P0420 on a Nissan Altima
P0420 means catalyst System Efficiency Below Threshold (Bank 1). On the Altima, a few causes come up far more than others.
What P0420 usually means on a Altima
P0420 is a signature Altima code because of the QR25DE's catalytic converter, which is integrated into the exhaust manifold close to the engine. As it ages the ceramic substrate can break down; in the worst cases loose material is drawn back into the cylinders and scores the bores - a documented Altima failure mode. More routinely, P0420 just means the converter has lost efficiency or the rear O2 sensor has gone lazy. Treat a P0420 on a high-mileage QR25 seriously and inspect for substrate breakup.
Most common causes on the Altima
- Degraded manifold-integrated converter (QR25)
The close-coupled cat's substrate breaks down with age; on the Altima this can shed material back into the cylinders.
- Lazy downstream O2 sensor
A slow rear sensor mimics a failed cat and is cheaper to replace.
- Upstream misfire damaging the cat
Ignition-coil misfires - common on the QR25 - overheat and glaze the converter.
- Exhaust leak before the rear sensor
A manifold or gasket leak skews the efficiency reading.
What P0420 costs to fix on a Altima
Because the converter is part of the exhaust manifold on the QR25, replacement is pricier and more labor-intensive than a bolt-in cat - often a several-hundred-to-four-figure job. Rule out the rear O2 sensor first with live data; it is far cheaper. If you find substrate breakup, have the cylinders inspected before installing a new manifold-cat so debris damage is not missed.
Year, engine and mileage change the answer. Start a case and Redline weighs these causes against what your car shows, then names the check that would settle it.
Altima owners also ask
Why is P0420 a bigger deal on a Nissan Altima?
The QR25's catalytic converter is built into the exhaust manifold near the engine; when its substrate breaks up, debris can be pulled back into the cylinders and cause scoring, so the code warrants a closer look than on many cars.
Can I fix Altima P0420 with just a rear O2 sensor?
Sometimes - if live data shows the rear sensor is lazy rather than the cat being dead, a sensor is the cheaper fix, but on the QR25 always inspect for converter substrate breakup first.
Other codes on the Nissan Altima
Related symptoms
Nissan Altima problems & recalls by year
Based on widely reported patterns for this model; individual causes vary by year, engine and history. Generic P0420 behavior is covered on the main P0420 page. Confirm with a scan tool before buying parts.
P0420 on your Altima?
Every problem becomes a Case. Tell Redline what your Altima is doing: it shows what the evidence supports, what’s still missing, and what to check next, before you approve a repair.
“Inner pad ~1 mm. Outer pad 3 mm.”
Next: check the slide pins move freely before buying parts.