Ford Explorer 1997 P0171/P0174 Diagnosis: Intake Leak or Fuel Trim Issue? - Live Data & Scanner Analysis

Have you cleaned the MAF sensor? This is often a common first step in diagnosing P0171/P0174 issues.

I've already cleaned the MAF using brake cleaner. The air filter is a new paper element — I’ve never had a kn filter before. Does this sound like a MAF problem? If it were faulty, wouldn't it show a MAF failure code?

If the MAF circuit or sensor itself is open, then yes — a failure code would typically appear. Otherwise, a reading of -100 on fuel trims is highly unusual. Does the vehicle run better with the MAF unplugged? If so, it's likely defective. Did cleaning the MAF make any difference? Are there any vacuum leaks or low fuel pressure issues?

After cleaning the MAF, the MAF reading increased to around 700 mg/s at idle. Fuel pressure is stable — 30 psi at idle and rising to 40 psi at 2000 rpm. I haven’t found a vacuum leak yet; spraying carb cleaner around the intake didn’t change idle or fuel trims, which remain stuck at -100. When running onboard monitor tests, the scanner shows 'not supported' for O2 sensors — all sensors are labeled with IDs but no clear mapping of what each one is. Is there a list available that explains sensor functions?

A fuel trim reading of -100 is abnormal — under lean conditions, you'd expect positive values, not negative. Are you certain the fuel pressure specification is correct? 30 psi at idle seems low; I’d expect at least 40 psi as a minimum. What scanner are you using? Fuel trims are calculated from O2 sensor data and other inputs — if those aren’t functioning properly, the trims won't be accurate. The rest of the monitors may show incomplete due to faulty fuel trims. How do the fuel trims or engine performance behave when the MAF is unplugged?

I agree — the fuel trim is extremely off. Only codes present are P0171 and P0174, which seems contradictory. I previously had a P0403 EGR code before replacing the DPFE sensor — that hasn’t returned since. I'm using an Autel GS500 scanner (available for $199 on PrincessAuto). The fuel trims stay at -100 constantly. My service manual lists idle fuel pressure as 30–45 psi, so I may be on the low side — it does increase to 40 psi at higher RPMs. After disconnecting MAF, the truck ran poorly: rough idle, couldn’t accelerate, and stalled after ~40 seconds — trims remained at -100. While I believe the scanner is comparable in price range to others (like the Equus 3130), it doesn't allow O2 testing on my Explorer. Does that mean the software might be flawed?

I suspect the scanner may not be fully functional — if O2 tests aren’t supported and fuel trims are off, the software likely has limitations. I recommend performing a vacuum leak test using a propane bottle with a hose connected to an intake port. The fact that idle doesn't change doesn’t rule out leaks — accurate O2 readings and fuel trim values would confirm this. It’s also worth checking with a dealer technician about the correct fuel pressure specification. A variation of 15 psi is too wide; I’d expect at least 40 psi as a baseline. A smoke test on the intake (injecting smoke through vacuum ports to detect leaks) could definitively rule out gasket failures or unintended fuel pressure increases.

The O2 test fails on my Explorer — it says 'not supported' — but works fine on a 2005 GMC at work. This suggests the scanner isn’t faulty, just limited by vehicle compatibility. The DPFE sensor is an OEM Ford part purchased from a dealership. Could you explain what a smoke test of the intake entails? I bought this scanner to monitor fuel trims while spraying around the intake — but if readings don’t change, it seems useless. I suppose I could eventually try replacing parts one by one, but that wouldn't improve my diagnostic skills. Issues like these are why I appreciate working on construction equipment and rental tools.

The scanner isn’t the issue — the software incompatibility is puzzling. Unless the engine is truly lean and O2 sensors aren't responding, it’s unlikely. A smoke test involves introducing a chemical smoke into vacuum ports to detect leaks — same method used in emissions testing. You don’t need to replace parts blindly — you may just need better diagnostic tools or professional help. There's no shame in getting dealer support or a certified shop to diagnose and repair the issue, especially if it’s beyond your current skill level.

If I perform a smoke test, can I assume that an intake leak would cause fuel trims to be positive (not negative)? Similarly, low fuel pressure should trigger the system to add more fuel — so fuel trims should shift positively under lean conditions. I was considering buying the Equus scanner on Amazon but it doesn’t ship to Canada.

Yes — when the engine is running lean, you'd expect positive fuel trim values, not negative ones.

Important point: Both O2 sensors are showing zero volts. Have the fuses been checked? Are there any loose wiring or connection issues?

Good observation — that’s a critical detail I missed!