车上诊断系统（On-Board Diagnostics，缩写为OBD，又译车载自动诊断系统），是一种设备于车中用以监控车辆运行状态和回报异常的系统，可于车辆的子系统出现问题时，产生故障代码和提醒信号通知车主和车厂诊断维修。1980年车上诊断系统发明后，早期仅能以指示灯形式回报故障发生与否。随着电脑科技的进步，目前已经能回报各式各样的实时数据和标准化故障代码（diagnostic trouble codes，DTC），使得汽车故障诊断维修方法发生翻天覆地的变化。

车上诊断系统的设计发想约起于1980年代中期的美国，当时发现配备空燃比控制系统的车辆如果排放污染超过管制值时，其含氧感知器通常也有异常，由此逐渐衍生出设计一套可监控各排放控制组件的系统，以早期发现可能超出污染标准的问题车辆。

On-board diagnostics (OBD) is an automotive term referring to a vehicle's self-diagnostic and reporting capability. OBD systems give the vehicle owner or repair technician access to the status of the various vehicle subsystems. The amount of diagnostic information available via OBD has varied widely since its introduction in the early 1980s versions of on-board vehicle computers. Early versions of OBD would simply illuminate a malfunction indicator light or "idiot light" if a problem was detected but would not provide any information as to the nature of the problem. Modern OBD implementations use a standardized digital communications port to provide real-time data in addition to a standardized series of diagnostic trouble codes, or DTCs, which allow one to rapidly identify and remedy malfunctions within the vehicle.