OBD-II trouble code
P0420: Catalyst System Efficiency Below Threshold (Bank 1)
The engine computer detected that the catalytic converter on Bank 1 is not reducing emissions as efficiently as expected. The check engine light is on, but the car is usually still drivable.
Quick facts
- System
- Powertrain
- Category
- Emissions / Catalyst
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $200 – $2,500
- DIY difficulty
- Intermediate DIY
What does P0420 mean?
The engine control module (ECM) monitors how well the catalytic converter is working by comparing readings from two oxygen sensors — one upstream of the catalyst (sensor 1) and one downstream (sensor 2). A healthy catalyst smooths the post-cat oxygen signal compared to the pre-cat signal. When the two signals look too similar over time, the ECM decides the catalyst has lost efficiency and sets P0420.
Bank 1 refers to the side of the engine that contains cylinder number 1. On an inline engine, there is only one bank, so P0420 still applies. On a V6, V8, or other V-configuration engine, P0420 specifically refers to the Bank 1 side, while P0430 covers Bank 2.
The code is most often associated with a worn catalytic converter, but it does not always mean the converter itself is bad. Oxygen sensor problems, exhaust leaks, and unresolved engine misfires can all produce the same code.
Common causes
- Failing or worn catalytic converter (most common after high mileage)
- Downstream oxygen sensor degraded or slow to respond
- Exhaust leak upstream of the catalytic converter
- Engine misfire that has damaged the catalyst over time
- Contaminated fuel or oil burning past the rings, fouling the catalyst
- Faulty engine coolant temperature sensor causing rich running
- Aftermarket high-flow catalytic converter that doesn't meet OEM efficiency
Symptoms
- Check engine light is on
- Vehicle will fail an OBD-II emissions test
- Slight reduction in fuel economy in some cases
- Often no driveability symptoms at all
Diagnostic steps
- 1.Scan the vehicle for other codes. Address misfire (P0300-series), lean (P0171/P0174), or oxygen sensor codes before assuming the catalyst is bad.
- 2.Inspect the exhaust system for leaks upstream of the catalytic converter, including the manifold and gaskets.
- 3.Compare voltage waveforms from the upstream and downstream oxygen sensors with a scan tool. A downstream sensor switching as quickly as the upstream is a strong sign of catalyst failure.
- 4.Check the downstream oxygen sensor with the manufacturer's specification. A slow or lazy sensor can cause P0420 even with a healthy catalyst.
- 5.If everything else checks out, the catalytic converter is the likely failure.
Repair cost
$200 – $2,500
Range is wide because the fix could be as simple as a $200 oxygen sensor replacement or as expensive as a $1,500-$2,500 OEM catalytic converter on a luxury or hybrid vehicle. Aftermarket cats run $300-$800 installed on most mainstream vehicles.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with catalytic converter replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.