P0400 Fault Diagnosis and Low MPG on 1998 Mitsubishi Mirage - OBD2 Check Engine Light Still On

Model: Mirage Fault Code: P0400 Posted: 2015-10-13 17:17

Hi everyone, I'm experiencing a persistent P0400 diagnostic trouble code on my 1998 Mitsubishi Mirage, which is also causing significantly reduced fuel efficiency. I've followed several troubleshooting steps recommended in this forum and have already replaced key components including the EGR valve, solenoid, cracked vacuum hoses, coolant temperature sensor, intake air temperature sensor, throttle body (with full vacuum line inspection), catalytic converter, and both front and rear O2 sensors. Despite these replacements, the check engine light remains on. I've also cleaned the intake manifold and performed a complete vacuum system check using the official vacuum diagram for my 1998 Mirage model. The issue persists, and I'm now seeking further insight from experienced users or technical experts to help identify the root cause of this P0400 code. Any detailed advice on possible causes—such as faulty wiring, sensor inaccuracies, or intake system leaks—would be greatly appreciated.