Nissan Check Engine Code
P0420 on Nissan
Catalyst System Efficiency Below Threshold (Bank 1)
Also covers: Infiniti
P0420 on Nissan: what makes it different
P0420 (Catalyst System Efficiency Below Threshold, Bank 1) is one of the most frequently reported codes on Nissan vehicles, particularly those powered by the 2.5L QR25DE four-cylinder and the 3.5L VQ35DE V6. Nissan's exhaust and catalyst design, combined with common upstream oxygen sensor issues and oil consumption problems on the QR25DE, make this code a recurring complaint among Sentra, Altima, and Maxima owners.
Most-affected engines
- 2.5L QR25DE I4 (Altima, Sentra, Rogue, X-Trail)
- 3.5L VQ35DE V6 (Altima, Maxima, Murano, Pathfinder, 350Z)
- 1.8L QG18DE I4 (Sentra, Almera — older models)
Common model years: 2002–2018
Most likely cause on Nissan
Degraded catalytic converter accelerated by excessive oil consumption from the QR25DE engine, or failed downstream oxygen sensor giving inaccurate catalyst efficiency readings on the VQ35DE.
Known Nissan engine-family issues
The QR25DE engine in Altimas and Sentras from the mid-2000s to early 2010s has a well-documented oil consumption issue where the piston rings allow oil to pass into the combustion chamber. This burned oil deposits phosphorus and other compounds onto the catalyst, degrading it prematurely. Simply replacing the catalytic converter without addressing oil consumption will result in the new catalyst failing again within a year or two. Always verify oil consumption rate before condemning the catalyst on these engines. The VQ35DE is generally more reliable in this regard but its dual-sensor catalyst monitoring is sensitive to even slightly lazy oxygen sensors.
Nissan-specific causes (most common first)
- Catalytic converter poisoned or degraded by oil burning — the QR25DE is known for elevated oil consumption that coats the catalyst substrate
- Failed or lazy downstream (post-cat) oxygen sensor providing incorrect catalyst efficiency signal
- Upstream air-fuel ratio sensor (front O2 sensor) drift causing the ECM to miscalculate catalyst efficiency
- Exhaust leaks near the manifold-to-catalyst junction on higher-mileage Altimas and Maximas, skewing sensor readings
- Genuine catalyst substrate breakdown on high-mileage VQ35DE engines, particularly where the vehicle has been used for many short trips
- Coolant intrusion from a failing head gasket on the QR25DE contributing to catalyst contamination
Nissan-specific diagnostic tip
Before replacing the catalytic converter on any Nissan QR25DE, perform an oil consumption test: record the oil level, drive 1,000 miles under normal conditions without adding oil, and recheck. If consumption exceeds about one quart per 1,000 miles, the catalyst is likely oil-fouled and will fail again after replacement. On VQ35DE models, use a scan tool to monitor the upstream and downstream O2 sensor waveforms simultaneously at operating temperature. The downstream sensor should show a slow, nearly flat voltage compared to the switching upstream sensor — if the downstream sensor is also switching rapidly, it may simply be a sensor fault rather than a catalyst failure.
Symptoms drivers report
- Check engine light
- Slight loss of power or fuel economy
- Rotten-egg smell (severe cases)
- Emissions test fail
Typical repair cost on Nissan
Most Nissan owners fix P0420 for between $120 and $1100, depending on which underlying cause turns out to be at fault. Start with the most-likely cause for your vehicle — Degraded catalytic converter accelerated by excessive oil consumption from the QR25DE engine, or failed downstream oxygen sensor giving inaccurate catalyst efficiency readings on the VQ35DE. — before throwing parts at it.
Nissan P0420 FAQ
My Nissan Altima with the 2.5L just had the catalytic converter replaced but P0420 came back after a few months — why?
On the QR25DE 2.5L, P0420 recurring after a cat replacement almost always points to ongoing oil consumption. The engine's piston rings allow oil into the combustion chamber, and that burned oil coats and poisons the new catalyst. The fix requires addressing the oil consumption — often through an engine rebuild or ring replacement — before the catalytic converter replacement will last.
How do I tell if it's the oxygen sensor or the actual catalyst causing P0420 on my Nissan?
Use a scan tool to watch the downstream (post-cat) O2 sensor voltage in real time. At operating temperature, it should hold a relatively steady voltage around 0.6–0.7 volts with only slow changes. If it's switching rapidly like the upstream sensor, the catalyst is likely bad. If it responds sluggishly or reads a fixed voltage that doesn't make sense, the sensor itself may be the problem. Replacing the cheaper downstream sensor first is a reasonable step when catalyst inspection looks okay visually.
Does P0420 on a Nissan mean I'll fail emissions?
In most jurisdictions, yes — an active P0420 code will cause an automatic emissions test failure because the catalyst monitoring system is reporting below-threshold efficiency. The MIL (check engine light) must be off and the catalyst readiness monitor must show complete for a pass. You'll need to diagnose and resolve the root cause before the light will go out and the monitor will reset.
Related codes
Drive a different make? See the general P0420 guide for cross-vehicle causes and symptoms.
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