2001 Mazda Millenia P1250 P1521 P0421 Fault Codes Not Clearing After O2 Sensor and VRIS Replacement
I own a 2001 Mazda Millenia with a 6-cylinder, non-supercharged engine. I've been consistently encountering three diagnostic trouble codes: P0421 (O2 sensor signal below threshold), P1521 (VRIS solenoid circuit fault), and P1250 (fuel pressure solenoid issue). I have already replaced all four oxygen sensors with new ones, and the VRIS (Variable Resonance Idle Solenoid) unit has been swapped out. Despite these replacements, the codes persist. They reappear after varying durations—sometimes immediately after driving, other times after a week or more. I expected the codes to clear following part replacements, but they do not. I'm seeking advice on potential underlying causes or troubleshooting steps that might resolve this issue without further costly repairs.
Did you purchase the replacement parts from an authorized dealer or an aftermarket brand? Additionally, please verify the electrical circuits connected to both the oxygen sensors and the VRIS solenoid—check for proper power supply, ground connections, and continuity to ensure no wiring faults are present.