Quick answer: Nissan Altima catalytic converter problems typically trigger a P0420 code, and on the Altima the real cause is frequently a failing downstream oxygen sensor or an original converter degrading past 120,000 miles. The 2.5L QR25DE engine’s known oil consumption on some years can also contaminate the converter.
The Altima is a common sedan to log a catalyst efficiency code. This guide explains what’s specific to the Altima – the model years most affected, the link to the QR25DE engine, and how to avoid an unnecessary converter replacement.
Why the Altima Logs Converter Codes
On the Altima, the 2.5L QR25DE four-cylinder is the most common engine, and some model years have a documented tendency toward oil consumption. Oil reaching the exhaust coats the converter’s substrate and reduces efficiency over time. More often, though, the trigger is simply a worn downstream oxygen sensor or an aging original converter.
For the full explanation of how the catalyst monitor works, see our main guide: P0420 – Catalytic Converter Efficiency Explained.
Most Affected Altima Model Years
- 2002-2006 (QR25DE): Early QR25 engines with oil consumption and aging converters.
- 2007-2012: Downstream O2 sensor wear and original converter degradation.
- 2013-2018: Generally reliable; sensor-related false triggers are most common.
Symptoms on the Altima
- Check engine light with a stored P0420.
- Slightly reduced fuel economy.
- Occasional sulfur smell from the exhaust.
- On oil-consuming engines, low oil between changes.
Common Causes on the Altima (Cheapest First)
- Faulty downstream oxygen sensor – the most common false trigger.
- Exhaust leak near the manifold or flex pipe.
- Oil consumption (QR25DE) – must be addressed or a new converter will fail again.
- Genuinely worn converter – common past 120,000 miles.
Check These Yourself First
- Check the oil level – important on QR25DE engines.
- Inspect the exhaust for leaks and soot marks.
- Note whether any misfire or lean codes are also present – fix those first.
Estimated Repair Costs for an Altima (2026)
| Job | Parts ($) | Labor ($) |
|---|---|---|
| Downstream O2 sensor | $40 – $130 | $60 – $110 |
| Exhaust leak repair | $20 – $90 | $80 – $150 |
| OEM catalytic converter | $400 – $900 | $150 – $300 |
Note: 2026 US estimates. On the Altima, confirm the oil consumption issue is under control before replacing the converter.
Frequently Asked Questions
Is the Altima’s oil consumption related to P0420?
It can be. Some QR25DE-powered Altimas consume oil, and oil entering the exhaust coats the catalyst substrate, reducing efficiency and eventually triggering P0420. Controlling oil use protects a new converter.
Why did my Altima’s P0420 return after a new converter?
Usually the underlying cause – a worn O2 sensor, an exhaust leak, or ongoing oil consumption – was never fixed. Diagnose those before assuming the replacement converter is defective.
Can I drive my Altima with P0420?
Yes, in most cases it drives normally. But if the code stems from a misfire or oil issue, prolonged driving can clog the converter, so address the root cause promptly.
Related: P0420 full guide · P0171 lean condition · P0300 misfire