P0420 on Acura: How to Diagnose & Fix
Catalyst System Efficiency Below Threshold (Bank 1)
1 Acura model · 1 documented issue · $300-$1,800 repair
P0420 on Acura by model: symptoms, diagnosis and fix
TSX owners across both generations report recurring P0420 (Catalyst System Efficiency Below Threshold, Bank 1) as the front catalytic converter loses efficiency. On the first-generation K24A2 cars this is frequently a downstream consequence of the oil-consumption problem - oil ash coats and poisons the catalyst substrate long before its normal service life. On the V6 cars the J-series family is separately documented for the catalyst substrate breaking apart, producing a loose rattle inside the converter shell. Beyond the check engine light, the practical consequence is a failed emissions inspection. Catalytic converters carry the federal 8-year/80,000-mile emissions warranty, which most of these cars are now well past.
Common Symptoms
- Check engine light with P0420
- Failed emissions/smog inspection
- Rotten-egg sulfur smell from the exhaust
- Rattle from the converter when tapped or on startup
- Reduced fuel economy or sluggish acceleration
How to Fix
Do not replace the converter on the code alone. Diagnose first: check for exhaust leaks ahead of the downstream sensor, then read upstream and downstream O2 sensor waveforms with a scan tool - a lazy or contaminated downstream sensor sets the same code and is a fraction of the cost. Fix any underlying oil consumption or misfire first, or the replacement converter will be poisoned too. If the converter is genuinely failed, replace it, and use a direct-fit unit that meets your state's emissions standard (California requires a CARB-compliant converter). No recall exists; check whether the vehicle is still inside the 8-year/80,000-mile federal emissions warranty before paying.
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 Acura?
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 Acura it is documented across 1 model: TSX.
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 Acura?
Published repair estimates on Acura range from $300 to $1,800 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.