert judgments. It can be used to compare the performance and behavior of various components, identifying the failure pattern and criticality [8]. The results of the reliability analysis are used to improve the design, manufacturing processes, and maintenance scheduling strategies [1]. In this regard, this study provides a reliability analysis and modelling of an HVAC system using field failure data, statistical techniques, and system reliability modelling. The analysis uses the field failure data collected from service stations across India. The primary goal of this study is to identify the critical components of the system, and in this regard, four objectives were set and are given below:  
* To develop a methodology/framework for reliability modelling of HVAC systems.