This paper presents an innovative application of IEEE1149.4 and the Integrated Diagnostic Reconfiguration (IDR) method to an Automotive Electronic Control Unit implemented as a fully-integrated mixed-signal system. The IEEE1149.4 test structure has been embedded and used on-line for interconnect monitoring and signal analysis. This provides higher resolution failure diagnostics enabling localised fault compensation. A novel On-Line Monitoring architecture is presented, that supports real-time testing of the critical circuit nodes. The paper concludes that fault tolerance can be integrated into mixed-signal electronic systems to handle key failure modes.