The software content in vehicles is increasing, including their system complexity, which leads to a higher probability of bugs appearing in a production vehicle. Consequently, there is an increasing need to observe what the production system is doing also once it is already in the field. This paper introduces an emerging topic of in-field diagnosis of microcontrollers in the automotive domain. This includes the architecture and how to address the safety and security challenges. We describe the necessary components of an in-field diagnosis architecture, including the required properties of an on-chip debug monitor. We provide several ways an on-chip debug monitor can be implemented by utilizing the available and, in some cases, unused system resources. With the described approach, we can utilize the same debug concepts and tools for local, remote, and in-field diagnosis.