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How to Interpret P0171 and P0172 OBD2 Codes in a 2007 Pontiac Wave Using PCMSCAN

Model: Pontiac Wave Fault Code: P2177 Posted: 2016-10-03 17:03

I'm working through interpreting the PCMSCAN output data from my 2007 Pontiac Wave, which has triggered P0171 (System Too Lean) and P0172 (System Too Rich) OBD2 diagnostic codes. I've exported the scan data to Excel for analysis but am struggling to identify the root cause. Can anyone help me understand what these codes might indicate based on the actual sensor readings and fuel trim values? I’ve included a PDF version of my log file since the original Excel format isn't supported here. The MIL (Malfunction Indicator Lamp) was reset and reactivated around frame 44. Any insights into how to better interpret these codes—especially in relation to oxygen sensor behavior or fuel trim inconsistencies—would be greatly appreciated.

Related fault codes

P2177

Comments (4)

Anonymous 2016-10-04 07:28

Could you share a freeze frame snapshot of the error conditions? The fuel trim values show an +18% lean reading and a -11% rich reading, which should not independently trigger P0171 and P0172. Are these codes set at different times in the log file or during distinct driving scenarios?

Anonymous 2016-10-04 17:33

Thanks for your input. My PCMSCAN version doesn't include freeze frame data—so I manually copied the relevant graphs into the PDF. The MIL came back on around frame 44 after being reset. Regarding diagnostics, would additional PIDs (like engine load, intake air temperature, or manifold pressure) be more helpful in diagnosing this issue?

Anonymous 2016-10-04 20:25

When the MIL reactivated, was it specifically a P0172 code? Both oxygen sensors were reading at maximum rich when the white mark was reached. However, the combined LT/ST fuel trim value was only -5%, which is insufficient to trigger a P0172 code. This suggests that fuel trim data alone may not be revealing the true issue. The presence of both lean and rich error codes on the same bank raises suspicion about an intermittent oxygen sensor. Could you monitor the pre-cat O2 sensor for switching cycles (high/low) over a 2-3 second period at idle? This would help determine if the sensor is responding properly.

Anonymous 2016-10-15 08:42

Thanks to cj1 for your detailed feedback. Unfortunately, my version of PCMSCAN (1.4.3) doesn’t support O2 sensor data—Palmer Performance charges CDN$100 for the updated version, which is beyond my budget. I also tried sourcing a digital oscilloscope but was unsuccessful. Since the vehicle has high mileage and the original O2 sensor appeared intact, I replaced it. After replacement, the P0171 and P0172 codes disappeared. This strongly suggests that the upstream oxygen sensor was not responding accurately—likely 'lazy' or intermittent. The experience confirms that faulty sensors can trigger misleading OBD2 codes even when fuel trim values appear normal. Thanks again for your guidance—the learning process continues.