P0420 on Infiniti: How to Diagnose & Fix
Catalyst System Efficiency Below Threshold (Bank 1)
1 Infiniti model · 1 documented issue · $600-$3,500 repair
P0420 on Infiniti by model: symptoms, diagnosis and fix
First-generation FX35s commonly set P0420 and/or P0430 (catalyst efficiency below threshold, bank 1/bank 2) as the two close-coupled converters degrade, often between 80,000 and 130,000 miles. Contributing causes on this engine are oil consumption coating the catalyst and rich running from failed coils/plugs or sensor faults. Dealers quote about $1,500 per converter plus labor, with the OEM manifold-integrated cats being the expensive part. Aftermarket high-flow cats compatible with the G35/350Z are a common lower-cost route, and replacing downstream O2 sensors alone often does not clear the code.
Common Symptoms
- Check engine light with P0420 and/or P0430
- Failed emissions inspection
- Sulfur / rotten egg smell from exhaust
- Reduced power or rattle from the converter when substrate breaks up
How to Fix
Confirm the code is genuine catalyst degradation: check for misfires, oil consumption, rich fuel trims and exhaust leaks near the rear O2 sensors first, and replace failed coils/plugs before buying converters. Then replace the affected converter(s); OEM runs $1,500-2,000 installed per side, aftermarket CARB-compliant units $300-550 each plus labor. Fix the upstream cause (oil consumption, misfire) or the new cats will fail again.
Sources for this issue (3)
P0420 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0420 mean on a Infiniti?
P0420 stands for "Catalyst System Efficiency Below Threshold (Bank 1)." P0420 means the engine computer determined the catalytic converter on Bank 1 isn't cleaning the exhaust as efficiently as it should. The computer compares the upstream and downstream oxygen sensors; a healthy converter stores and releases oxygen, so the downstream sensor should read fairly steady, while a worn one lets its readings mirror the upstream sensor. When that efficiency falls below a set threshold, this code is set. While it most often points to a worn-out catalytic converter, it can also be triggered by upstream problems like a faulty oxygen sensor or an exhaust leak, so the converter isn't always the actual fault. On Infiniti it is documented across 1 model: FX35.
What are the most common causes of P0420?
The most common causes of P0420 are: Worn-out or failing catalytic converter (Bank 1), Faulty or aged downstream (or upstream) oxygen sensor, Exhaust leak before or near the oxygen sensors, Engine running rich or lean (fuel trim issues), Misfires or oil/coolant contaminating the converter, Damaged or aftermarket low-quality catalytic converter, Faulty oxygen sensor wiring or connectors, Software/calibration needing update (on some vehicles). The specific cause varies by vehicle.
How much does it cost to fix P0420 on a Infiniti?
Published repair estimates on Infiniti range from $600 to $3,500 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
Looking for P0420 on a different make?
View P0420 across all makes →Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.