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1999 Astro Van P0172 and P0175 Codes: Fuel Pressure, MAF, and EGR Issues Explained

Model: 99 Astro Fault Code: P0172 Posted: 2010-08-21 08:57

I recently purchased a 1999 Chevrolet Astro with 190,000 miles on the odometer. During my test drive, no diagnostic trouble codes (DTCs) were present, but the engine exhibited rough idling. On the return trip home, I noticed the throttle failed to respond beyond 1/4th throttle—no acceleration past that point. I initially suspected a failing fuel pump, and confirmed this issue: initial fuel pressure was low when starting, with pressure dropping as the pump warmed up. The vehicle also previously had its catalytic converter removed, which I replaced along with the fuel pressure regulator during my repairs. After these fixes, the van now displays P0172 (System Too Rich Bank 1) and P0175 (System Too Rich Bank 2), indicating a rich air-fuel mixture. After reviewing numerous forum posts, I replaced both the Mass Airflow (MAF) sensor—originally installed—and the Manifold Absolute Pressure (MAP) sensor, as both were found to be dirty or degraded. The vehicle is now running smoothly with no idle roughness or performance issues. I’ve ruled out oxygen sensors failing simultaneously, as that would be highly unlikely and inconsistent with code patterns. Other potential causes mentioned include a stuck EGR valve or carbon buildup in the intake system. While these are plausible, I believe the fuel pressure regulator may still be contributing to the rich condition. I’m seeking input on whether further diagnostics—such as checking for fuel pressure bleed-off or temporarily blocking the EGR valve—are warranted before considering additional repairs.

Related fault codes
P0172P0175
Comments (2)
Anonymous 2010-08-21 17:11

The fuel pressure regulator is likely leaking, causing a loss of pressure over time. This is a common cause of rich mixtures in older Astro models. I recommend installing a digital fuel pressure gauge to verify that the system maintains consistent pressure under load. Additionally, try temporarily blocking the EGR valve and see if the P0172/P0-175 codes clear—this can help isolate whether EGR-related issues are contributing to the problem.

Anonymous 2010-08-21 18:05

I don’t know the original O2 sensor brand, as I just acquired the vehicle. A technician previously scanned it and reported a high coolant temperature sensor (CTS) voltage reading of 2.2V when warm—suggesting the ECU may believe the engine never reaches normal operating temperature. I’ve since replaced the CTS with a new one and will drive the van for at least one full day to monitor if this resolves the issue. If not, I’ll need to remove the intake manifold (the 'doghouse') to inspect fuel pressure bleed-off and ensure proper system integrity.