P0171, P0300

Hello! Currently tackling P0171 and P0300 on my 2006 BUICK LACROSSE 3.8L V6.

A few weeks back, the check engine light illuminated with a P0171 code. Swiftly, I took it to the shop, where they opted for a throttle body cleanse and a MAF sensor replacement. Despite claiming a leak-free outcome from a smoke test, the very next day witnessed a return of the P0171 code, accompanied by erratic idling, rough acceleration, and sporadic RPM sputtering. Another round at the shop involved a repeated smoke test, yielding no leaks, and a second cleansing of the throttle body. Unsurprisingly, the code resurfaced within 24 hours. Choosing a different approach, I consulted a family mechanic who suggested replacing the upstream O2 sensor. Post-change, the code reappeared within an hour.

Using my scan tool, I checked freeze frames, the code is only being thrown at idle. Fuel trims seem okay, while driving on the highway, and when driving for long periods.

First Freeze Frame:

* Engine RPM: 739 RPM
* Engine Coolant Temperature: 181°F
* Closed Loop
* Long-term fuel trim: 9.38%
* Short-term fuel trim: 12.5%
* MAF air flow rate: 4.25 g/s

Second Freeze Frame:

* Engine RPM: 726 RPM
* Engine Coolant Temperature: 203°F
* Closed Loop
* Long-term fuel trim: 14.06%
* Short-term fuel trim: 3.91%
* MAF air flow rate: 4.08 g/s

Third Freeze Frame:

* Engine RPM: 759 RPM
* Engine Coolant Temperature: 206°F
* Closed Loop
* Long-term fuel trim: 13.28%
* Short-term fuel trim: 3.13%
* MAF air flow rate: 3.97 g/s

Having exhausted my options, I decided to clear the codes one last time. Alas, the P0171 code persisted. A day later, during a smooth start, the car exhibited internal shaking, accompanied by a blinking check engine light. The P0300 code was now being thrown. Rushing to my family mechanic, we discovered some spark plugs lacked any spark. Given the car's 165,000 miles and its factory spark plugs and wiring, I will be replacing the coils, spark plugs, and wiring. The ignition module remains under consideration.

Is it reasonable to assume that the P0171 triggered the P0300, or was it merely a precursor? Additionally, I plan to replace the PCV valve due to its affordability and simplicity.  


Any tips on this P0171, or P0300?  


Thank you!

It is most likely a vacuum leak because of the low RPM (Idle condition). Verify that the O2  functions as designed, raise the RPM and look for fuel trim improvement. If improved, clamp the pcv valve hose and note fuel trim again at idle. Make sure the oil filler cap is secure and in good condition and the oil filler tube is installed correctly.

Good to see you active on the forum Monde!