P0420 on Audi
Catalyst System Efficiency Below Threshold (Bank 1)
P0420 on Audi vehicles indicates catalyst system efficiency below threshold (bank 1). Au7o has documented this code across 1 Audi model — most commonly on R8. 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. Typical repair costs on Audi range from $3,000 to $10,000, depending on the specific model and root cause.
Common Causes of P0420
- •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)
P0420 on Audi by Model
Audi R8(1 issue)
- Catalytic Converter Failure Requiring Engine-Out Replacement2008-2023
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.
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View P0420 across all makes →Frequently Asked Questions
What does P0420 mean on 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 specifically, this code is documented across 1 model.
What causes P0420 on Audi vehicles?▼
Common causes on Audi: 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. Specific causes vary by model and year — see the per-model sections below.
How much does it cost to fix P0420 on a Audi?▼
Repair costs on Audi range from $3,000 to $10,000, depending on the specific model and root cause.
Which Audi models have P0420 documented?▼
Au7o has documented P0420 on 1 Audi model: R8.