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2018 MKX Reserve: Check Engine Light with P0171 and P0174 Codes

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Steve N. · rep 0 · May 22, 7:56 AM · 1 replies · solved · Question

Vehicle: 2018 Lincoln MKX Reserve

Hey everyone, I'm hoping someone can help me out with my 2018 Lincoln MKX Reserve. Lately, the check engine light has come on, and when I scanned it, I got codes P0171 and P0174, indicating System Too Lean on both banks.

The car feels a bit rough at idle, and I've noticed a slight hesitation when accelerating. Fuel economy seems to have dipped a little too. It's got about 75,000 miles on it and has been pretty reliable until now.

Has anyone experienced this with their MKX or a similar Lincoln model? What should I be looking for first? Any advice on troubleshooting these lean codes would be greatly appreciated before I take it to the dealer.

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 7:56 AM

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Hi Chris, the P0171 (System Too Lean Bank 1) and P0174 (System Too Lean Bank 2) codes on your 2018 Lincoln MKX Reserve indicate that the engine's computer is detecting too much air or not enough fuel in the combustion process for both sides of the engine. This is a common issue with several potential causes.

Here are the most common causes and diagnostic steps:

  1. Vacuum Leaks: This is a very frequent cause for both banks to run lean. Inspect all vacuum lines, the PCV valve and hoses, intake manifold gaskets, and the brake booster hose for cracks, disconnections, or leaks. A smoke test is the most effective way to find these.
  2. Mass Air Flow (MAF) Sensor: A dirty or faulty MAF sensor can send incorrect air readings to the engine control module (ECM), leading to an improper fuel mixture. Try cleaning the MAF sensor with specialized MAF cleaner, ensuring it's completely dry before reinstallation.
  3. Fuel Delivery Issues:
    • Low Fuel Pressure: A weak fuel pump, clogged fuel filter, or faulty fuel pressure regulator can cause insufficient fuel delivery to the injectors. You'd need a fuel pressure gauge to test this.
    • Clogged Fuel Injectors: While less common for both banks simultaneously, partially clogged injectors can reduce fuel flow.
  4. Oxygen (O2) Sensors: While possible, it's less likely for both upstream O2 sensors to fail simultaneously and cause lean conditions on both banks. However, faulty O2 sensors can provide incorrect readings, leading the ECM to adjust fuel trim incorrectly.
  5. Exhaust Leaks: Leaks in the exhaust system before the upstream O2 sensors can draw in ambient air, tricking the O2 sensors into thinking the engine is running lean.

When to see a shop: If you've checked for obvious vacuum leaks and cleaned the MAF sensor without success, or if you don't have the tools to perform a fuel pressure test or a smoke test, it's best to take your MKX to a qualified mechanic. Ignoring persistent lean conditions can lead to long-term engine damage, including catalytic converter failure, due to increased combustion temperatures.

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