The largest online repair guide library for every car

About us
Contact support
FAQ
Sign up
Sign in

2018 Altima 2.5 SL Check Engine Light - P0420 Code

0

Mike T. · rep 0 · May 25, 1:52 AM · 1 replies · solved · Question

Vehicle: 2018 Nissan Altima 2.5 SL

Hey everyone,

My 2018 Nissan Altima 2.5 SL just threw a Check Engine Light (CEL) yesterday. I scanned it with my OBD-II reader and got a P0420 code: 'Catalyst System Efficiency Below Threshold (Bank 1)'.

The car seems to be running okay, no major performance issues, but I feel like my fuel economy might be slightly worse than usual. I'm at about 95,000 miles. What's the most common cause for this code on an Altima, and what should I look into first before taking it to a shop?

Any advice on DIY checks or common culprits would be greatly appreciated!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 1:52 AM

0

ClearTheCode admin answer:

The P0420 code on your 2018 Nissan Altima indicates that your catalytic converter isn't operating as efficiently as it should be. While it points to the converter, other issues can trigger this code. Here's a breakdown:

Common Causes:

  • Failing Catalytic Converter: This is the most direct cause. Over time, converters can degrade and lose efficiency.
  • Oxygen Sensors (O2 Sensors): Specifically, the downstream (post-catalyst) O2 sensor monitors the converter's efficiency. A faulty sensor can send incorrect readings.
  • Exhaust Leaks: Leaks before or near the O2 sensors can introduce unmetered air, skewing sensor readings.
  • Engine Misfires or Rich/Lean Conditions: Persistent engine problems that cause unburnt fuel to enter the exhaust can damage the catalytic converter over time.

Diagnostic Steps:

  1. Check for Exhaust Leaks: Visually inspect your exhaust system for any cracks, holes, or loose connections, especially around the O2 sensors. Listen for any hissing sounds when the engine is running.
  2. Monitor O2 Sensor Data: If your scanner allows, check the live data for both upstream and downstream O2 sensors. The upstream sensor should fluctuate rapidly, while the downstream sensor should show a relatively stable, higher voltage reading if the converter is working properly. If the downstream sensor mirrors the upstream, the converter is likely inefficient.
  3. Inspect O2 Sensors: While less common for P0420 directly, sometimes a damaged or fouled O2 sensor can give false readings.

When to See a Shop:

  • If you've checked for exhaust leaks and O2 sensor data suggests a failing converter, professional diagnosis is needed. Replacing a catalytic converter is a significant expense, so ensure it's the root cause.
  • If you're uncomfortable performing the diagnostic steps or don't have access to an advanced scanner, a qualified technician can perform a comprehensive exhaust system and emissions test.
  • Ignoring a P0420 can lead to failed emissions tests and potentially worse engine performance over time.
Menu

Help & account