1999 Mazda 626 P0171 Code: Fix for Lean Mixture Bank 1 Due to EGR Valve Gummy Deposits

I've been troubleshooting this issue for a while — my 1999 Mazda 626 is showing a P0171 code indicating a lean mixture in bank 1. I started by cleaning the MAF sensor, which was already clean and showed no issues. After clearing the codes with an Equus scanner, the check engine light returned, and I noticed occasional idle stumbles. I checked for obvious vacuum leaks and found none. However, the I/M monitors indicated that the EGR circuit had not returned to operational status. Suspecting a problem with the EGR system, I removed the EGR valve and discovered gummy deposits preventing the pintle from seating properly. The valve has an extremely weak return spring — it's very sensitive to contamination. I disassembled the actuator motor for inspection but found no issues. Instead, I cleaned the valve pintle using throttle body cleaner and gently scraped away debris with a Xacto knife. After this repair, the car runs smoothly. The issue appears to be related to exhaust gas condensation in cold weather or during short trips — when the engine doesn't reach optimal temperature. Taking the vehicle out on highway driving helps clear the system. The real culprit? A vacuum leak caused by a dirty EGR valve. This is now fully resolved, and the P0170 code has been cleared with no recurrence.