P0171 and P0174 Fault Codes on 2001 Cadillac Deville - Lean Mixtures and Idle Issues
I recently received check engine light activation accompanied by OBD-II fault codes P0171 and P0174 on my 2001 Cadillac Deville. The car has 62-000 miles on the odometer. I had the Idle Air Control (IAC) valve replaced about two weeks ago, but the issue returned last week. The most noticeable symptom is that the vehicle accelerates on its own without any input from the accelerator pedal. Prior to the IAC replacement, a mechanic cleaned the Mass Airflow (MAF) sensor and the car ran normally. After the IAC installation, the problem reappeared. I took it to a Cadillac dealership, where they installed a new IAC valve — yet the issue persists. Given that the MAF sensor was previously serviced, could it be failing again? Or are there other potential causes such as vacuum leaks or intake manifold issues? The fault codes point toward a lean air-fuel mixture, which may stem from an underlying system malfunction. Any insights or troubleshooting tips would be greatly appreciated.
P0171 and P0174 typically indicate a lean condition in the air-fuel mixture. While a faulty MAF sensor is possible, vacuum leaks or issues with the EGR valve are also common causes. The 2001 Cadillac Deville with a 4.6L V8 engine may be especially susceptible to intake manifold cracks or backfire valve malfunctions. A vacuum leak can cause the IAC to overcompensate, leading to erratic idle and uncontrolled acceleration. Additionally, a stuck-open intake manifold backfire valve could contribute. Although a faulty Throttle Position Sensor (TPS) might cause irregular idling, the presence of lean codes suggests a more fundamental issue with air intake or fuel delivery. Keep me posted on your findings — troubleshooting this requires systematic checks.