P0420: Catalyst System Efficiency Below Threshold (Bank 1)

What does P0420 mean?

Your car has an oxygen sensor before the catalytic converter (upstream) and one after it (downstream). A healthy converter smooths out the oxygen swings, so the downstream sensor reads fairly steady. When the downstream signal starts to look too much like the upstream one, the engine control module (ECM) concludes that the converter's efficiency has dropped below its threshold and stores P0420.

Bank 1 is the side of the engine that contains cylinder 1. On a four cylinder engine there is only one bank. If the same fault is detected on the other side of a V6 or V8, you will see P0430 instead.

P0420 is one of the most common reasons vehicles fail OBD based emissions inspections, so it is worth fixing properly rather than just clearing it.

Common symptoms

• Check engine light on, usually steady rather than flashing
• Often no noticeable change in how the car drives
• Failed emissions or inspection test
• Sometimes a sulfur or rotten egg smell from the exhaust
• Reduced power or a rattle from under the car if the converter is physically damaged or clogged

Common causes

The causes below are the ones commonly seen with P0420. The actual cause can vary by vehicle, engine and mileage, so treat this list as a starting point, not a diagnosis.

• An aged or damaged catalytic converter
• A faulty or slow downstream (or upstream) oxygen sensor
• Exhaust leaks near the oxygen sensors or converter
• An engine problem that overheated or contaminated the converter, such as misfires, running rich, or burning oil or coolant
• Wiring or connector problems at the oxygen sensors
• Outdated engine software on some vehicles (check for service bulletins)

What to check with a scanner

• Look for other codes first. Misfire codes (P0300 to P0308) or fuel trim codes (P0171, P0172) should be fixed before you judge the converter.
• In Live data, compare the upstream and downstream O2 sensor graphs. A downstream sensor that switches almost as fast as the upstream one is consistent with low converter efficiency.
• Check Mode 06 for the catalyst monitor test result if your vehicle reports it. It can show how close the result is to the limit.
• Freeze frame data shows the conditions when the code set, which helps when checking for exhaust leaks or temperature related faults.

How to read and clear P0420 with OBD Master Pro

1. Plug in your adapter. Connect a generic ELM327 (or ELM329) OBD2 adapter to the diagnostic port, usually under the dashboard on the driver's side. Bluetooth Classic, Bluetooth LE, Wi-Fi and USB OTG adapters are supported.
2. Switch the ignition on. Turn the key to ON or start the engine, depending on your car.
3. Connect in OBD Master Pro. Open the app, go to Connection and pick your adapter. Classic Bluetooth adapters connect from the Bluetooth tab, Bluetooth LE adapters from the BLE tab with no pairing needed. Use the generic OBD2 / EOBD profile or your car's brand profile.
4. Read the codes. Open the fault codes screen to see stored, pending and permanent codes. Confirm P0420 is listed and note any other codes, since they often explain the cause.
5. Check the details. Look at freeze frame data, Live data and, where your car supports it, Mode 06 test results (see the checks above).
6. Fix, then clear. After the repair, clear the codes to turn off the check engine light. Clearing also resets the emissions readiness monitors, so drive normally and check the readiness screen before any inspection. Permanent codes cannot be erased with a scan tool; they clear on their own once the car's self test passes.

Tip: you can also look up P0420 and other codes in the app's DTC search without connecting to a car. If your adapter will not connect, use Connection > Share connection log and email it to support. Many cheap ELM327 clones only handle basic engine OBD2, which is all you need for P0420.

When to see a mechanic

Get professional help if any of these apply:

• The check engine light is flashing (this points to a misfire that can damage the converter)
• You smell fuel, hear a rattle, or notice a big loss of power
• The code returns soon after a converter or O2 sensor replacement
• You need to pass an inspection and are unsure of the cause

If you are unsure, a qualified technician can confirm the cause with tests such as smoke testing, pressure testing or scope readings. Clearing a code does not repair the fault.

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