Nissan Altima P0420: Catalyst System Efficiency Below Threshold (Bank 1)
TL;DR
Nissan Altima P0420: often a failing converter on high-mileage 2.5L engines, but rule out the rear O2 sensor first. Shop cost: $250 (sensor) to $1,500 (converter).
Why Nissan Altima owners get P0420
The 2.5L Altima can develop real converter degradation with mileage, but a sluggish rear O2 sensor can also set P0420. Because the converter is costly, confirm with live O2 data and clean fuel trims before replacing it; address any underlying lean/rich condition first.
Most likely causes on the Altima
| Likely cause | Probability | Notes |
|---|---|---|
| Worn catalytic converter (high-mileage 2.5L) | 50% | |
| Faulty downstream O2 sensor | 30% | Rule out first |
| Underlying fuel-trim issue aging the cat | 10% | |
| Exhaust leak near the sensor | 10% |
Nissan Altima-specific known issues
- High-mileage 2.5L Altimas do see genuine converter failures behind P0420.
- Still test the rear O2 sensor first — it is far cheaper than the converter.
- Fit a CARB-compliant direct-fit converter to avoid re-triggering the code.
Nissan Altima P0420 repair cost
| Typical shop cost (Altima) | $250–$1,500 |
|---|---|
| DIY range (generic) | $40–$250 |
Ranges vary by region, engine and parts grade.
Owner tips
- Insist on live O2 data before approving a converter.
- Fix any paired lean/rich codes first.
- Use a quality direct-fit converter, not a cheap universal one.
Full P0420 diagnostic guide
For the complete step-by-step diagnosis, all causes, and repair walkthrough that applies to every vehicle, see the main guide: P0420 — Catalyst System Efficiency Below Threshold (Bank 1).
Nissan Altima P0420 — FAQ
Is P0420 on a Nissan Altima the converter?
On higher-mileage 2.5L engines it often is, but the rear O2 sensor should be tested first since it is much cheaper.
How much to fix P0420 on an Altima?
About $250 if it is the rear O2 sensor, up to roughly $1,500 for the catalytic converter.
Can I drive my Altima with P0420?
Short-term yes, with a steady light and no misfires. You will fail emissions until repaired.