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P0172 and P0175 O2 Sensor Codes on 2006 Ford Taurus: Rich Fuel Condition, Poor MPG, and Sluggish O2 Sensors

Model: Taurus Fault Code: P0172 Posted: 2015-09-18 02:02

My 2006 Ford Taurus is throwing diagnostic trouble codes P0172 and P0175, indicating a rich fuel condition. Fuel economy has dropped significantly—from 28 mpg to just 18 mpg. Long-term fuel trim is at -24%, with short-term fuel trim fluctuating between -9% and 0%. The forward oxygen (O2) sensors for both engine banks are sluggish, reading between 100 and 875, while the rear O2 sensors remain stable at 875. Fuel pressure drops from 44 psi at KOEO to 39 psi after five minutes, and remains at 39 psi when the engine is running. Engine operating temperature is consistently around 195°F. I previously cleaned the MAF sensor with a dedicated cleaner and allowed it to dry thoroughly before driving—no improvement was seen. The MAF reads 27.28 inches of mercury, which is 2.14 inches below the barometer reading (29.42 inHg). I compared this to my 2006 Taurus with a known healthy engine; both vehicles show identical readings, so I believe the MAF sensor is functioning correctly. The fuel injectors were professionally cleaned one year ago, and I recently replaced spark plugs and plug wires. The vehicle drives and accelerates normally, though idle is slightly rough. I am not inclined to replace parts without diagnosing root causes—so any technical insights would be greatly appreciated. I suspect either a leaking fuel injector or a faulty fuel pressure regulator may be causing the rich condition and sluggish O2 sensor response. I’ve reviewed freeze frame data from when the codes were set, and it shows a load value of 21.1%, coolant temperature at 172°F, engine RPM at 1573, vehicle speed at 45 mph, and fuel rail pressure at 39 psi. I have also tested the battery (630 CCA) and alternator (charging at 14.5 volts), both of which appear healthy. I’ve ruled out excessive blow-by, oil leaks, or air filter restrictions. The head gaskets were replaced three years ago with valves ground during that service. I plan to inspect plugs this weekend and will update the forum on my findings.

Related fault codes

P0172P0175

Comments (9)

Anonymous 2015-09-18 05:53

The fuel pressure drop within five minutes is unusual—though not extreme—but it's worth checking the freeze frame data to see exactly when the P0172 and P0175 codes were set. This can help pinpoint whether the issue occurs under specific driving conditions.

Anonymous 2015-09-18 09:55

Start with simple, low-cost fixes: try adding a bottle of Techron or Seafoam fuel additive. You’ve already cleaned the MAF—did you use a dedicated MAF cleaner? Check for excessive blow-by by inspecting PCV system operation. Ensure your air filter and intake ducts are clean and not clogged. Avoid using oiled air filters. Old-school tip: check spark plugs—they can reveal key clues about engine performance. Also, don’t rely on dashboard gauges—engine temperature readings from a scan tool are more accurate. Confirm no fluid leaks in transmission or engine.

Anonymous 2015-09-18 12:28

I use Seafoam once a month in the fuel tank. I did apply MAF cleaner, and my air filter is new with regular cleaning after every oil change. I replace filters at least twice per year depending on local dust levels. I still maintain oil changes every 3000 miles. I’ve learned not to trust dashboard gauges—actual engine temperature from a scan tool matters more. No fluid leaks anywhere. Transmission runs smoothly and has been serviced annually. Engine shows no blow-by, with only 86,000 miles on the clock. Head gaskets were replaced three years ago, with valves ground during that service. I haven’t pulled freeze frame data yet due to doctor appointments—will do this weekend if time permits. I’ll check spark plugs this weekend before winter painting begins. Thanks for your input!

Anonymous 2015-09-18 13:43

Freeze Frame Data: P0172 — Fuel System 1 in Closed Loop, Load Value at 21.1%, Coolant Temp at 172°F, Short Term FT1 at -2.4%, Long Term FT1 at -24.3%, Short Term FT2 at -9.4%, Long Term FT2 at -24.3%. Fuel Rail Pressure is 39 psi. Engine RPM: 1573, Vehicle Speed: 45 mph. This data was captured when I left the parking lot—engine not yet warmed up. The readings remain consistent even when engine temperature reaches 195°F.

Anonymous 2015-09-18 13:58

I recall that on my 2003 Explorer, air charge temperature came from the MAF sensor. Does your scan tool show air charge temperature or ambient/battery temperatures? Can you warm up the engine and then run it at 2000–2500 RPM—does the rear O2 sensor begin to respond more dynamically? Also, check battery health and alternator output for AC voltage.

Anonymous 2015-09-19 01:49

Air charge temperature was 48°F tonight; ambient temperature was also 48°F—both from scan tool readings. The rear O2 sensor ranges between 100 and 875. I briefly disconnected a vacuum line to induce a lean condition—it helped slightly, but long-term fuel trim only dropped to -11%. Short-term fuel trim returned to normal (±5% of zero). O2 sensor response improved temporarily. I also tried removing the fuel cap to check for purge solenoid issues—no improvement. The car remained rich. Battery is 2 years old with a 630 CCA rating. I’ll verify alternator AC voltage soon.

Anonymous 2015-09-19 02:44

Is this a returnless fuel system or one with a pressure regulator? Try disconnecting the MAF sensor—does idle improve or does O2 sensor behavior change when idling at warm temperature? You can also test vacuum levels using a gauge. A collapsed PCV hose or elbow may cause rich conditions. A leaking EGR valve can command more fuel due to incorrect air mass readings—worth checking, especially if the MAF is suspected as root cause. When unplugging the MAF, the engine relies on throttle position and RPM for fuel trim calculations—check O2 sensor response at cruise speed.

Anonymous 2015-09-20 05:45

This vehicle has a returnless fuel system. After disconnecting the MAF, idle behavior did not improve—I was unable to drive with it due to other commitments. The PCV hose and elbow are in good condition. I haven’t tested vacuum with a gauge yet, though when I disconnected a line to induce lean conditions, the response appeared normal.

Anonymous 2015-09-23 13:33

I want to thank everyone for their helpful suggestions. The root cause was found accidentally: while stopping at an auto parts store to drop off vegetables, my car wouldn’t start. A load tester showed only 650 CCA—below the expected level. Using a booster pack, it started immediately. Alternator tested at 14.5 volts and was charging properly. After shutting down and waiting five minutes, the car restarted without issue. Battery posts and cables were clean. I discovered a swollen spot on the positive battery cable about 6 inches from the starter—insulation split, internal wire turned green. This is a critical lesson: never assume battery cables are failure-proof. After fixing this connection, fuel trims normalized, O2 sensors responded properly, and fuel pressure rose to 45 psi. Everything now runs smoothly. Thanks again—this was an unexpected but valuable troubleshooting experience.