What Does P0420 Mean on a 2018 Nissan Altima?
A P0420 code on your 2018 Nissan Altima means the ECM has detected that the Bank 1 catalytic converter is not converting exhaust pollutants efficiently. The 2018 Altima is the last year of the L33 generation and comes with either the 2.5L 4-cylinder QR25DE engine (179 hp) or the 3.5L V6 VQ35DE engine (270 hp). The 2.5L uses an integrated exhaust manifold converter, while the V6 has a more traditional setup with separate manifolds and converters.
With the 2018 model now several years old and potentially carrying higher mileage, P0420 is one of the more common codes that appears. The good news is that diagnosis and repair are well understood on this generation of Altima.
Symptoms You Might Notice
- Check engine light on (steady)
- Decreased fuel economy
- Rotten egg or sulfur smell from exhaust
- Possible slight loss of power
- Failed emissions inspection
Common Causes on the 2018 Altima
1. Worn Catalytic Converter
By 2024-2026, many 2018 Altimas have 60,000 to over 100,000 miles. At this point, the converter's internal catalyst material is naturally wearing out. The QR25DE engine's converter is particularly susceptible after high-mileage city driving. This is the most common cause of P0420 on higher-mileage 2018 Altimas.
2. Failing Downstream O2 Sensor
The downstream oxygen sensor on the 2018 Altima sits in a harsh environment and degrades over time. A sluggish sensor that responds slowly to exhaust gas changes can mimic a failing converter. At the 2018 Altima's age, sensor replacement is a smart diagnostic step before condemning the converter.
3. Exhaust Flex Pipe Leak
The flex pipe on the 2018 Altima is a known weak spot, especially in northern states with road salt. A cracked or perforated flex pipe allows ambient air to enter the exhaust stream, confusing the downstream O2 sensor. Inspect from underneath for soot deposits and listen for exhaust noise.
4. Oil Consumption and Converter Contamination
Some QR25DE engines in this generation experience elevated oil consumption. Oil entering the exhaust stream coats the converter substrate and reduces its ability to catalyze reactions. If you are adding oil between changes, this could be the root cause.
How to Diagnose the Problem
- Check for related codes — Misfire codes, O2 sensor codes, or EVAP codes can point to the root cause of converter failure.
- Test the downstream O2 sensor — Watch the downstream sensor on a scan tool. A steady 0.6-0.8V reading is normal. Oscillating voltage means the converter is failing.
- Inspect the flex pipe and exhaust — Look for cracks, rust-through, and exhaust leaks along the entire system.
- Check oil consumption — Monitor oil level over 1,000 miles. If you are losing more than half a quart, oil contamination may be the cause.
- Temperature test — Converter outlet should be 50-100°F hotter than inlet during normal driving.
Repair Options
Professional Repair
An independent shop familiar with Nissans can diagnose and repair for less than a dealer. Expect $80-$130 for diagnosis. OEM and aftermarket converters are widely available for the 2018 Altima, keeping costs reasonable. The 2.5L converter replacement is straightforward; the 3.5L V6 takes more labor due to dual converters.
DIY Repair
O2 sensor replacement is a solid DIY job. The converter replacement on the 2.5L is moderate difficulty — the integrated manifold unit bolts directly to the engine. The flex pipe can be replaced with a bolt-in section from most auto parts stores. Budget a weekend for a converter job.
Estimated Repair Costs
- Catalytic converter replacement (2.5L): $500 - $1,100
- Catalytic converter replacement (3.5L): $700 - $1,500
- Downstream O2 sensor: $80 - $200
- Flex pipe replacement: $150 - $350
- Oil consumption repair: $500 - $2,500
Prevention Tips
- Monitor oil consumption closely and address it promptly
- Use quality fuel to reduce converter contamination
- Replace spark plugs at the recommended interval to prevent misfires
- Inspect the exhaust system annually for rust and damage
- Fix any engine performance issues before they damage the converter