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P0172 Fault Diagnosis for 1999 Saturn SL2: Running Rich, Engine Warm-Up Issues

Model: 99 Saturn SL2 Fault Code: P0172 Posted: 2014-03-30 13:01

I'm seeking help diagnosing a P0172 code on my 1999 Saturn SL2. The vehicle is running rich and the check engine light comes on after the engine reaches operating temperature. I've already replaced the flex pipe, resonator, oxygen sensor, spark plugs, throttle body, EGR valve, and fixed all broken vacuum hoses. I've also performed a thorough cleaning of the intake system and confirmed all vacuum lines are properly connected. Despite these repairs, the P0172 code persists after the engine warms up. The vehicle runs roughly at idle and has a hard start when hot. I'm looking for further diagnostic steps to identify whether this is due to fuel pressure issues, faulty injectors, sensor inaccuracies (like MAF or ECT), or another underlying problem. Any advice on next steps—such as fuel pressure testing, leak-down tests, or checking the mass air flow (MAF) sensor—would be greatly appreciated.

Related fault codes

P0172

Comments (8)

Anonymous 2014-03-30 13:05

1999 Saturn SL2 - common model with known P0172 issues related to fuel mixture and warm-up behavior.

Anonymous 2014-03-30 13:25

P0172 indicates a rich air-fuel mixture. Check if the engine reaches 200°F during operation. Confirm all spark plugs are firing properly—look for fouling or carbon buildup. Inspect PCV system for excess oil, ensure the air filter is clean and correctly installed. Perform a fuel pressure test and leak-down test to identify any leaking injectors; this will also help verify the function of the fuel pressure regulator.

Anonymous 2014-03-31 23:18

If you're seeing both P0172 and P0175, consider replacing the Mass Air Flow (MAF) sensor. It's critical to replace it only when the engine is cold—using a hot engine can affect readings. After replacement, clear the codes and monitor for recurrence. I've personally resolved this issue on my 2002 Toyota Land Cruiser with similar symptoms.

Anonymous 2014-04-02 18:26

This symptom often appears after the engine has warmed up and is idling. If you move the vehicle slightly, the P0340 code may appear—this suggests a potential issue with ignition timing or plug wires. I recently replaced my plug wires due to similar symptoms when starting the car.

Anonymous 2014-04-02 19:27

High fuel pressure is a likely cause. A crushed or blocked return line can lead to rich mixture conditions. Check for leaking fuel injectors and a broken diaphragm in the fuel pressure regulator. While a faulty oxygen sensor usually sets a code, rare cases may result in a shift to rich without triggering an O2 monitor failure. Do you experience hard starts when the engine is hot?

Anonymous 2014-07-02 14:02

We've replaced both oxygen sensors, EGR valve, catalytic converter, MAP sensor, fuel injectors, thermostat, and fuel filter—along with spark plugs and coil pack. Despite these efforts, P0172 persists after the engine warms up. Any additional troubleshooting steps or suggestions would be very helpful.

Anonymous 2014-07-02 14:09

The vehicle is running rich. As mentioned in earlier posts, check fuel pressure, fuel regulator function, and leak-down tests. Also verify the Engine Coolant Temperature (ECT) sensor reading—what temperature is the PCM detecting? This can impact mixture calculations.

Anonymous 2014-07-02 14:13

Fuel pressure was tested and found normal; the fuel filter was replaced (note: the 1999 Saturn SL2 has a built-in fuel pressure regulator in the filter). All four fuel injectors were replaced as two were leaking. My gauge shows engine temperature between 1/4 and 1/2 mark—consistent with warm-up phase, but still experiencing P0172.