Service engine light is on code 0421

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P0420 GMC - Catalyst System Low Efficiency Bank 1 ' Add Comment
Possible causes
- Three way catalyst converter Bank 1
- Exhaust tube
- Intake air leaks
- Fuel injectors may be faulty
- Fuel injector leaks
- Spark plugs may be faulty
- Improper ignition timing
- Powertrain Control Module (PCM)
What does this mean?
Tech notes
As the code description implies the P0420 code means that the vehicle's control module has detected that the three-way catalytic converter is not working properly (is not as efficient as the factory is expecting). Replacing the oxygen (O2) sensors may sometimes fix the code, but in most cases the catalytic convert needs to be replaced to fix the problem. If the P0420 code is combined with other codes, try fixing the other codes first.
What does this mean?
When is the code detected?
If a condition exists which causes the Powertrain Control Module (PCM) to detect excessive Bank 1 HO2S #2 activity outside of an acceptable range for an extended period of time, the PCM will set the P0420 code. This indicates that the three-way catalytic converters oxygen storage capacity is below a threshold considered acceptable.
Possible symptoms
- Engine Light ON (or Service Engine Soon Warning Light)
P0420 GMC Description
In order to maintain a reasonably low emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides of Nitrogen (NOx), the engine controls system uses a three-way catalytic converter. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas. This reaction converts them into harmless water vapor and Carbon Dioxide. The catalyst also reduces NOx, converting it to Nitrogen. The Powertrain Control Module (PCM) monitors this process using Bank 1 HO2S #2. Bank 1 HO2S #2, located in the exhaust stream past the three-way catalytic converter, produces an output signal which indicates the oxygen storage capacity of the catalyst. This determines the catalysts ability to convert exhaust emissions effectively. If the catalyst is functioning correctly, the Bank 1 HO2S #2 signal will be far less active than that produced by Bank 1 HO2S #1.

What is the code?

that is a low efficency bank 1 catalyst code, basically you need a new catalytic convertor

Exhaust Converter on Bank #1 side, is worn out,needs
to be replaced

PO420 &PO430 are Converters Below Efficiency

Should have lasted another year or two

I suspect you don't keep up on what the car
required,air filter,spark plugs,O2 Sensors & the like.

Your fuel trim & ignition issues over the years ruined it

OBD2 Diagnostic Test Code (DTC) P0421 - Warm Up Catalyst Efficiency Below Threshold (Bank 1)


Basically this means that the oxygen sensor downstream of the catalytic converter on bank 1 is detecting that the catalyst is not working as efficiently as it should be. (Too much free oxygen in the exhaust downsream of the converter)

This can be caused by leaks in the exhaust system between the upstream O2 sensor and the downstream O2 sensor, but is usually caused by a converter that is chemically inactive or contaminated.

You didn't mention the year or mileage on your vehicle, but it should be noted that many manufacturers cover the catalyst under warranty for extended periods of time/mileage. If you live in the US, the manufacturer MUST warranty the catalyst for a MINIMUM of 8 years/80,000 miles per EPA mandates.

I'm not sure you understand the difinition of OBD2 fault code P0420, it means that Catalytic converter in the exhaust that is connected to the engine cylinder bank that contains number one engine cylinder has fallen below the a allowed threshold of cleaning pollunts from the engines exhaust stream. I would be more specific but you have not given model or year of the Mercury you have.
Here is the full difinition of OBD2 fault code P0420 from the factory service information system.


P0420 - Catalyst System Efficiency Below Threshold (Bank 1)

Possible causes - Three way catalyst converter worn out
- Intake air leaks
- Fuel injectors may be faulty
- Fuel injector leaks
- Spark plugs may be faulty
- Improper ignition timing
- Engine Control Module

Hi and welcome to FixYa!

Code p0420 means Catalyst System Efficiency Below Threshold (Bank 1). This will surely needs attention to resolve the problem.

Hope this helps and thank you for using FixYa! Have a good one!

sound like a injector problem

Replace the catalytic converter.