P0420 on Mazda
Catalyst System Efficiency Below Threshold (Bank 1)
P0420 on Mazda vehicles indicates catalyst system efficiency below threshold (bank 1). Au7o has documented this code across 2 Mazda models — most commonly on CX-5, RX-8. 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 Mazda range from $500 to $1,500, 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 Mazda by Model
Mazda CX-5(1 issue)
- Excessive Engine Oil Consumption2013-2022
Defective valve stem seals allow oil to leak into combustion chambers, causing excessive oil consumption and potential catastrophic engine failure if oil levels drop critically low. Issue primarily affects 2013-2014 models with resurgence in 2021-2022.
Mazda RX-8(1 issue)
- Catalytic Converter Clogging2004-2012
The RX-8 catalytic converter clogs prematurely due to the rotary engine's naturally higher oil and fuel consumption. Unburned fuel and oil residue accumulate in the catalyst substrate, restricting exhaust flow. A clogged cat causes power loss, overheating, and can destroy the engine if backpressure becomes severe. Catalytic converter failure is often a secondary symptom of apex seal wear.
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View P0420 across all makes →Frequently Asked Questions
What does P0420 mean on Mazda?▼
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 Mazda specifically, this code is documented across 2 models.
What causes P0420 on Mazda vehicles?▼
Common causes on Mazda: 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 Mazda?▼
Repair costs on Mazda range from $500 to $1,500, depending on the specific model and root cause.
Which Mazda models have P0420 documented?▼
Au7o has documented P0420 on 2 Mazda models: CX-5, RX-8.