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2006 Pontiac Montana P0172 Code: STFT and O2 Sensor Anomalies Explained

Model: 2006 Montana Fault Code: P0172 Posted: 2016-01-02 15:14

I'm experiencing intermittent appearance of the P0172 code on my 2006 Pontiac Montana with a 3.9L V6 SFI engine. The issue occurs under specific driving conditions and is accompanied by unusual readings from the STFT (Short-Term Fuel Trim) and O2 sensor. Today, while testing in the driveway to assess interior heating performance (which I suspect may be related to a leaking timing cover gasket—another topic), I observed the following: - When revving the engine to 3000 RPM and holding it steady, the service engine light begins flashing after a few seconds. - Releasing the throttle causes the check engine light to go solid. - During this period, the STFT1 gauge on my OBD-II Bluetooth dongle (using Torque Lite on Android) flatlines at 0. This occurs simultaneously with the flashing MIL. - After a brief delay, the STFT value resumes normal fluctuation and the service light stabilizes. The O2 sensor voltage shows significant oscillation—jumping between 0.2V and 0.8V every few seconds—without any noticeable change when the check engine light is flashing. Given my limited background in automotive electronics, I find this range unusually dramatic and suspect it may indicate a faulty sensor or loose connection. The issue can be reproduced several times but only under cold-to-warm engine conditions; as the engine warms up, the behavior disappears. I am currently considering a loose wiring harness, a failing O2 sensor, or an electrical fault in the fuel trim system. The presence of P0172 (Rich Fuel Condition) suggests over-fueling, which could stem from issues like dirty injectors, high fuel pressure, or a malfunctioning MAF sensor. Update: I repeated the test at 3000 RPM—within seconds, STFT dropped to zero, O2 stabilized at 0.9V, and the service light flashed. Releasing the throttle caused the light to go solid, STFT resumed normal readings, and the engine idled slightly rough. I'm now wondering if this behavior is due to a misfire or sensor data processing issue—especially since the O2 voltage seems inconsistent with typical operation. I suspect my cheap USB OBD-II dongle may not provide full diagnostic depth, but I want to confirm whether such erratic readings are normal for a 3.9L V6 SFI engine.

Related fault codes
P0172
Comments (8)
Anonymous 2016-01-02 17:31

Hmmm... I can't upload .txt or .csv files directly. Here's a short log of STFT values showing the zero readings. The timestamps in the last column are approximate. https://docs.google.com/spreadsheets/d/...sp=sharing

Anonymous 2016-01-03 09:31

A flashing check engine light typically indicates a misfire—possibly P0300. GM has a technical service bulletin (TSB) on dirty fuel injectors; a drop test would be needed to confirm. A faulty MAF sensor can also trigger P0172. What are your long-term fuel trims (LTFTs) like? Are you experiencing excessive fuel pressure?

Anonymous 2016-01-03 13:14

The only detected code is P0172. The check engine light only flashes when STFT data flatlines at zero, which suggests a direct correlation between the misfire event and fuel trim behavior.

Anonymous 2016-01-03 13:53

The PCM only flashes the MIL if there's a misfire capable of damaging the catalytic converter. Testing is required to confirm this. What scan tool are you using? STFT data is processed by the scan tool and may not reflect raw sensor values—some tools have known limitations. I use a dedicated scan tool that graphs both LTFT and STFT across RPMs and load conditions.

Anonymous 2016-01-03 14:21

This is interesting—I've never seen a flashing MIL under these conditions before. I was only revving the engine to warm up the engine for diagnosing a cabin heating issue. My setup uses a basic USB OBD-II dongle with Torque Lite on Android. It's possible my tool lacks full diagnostic depth. If STFT is derived from O2 sensor data, then focusing on the O2 readings makes sense. The O2 voltage bounces between 0.2V and 0.8V every few seconds—does this behavior appear normal in a healthy system?

Anonymous 2016-01-03 16:56

Address P0172 (rich fuel condition) first: check fuel pressure under KOEO conditions for leaks or excessive pressure. Clean the MAF sensor, especially if using an oiled filter. FYI—flashing MIL often indicates catalytic converter damage, which is typically caused by misfires or over-fueling.

Anonymous 2016-01-04 19:24

Is this a 3.5L flex-fuel engine? Fuel pressure issues would affect both P0172 and P0175 on both fuel banks.

Anonymous 2016-01-04 20:29

Correction: This is actually a 2007 Pontiac Montana SV6 EXT, not 2006. The engine is a 3.9L V6 with SFI injection system.