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2018 Discovery HSE - CEL with P0171/P0174 and rough idle

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Paul Z. · rep 0 · May 22, 6:26 AM · 1 replies · solved · Question

Vehicle: 2018 Land Rover Discovery HSE

Hey everyone,

I'm having some trouble with my 2018 Land Rover Discovery HSE, 3.0L Supercharged. The Check Engine Light came on a few days ago, and I've noticed a rough idle, especially when cold. It also feels like it's lost a bit of power.

I hooked up my OBD-II scanner and pulled two codes: P0171 (System Too Lean, Bank 1) and P0174 (System Too Lean, Bank 2). Both banks are showing lean, which seems a bit odd to me, as usually it's just one.

I've checked the gas cap, and it seems fine. No obvious vacuum leaks that I can hear, but I haven't done a smoke test yet. I'm wondering if it could be a Mass Air Flow (MAF) sensor, or perhaps something with the fuel system, given both banks are affected.

Any ideas on where I should start looking? Could it be O2 sensors, or something more serious? Thanks for any help!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 6:26 AM

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ClearTheCode admin answer:

The P0171 and P0174 codes indicate that your engine's computer (ECM) is detecting a lean condition on both banks, meaning there's too much air or not enough fuel in the combustion mixture. This is a common issue with several potential causes.

Here are the most common causes and diagnostic steps:

  1. Vacuum Leaks: This is the most frequent culprit for lean codes on both banks. Look for cracked or disconnected vacuum hoses, a faulty PCV valve or its hoses, or a leaking intake manifold gasket. A professional smoke test is often the best way to find these elusive leaks.
  2. Mass Air Flow (MAF) Sensor: A dirty or failing MAF sensor can send incorrect air mass readings to the ECM, leading it to inject less fuel than needed. Try cleaning the MAF sensor with a specialized MAF cleaner. If the problem persists, monitoring its live data with a scanner can help determine if it's reporting plausible values.
  3. Fuel Delivery Issues:
    • Weak Fuel Pump: A failing fuel pump may not deliver adequate fuel pressure to both banks.
    • Clogged Fuel Filter: While less common on newer vehicles with long-life filters, a restricted fuel filter can starve the engine of fuel.
    • Dirty Fuel Injectors: Though less likely to affect all injectors equally across both banks simultaneously, partially clogged injectors can contribute to a lean condition.
  4. Exhaust Leaks: Leaks in the exhaust system before the oxygen sensors can draw in outside air, tricking the O2 sensors into reporting a lean condition.

When to see a shop:

  • If you've performed basic checks like visually inspecting vacuum lines and cleaning the MAF sensor, and the codes or symptoms persist.
  • If you lack the tools for advanced diagnostics such as a fuel pressure gauge or a smoke machine.
  • If the rough idle worsens, or if you experience significant loss of power, as prolonged lean conditions can lead to catalytic converter damage or other engine issues.
  • A qualified Land Rover specialist can perform comprehensive diagnostics, including fuel system pressure tests, advanced live data analysis, and professional smoke tests, to accurately pinpoint and resolve the issue.
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