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1999 Ford Windstar 3.8L with O2 Sensor and DPF Emissions Codes: Diagnosing 0401, 0171, and 0172 After Intake Manifold Repair

Model: 99 3.8L Windstar Fault Code: P0171 Posted: 2006-06-06 23:38

I recently completed a full intake manifold gasket replacement on my 1999 Ford Windstar with the 3.8L engine at approximately 104,000 miles about one month ago. This repair successfully resolved the previous 0171 and 0172 O2 sensor-related codes by eliminating a vacuum leak. However, a persistent 0401 code (exhaust oxygen content) has reappeared. During the manifold disassembly, I thoroughly cleaned all components—including valves, intake surfaces, and injectors—due to significant carbon buildup. After cleaning, the vehicle ran smoothly for several days without any diagnostic reader needed, with no visible engine performance issues. However, within a few days, new codes have returned: 0171 (lean condition), 0401 (exhaust oxygen sensor issue), and 0402. I now suspect that the original problem may not have been fully resolved due to neglected manifold bolt replacement—a known common issue in older Windstar models. I'm concerned whether this could invalidate my new intake gaskets or indicate a deeper system failure. Additionally, my DPFE (Differential Pressure Feedback Emission) sensor consistently reads 5 volts under all conditions—neither dropping to the recommended 0.20–0.70 volts when cold nor showing any variation during engine warm-up. Despite these diagnostic codes, the vehicle runs smoothly with no rough idle or performance loss. The only noticeable symptom is the illuminated check engine light. I'm seeking advice on whether replacing the DPFE sensor is a viable fix and if there are other components—such as EGR valve function, MAF sensor sensitivity, or air filter quality—that might be contributing to these emissions-related faults.

Related fault codes
P0171P0401
Comments (11)
Anonymous 2006-06-07 09:02

Start by replacing the DPFE sensor. A steady 5-volt reading suggests a possible open circuit or faulty wiring—this could be causing the 0401 and 0171 codes. Ensure you're using an OEM-quality sensor, such as the Ford-branded unit, to avoid compatibility issues.

Anonymous 2006-06-07 13:13

The DPFE sensor is often referred to interchangeably with the EGR pressure sensor in some forums. While both are related to emissions control, they serve different functions. The EGR pressure sensor costs $32 at AutoZone, compared to $101 from a dealer—worth checking if it's actually the correct component for your 1999 Windstar.

Anonymous 2006-06-07 13:35

I believe they are related but not identical. The WELLS brand (sometimes listed as WESS) has been reported to have poor reliability in some cases. I recommend verifying the sensor brand with your dealer—some emissions warranties may cover replacements. Before replacing the sensor, also test the EGR valve using a vacuum pump and inspect all associated lines and solenoids for blockages or leaks. This can help rule out mechanical issues before assuming it's a sensor failure. Good luck and feel free to update your progress here!

Anonymous 2006-06-08 15:37

I'm testing the DPFE sensor tonight—purchased the original Ford OEM unit. Will report back after driving for a few days with no codes.

Anonymous 2006-06-09 13:56

Replacing the DPFE sensor resolved the issue temporarily—at least long enough to pass smog inspection. Thanks for the tip! We had a previous replacement at a local shop, but after removing the sensor we discovered it was actually a WELLS brand unit—not the Ford OEM one. If you're experiencing these symptoms, I strongly recommend investing in a genuine Ford-branded DPFE sensor—even if it goes against personal preference. It's been proven to resolve emissions faults reliably. Thanks for the helpful advice!

Anonymous 2006-06-10 06:31

Is WELLS the same as WESS? Are they interchangeable?

Anonymous 2006-06-11 15:37

I'm not certain. The shop receipt didn't list the brand, and the sensor had no markings. I was out of town when my wife took it to the shop. That said, during closed-loop operation, the sensor only read between 0.1 and 0.3 volts—far below a full volt. I verified these readings using a Genesis scanner, and everything appears within specifications. The vehicle runs fine, so I believe the issue may be related to sensor calibration or wiring rather than failure.

Anonymous 2006-11-07 16:45

This is now my wife's van—my husband left in July for his second tour in Iraq and, as it turns out, the check engine light came back on the very next day. Any new suggestions? I'm hoping to fix this before he returns in March.

Anonymous 2006-11-07 17:47

Has the same set of codes returned since July? If so, it might point to a recurring issue—consider getting a full diagnostic scan. A persistent problem may require deeper inspection beyond just sensor replacement.

Anonymous 2006-11-09 00:31

Yes—the last time we thought everything was fixed, a full diagnostic showed normal readings. About two weeks later, the check engine light returned. My husband had to go on vacation in Iraq, so I’ve left it unattended. Now I’m considering taking it to my local community college for hands-on diagnostics as a student project. Could an aftermarket K&N air filter be causing issues? I've seen references suggesting that some K&N filters can trigger MAF sensor errors or cause lean conditions—especially in older 3.8L Windstars.

Anonymous 2006-11-09 07:22

Have any of you installed a K&N filter? If so, remove it and dispose of it—it's likely the root cause of your MAF-related codes and emissions faults. Many users report that aftermarket air filters interfere with airflow sensing, leading to false lean readings and triggering O2 sensor errors. Consider switching back to an OEM or high-quality stock filter.