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2018 Discovery Sport HSE - Check Engine Light P0420 & Rough Idle

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Mark S. · rep 0 · May 22, 6:26 AM · 1 replies · solved · Question

Vehicle: 2018 Land Rover Discovery Sport HSE

Hey everyone,

I'm having an issue with my 2018 Land Rover Discovery Sport HSE and hoping someone here might have some insight. The Check Engine Light came on a few days ago, and since then, I've noticed the engine running a bit rough, especially at idle. It's not stalling, but it definitely feels less smooth than usual.

I used my OBD-II scanner and pulled a P0420 code: 'Catalyst System Efficiency Below Threshold (Bank 1)'. I've heard this can be a few different things, from a bad O2 sensor to the catalytic converter itself.

Before I take it to the dealer, I wanted to see if anyone has experienced this specific code or symptoms with their Discovery Sport or similar Land Rover models. Are there any common culprits I should check first? I'm trying to avoid an expensive repair if it's something relatively simple. Any advice would be greatly appreciated!

✓ Accepted solution

ClearTheCode Admin

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

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ClearTheCode admin answer:

The P0420 code, 'Catalyst System Efficiency Below Threshold (Bank 1),' indicates that your vehicle's catalytic converter is not operating at optimal efficiency. While it points to the catalyst, it's crucial to understand that other components can trigger this code.

Here's a breakdown of potential causes and steps:

  1. Oxygen Sensors (O2 Sensors):

    • Cause: Faulty upstream or downstream O2 sensors can send incorrect readings to the engine control unit (ECU), leading it to believe the catalytic converter is inefficient.
    • Action: Inspect both pre-catalytic (upstream) and post-catalytic (downstream) O2 sensors for damage or contamination. A scanner capable of live data can help monitor their readings.

  2. Exhaust Leaks:

    • Cause: Leaks in the exhaust system before the downstream O2 sensor can allow unmetered air in, skewing sensor readings and triggering the P0420.
    • Action: Carefully inspect the exhaust manifold, exhaust pipes, and gaskets for any signs of leaks (soot marks, hissing sounds).

  3. Catalytic Converter Failure:

    • Cause: Over time, catalytic converters can become clogged or internally damaged, losing their ability to properly convert harmful emissions. This is often a result of other underlying engine issues (e.g., misfires, excessive oil consumption).
    • Action: If O2 sensors and exhaust leaks are ruled out, the catalytic converter itself might be failing. This is a more costly repair.

  4. Engine Misfires or Fuel System Issues:

    • Cause: Persistent engine misfires or issues like a rich fuel mixture can send unburnt fuel into the catalytic converter, causing it to overheat and fail prematurely.
    • Action: Check for any other pending or active codes related to misfires (P030x) or fuel trim issues. Address these primary problems first.

When to see a shop: If you've checked for obvious exhaust leaks and confirmed O2 sensor functionality (if you have the tools), and the code persists, it's best to consult a qualified Land Rover specialist. Diagnosing catalytic converter issues often requires specialized equipment to measure exhaust gas composition and back pressure. Driving with a failing catalytic converter can lead to further engine damage and significantly increased emissions.

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