P0420 on Audi: How to Diagnose & Fix
Catalyst System Efficiency Below Threshold (Bank 1)
1 Audi model · 1 documented issue · 1 critical · $3,000-$10,000 repair
P0420 on Audi by model: symptoms, diagnosis and fix
The R8 runs four catalytic converters, with two primary cats integrated into the exhaust manifolds. Internal cat breakdown is a recurring failure that throws catalyst-efficiency codes and fails emissions testing. Because the primary cats are part of the manifold assembly deep in the engine bay, replacing them typically requires pulling the engine, making this one of the most expensive non-engine repairs on the car. Some owners also report cracked manifolds on certain 2010-2012 V10s.
Common Symptoms
- Catalyst-efficiency check-engine light
- Failed emissions/smog test
- Loss of power or sulfur/rotten-egg smell
- Rattle from the exhaust (broken substrate)
- Elevated exhaust temperatures
How to Fix
Confirm the failed bank with O2/catalyst-efficiency data and a back-pressure check before condemning. OEM repair means replacing the catalytic-converter-with-integrated-manifold assembly (engine-out labor); many owners opt for high-flow aftermarket or de-catted manifolds where legal. Verify no upstream misfire/oil-consumption issue is destroying the new cats.
Sources for this issue (1)
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 Audi?
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 Audi it is documented across 1 model: R8.
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 Audi?
Published repair estimates on Audi range from $3,000 to $10,000 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.