P0420 fault not going away

Did you reset the code?

Try some of this to clean the fouling in the rest of the system,

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Hi,
The code indicates a fault detected in the catalytic converter (bank1). It could just be an oxygen sensor, but if your truck is over 6-7 years old, I would say you need to replace the converter.You could try a new upstream O2 sensor on the left cylinder bank manifold first, it will be much cheaper if that would be the cause.....Mike

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

a p0420--means one of the following has happened. an oxygen sensor is not reading properly, the engine coolant sensor is not working properly, damaged or leaking exhaust manifold /catalytic converter/ exhaust pipe, retarded spark timing, the oxygen sensors in front and back of converter are reporting too similar of readings. A P0420 - P0430 are pointing at the catalytic converter, and changing the Oxygen Sensors would be the major choice. good-luck!!

Diagnostic code produces P0420 catalyst system efficiency below threshold-Bank 1. Probable causes are listed as follows:
1-air leak in exhaust before rear 02 sensor, 3-Fuel system fault,
4-faulty catalytic converter.

The code PO420 you have is Catalyst System efficiency is below threshold bank one. Usually replacing the O2 sensor that's the front most in the exhaust system will cure your problem,

Your Check Engine light is on and you find a P0420 code for a catalytic converter fault. Does that really mean your converter has reached the end of the road and needs to be replaced? A new converter can cost $600 to $1000 or more if a new car dealer or repair shop does the job, or maybe half that amount if you do it yourself. It's an expensive fix that may or may not be necessary. The problem, in most cases, is an emissions issue, not a performance issue that affects the way the engine runs.

A P0420 diagnostic trouble code is a "generic" fault code that is set when the Onboard Diagnostic II (OBD II) system sees a drop in converter efficiency. The OBD II system monitors catalyst efficiency by comparing the switching activity of the upstream and downstream oxygen sensors in the exhaust. The upstream O2 sensor in theexhaust manifold reflects the condition of the exhaust gases as they exit the engine. The downstream O2 sensor in or behind the catalytic converter reflects the condition of the exhaust after it passes through he converter.

The catalytic converter is like an after-burner. It oxidizes (burns) any residual fuel vapors (unburned hydrocarbons or HC) in the exhaust. It also burns any carbon monoxide (CO) in the exhaust. The exhaust must meet federal emission standards, and if a problem exists that causes emissions to exceed the federal limits by 150%, the OBD II system is supposed to catch the fault, set a code and turn on the Check Engine light.

Thanks

go to this link for fault code P0420
http://www.obd-codes.com/p0420
all this read the catalytic converter is most likely worn out.
This Definition of P0411 is : Secondary Air Injection System Incorrect Flow Detected
What does this mean? u have a electric air pump that pumps fresh air into the exhaust system during cold start, this reduces exhaust emissions, that pump is defective, replace it.

In the exhaust line...


DTC P0420: 1/1 CATALYTIC CONVERTER EFFICIENCY
NOTE:For component location, see COMPONENT LOCATIONS . For connector identification, see CONNECTOR IDENTIFICATION . For circuit identification and wiring diagram, see WIRING DIAGRAMS article. After each repair procedure has been completed, reconnect all components. Perform POWERTRAIN VERIFICATION TEST VER-5 under VERIFICATION TESTS to ensure system is functioning properly.

Monitored & Set Conditions
Catalytic converter efficiency is monitored after engine warms up to 158°F (70°C), throttle has been open for 180 seconds at vehicle speed more than 18 MPH, with engine speed at 1200-1700 RPM and MAP vacuum 15-21 in. Hg. DTC will set in Powertrain Control Module (PCM) memory when PCM senses catalyst efficiency deteriorated to a predetermined value. Possible causes for DTC to set are: intermittent condition, exhaust leak, engine mechanical problem, defective catalytic converter or upstream Heated Oxygen Sensor (HO2S) is older than downstream HO2S.
Testing
NOTE:1/1 HO2S may also be referred to as an upstream oxygen sensor. On 2.4L, 1/1 HO2S is located on exhaust manifold flange. On 3.3L and 3.8L, 1/1 HO2S is located on right exhaust manifold flange (manifold closest to firewall).
NOTE:If any O2 sensor DTCs are set along with P0420, diagnose those DTCs before continuing with this test. See DIAGNOSTIC TROUBLE CODE DEFINITIONS .

Turn ignition on. Using scan tool, read DTCs. If Good Trip counter is displayed for DTC P0420 and displayed count is "0", go to next step. If Good Trip counter is not displayed for DTC P0420 or displayed count is not "0", go to step 7 .
Inspect catalytic converter for damage, discoloration caused by overheating and for converter coming apart internally. Replace catalytic converter as necessary. If catalytic converter is okay, go to next step.
Start engine and allow it to idle. Check for exhaust leaks between engine and 1/1 HO2S. Repair exhaust system as necessary. If exhaust system is okay, go to next step.
Check exhaust for excessive smoke caused by coolant entering combustion chambers or high oil consumption. If excessive smoke caused by coolant entering combustion chambers or high oil consumption does not exist, go to next step. If excessive smoke caused by coolant entering combustion chambers or high oil consumption exists, repair engine as necessary and replace catalytic converter.
NOTE:A new downstream HO2S used along with an aging upstream HO2S may cause DTC P0420 to set.

If downstream HO2S has been replaced without replacing upstream HO2S, replace upstream HO2S. If downstream HO2S has not been replaced recently or downstream HO2S has been replaced along with upstream HO2S, go to next step.
At this time, catalytic converter is assumed to be defective. Replace catalytic converter.
NOTE:A new downstream HO2S used along with an aging upstream HO2S may cause DTC P0420 to set. Review vehicle repair history and repair as necessary.

Conditions for DTC to set do not exist or fault is an intermittent problem. Start engine and allow it to idle. Ensure engine is at normal operating temperature. Using scan tool, monitor parameters related to DTC while wiggling connectors and wiring harness. If value changes while wiggling connectors and wiring harness, repair connectors and wiring harness where wiggling caused value to change. If value does not change, review Freeze Frame data using scan tool. Attempt to duplicate condition that caused DTC to set. Check for any related technical service bulletins that may apply. Visually inspect related connectors and wiring harness. Repair connectors and wiring harness as necessary. If no problems are indicated, test is complete.

Good luck.

The code PO420 you have is Catalyst System efficiency is below threshold bank one. Usually replacing the O2 sensor that's the front most in the exhaust system will cure your problem,

Your Check Engine light is on and you find a P0420 code for a catalytic converter fault. Does that really mean your converter has reached the end of the road and needs to be replaced? A new converter can cost $600 to $1000 or more if a new car dealer or repair shop does the job, or maybe half that amount if you do it yourself. It's an expensive fix that may or may not be necessary. The problem, in most cases, is an emissions issue, not a performance issue that affects the way the engine runs.

A P0420 diagnostic trouble code is a "generic" fault code that is set when the Onboard Diagnostic II (OBD II) system sees a drop in converter efficiency. The OBD II system monitors catalyst efficiency by comparing the switching activity of the upstream and downstream oxygen sensors in the exhaust. The upstream O2 sensor in theexhaust manifold reflects the condition of the exhaust gases as they exit the engine. The downstream O2 sensor in or behind the catalytic converter reflects the condition of the exhaust after it passes through he converter.

The catalytic converter is like an after-burner. It oxidizes (burns) any residual fuel vapors (unburned hydrocarbons or HC) in the exhaust. It also burns any carbon monoxide (CO) in the exhaust. The exhaust must meet federal emission standards, and if a problem exists that causes emissions to exceed the federal limits by 150%, the OBD II system is supposed to catch the fault, set a code and turn on the Check Engine light.

Thanks

The code PO420 you have is Catalyst System efficiency is below threshold bank one. Usually replacing the O2 sensor that's the front most in the exhaust system will cure your problem, Your Check Engine light is on and you find a P0420 code for a catalytic converter fault. Does that really mean your converter has reached the end of the road and needs to be replaced? A new converter can cost $600 to $1000 or more if a new car dealer or repair shop does the job, or maybe half that amount if you do it yourself. It's an expensive fix that may or may not be necessary. The problem, in most cases, is an emissions issue, not a performance issue that affects the way the engine runs.
A P0420 diagnostic trouble code is a "generic" fault code that is set when the Onboard Diagnostic II (OBD II) system sees a drop in converter efficiency. The OBD II system monitors catalyst efficiency by comparing the switching activity of the upstream and downstream oxygen sensors in the exhaust. The upstream O2 sensor in theexhaust manifold reflects the condition of the exhaust gases as they exit the engine. The downstream O2 sensor in or behind the catalytic converter reflects the condition of the exhaust after it passes through he converter.
The catalytic converter is like an after-burner. It oxidizes (burns) any residual fuel vapors (unburned hydrocarbons or HC) in the exhaust. It also burns any carbon monoxide (CO) in the exhaust. The exhaust must meet federal emission standards, and if a problem exists that causes emissions to exceed the federal limits by 150%, the OBD II system is supposed to catch the fault, set a code and turn on the Check Engine light.
Thanks.