1999 Mazda 626 2.0L Engine Codes P0400 and P1507: EGR Flow & Idle Air Control Issues
I own a 1999 Mazda 626 with a 2.0-liter 4-cylinder engine and automatic transmission. The vehicle runs smoothly under normal conditions, but the check engine light has been on for some time. A diagnostic scan revealed two fault codes: P0400 (EGR Flow Malfunction) and P1506 (Idle Air Control System Overspeed Detected). When I purchased the car last year, the air conditioning was not working, so I replaced the compressor. After installing the new unit, I noticed that running the A/C caused the engine to idle very low—sometimes even stalling—which led me to adjust the idle speed at the throttle body. The vehicle has run reliably for a year since then. Could this modification have contributed to the P1506 code? I plan to first remove and clean the EGR valve, focusing on clearing carbon buildup in the EGR port before replacing the valve entirely. Any insights or recommendations would be greatly appreciated.
The EGR system could be contributing to the idle control issue. Tampering with the throttle body may also be a factor. It's important to properly set the minimum airflow at the throttle body or consider using an untouched, factory-original unit. Always ensure the EGR valve is flowing correctly through its ports before concluding it’s faulty.