Can a plugged catalytic converter lead to engine noise and loss of power due to back pressure?

P0420 code is catalyst system efficiency below threshold bank one. That means that the computer has detected that a catalytic converter is not functioning properly. This code can be set by a faulty O2 sensor but when it is complemented by a loss of power, it is most likely a clogged catalytic converter. A back pressure test should be performed to verify this. I would suggest taking action as soon as possible because if it is in fact a clogged catalytic converter, that causes components in the engine to overheat and may result in requiring major engine repair

Evaporative purge solenoid wouldn't cause lack of power , You have plugged catalytic converter . Have exhaust system back pressure tested .

Owen, is the engine light on or any codes stored in computer as this info would be beneficial in diagnosing problem.
My guess is your catalytic converter has begun to plug, which gives excess back pressure, robbing engine power.
check the links below.
Signs of clogged bad catalytic converter

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6 Symptoms of Failing Catalytic Converter DoItYourself com

Symptoms Clogged Catalytic Converter Loss of Power

Hello Jamal, I had a catalytic converter go bad. I took it to a muffler shop. They put the car on a hoist while engine was running and used a cutting torch to blow a small hole before and after the converter to compare back pressures. My converter was plugged up .

defective catalytic convertor. It may be plugged up.Make sure ALL the exhaust line from intake manifold(at engine block) to final exhaust pipe are not clogged up.You have reverse exhaust pressure going back towards engine manifold.

Low fuel pressure or a misfire.

Your catalytic converter sounds like its clogged...When clogged it restricts exhaust and puts excessive back pressure on motor causing loss of power ,especially when engine is under load.(going up hill).ONE WAY TO SEE IF THIS IS THE PROBLEM IS TO LOOK UNDER VEHICLE WHEN PROBLEM OCCURS...tHE EXHAUST PIPE BEFORE THE CATALYTIC CONVERTER WILL BE GLOWING DUE TO RESTRICTION....

SECTION 309-00: Exhaust System 2003 Focus Workshop Manual DESCRIPTION AND OPERATION Procedure revision date: 01/24/2003 Exhaust System Printable View (214 KB) Vehicles with 2.0L Zetec-E engine
Item Part Number Description 1 - Bracket 2 - Gasket 3 - Catalytic converter 4 - Bracket 5 - Gasket 6 - Front muffler 7 - Rear muffler 8 - Hanger 9 - Flexible pipe

Vehicles with 2.0L SPI engine
Item Part Number Description 1 - Gasket 2 - Catalytic converter 3 - Bracket 4 - Gasket 5 - Front muffler 6 - Rear muffler 7 - Hanger 8 - Bracket 9 - Flexible pipe

Vehicles with 2.3L PZEV Engine
Item Part Number Description 1 5G232 Catalytic converter 2 5K222 Mounting bracket 3 5K282 Head shield 4 9451 Gasket 5 5A262 Insulator 6 5E292 Converter and muffler assembly 7 9448 Gasket 8 5G203 Flex Pipe NOTE: The exhaust systems shown in this section are production systems.
The exhaust systems are of a two-piece construction for production and five-piece construction for service.
Catalytic Converter
The catalytic converter fitted to the Focus range resembles a small muffler. Because of the high temperatures at which the catalytic converter operates, the vehicle body areas around the catalytic converter are protected by heat shields. The catalytic converter controls three emissions; Hydro carbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx).
Catalyst deactivation due to poisoning
Sulphur (S) in the fuel can cause deactivation of the catalyst.
Lead (Pb) in fuel can cause a severe deactivation at levels above 5 mg per liter of fuel. It is therefore essential that vehicles with catalytic converters always operate on unleaded fuel.
Zinc (Zn) and phosphorus (P) present in some oils, can cause deactivation if oil consumption is high.
The effect of sulphur is felt most if it is released from the catalytic converter as hydrogen sulphide (H2S). This has an unpleasant odor similar to rotten eggs. Changing the fuel manufacturer may help in reducing this phenomena.
To minimize H2S emissions, it is essential that the idle CO level is within specification and that the engine emission system is functioning correctly. For additional information, refer to Section 303-08A or Section 303-08B.
CAUTION: If temperatures exceeding 900°C are achieved the catalyst substrates will start to deteriorate. It is important that the cause is defined and rectified if subsequent melts are to be prevented.
Catalyst melts are normally accompanied by loss of power due to excessive back pressure. For additional information, refer to Section 303-07A or Section 303-07B.
High substrate temperatures are caused by the ignition or the fuel systems. For additional information, refer to Section 303-07A or Section 303-07B.
Misfires at high engine speed may result in an almost instantaneous melt.

LOST OF POWER CAN BE CAUSED BY MANY THINGS.BAD PLUGS AND WIRES.LOW FUEL PRESSURE CAUSED BY STOPPED UP FUEL FILTER OR WEAK FUEL PUMP.TRANSMISSION SLIPPING DUE TO LOW FLUID LEVEL.DRAGGING BRAKES DUE TO PARKING BRAKE ADJUSTMENT INCORRECT.CLOGGED EXHAUST SYSTEM LIKE MUFFLER OR CATALYTIC CONVERTER.WORN OUT ENGINE DUE TO LOW COMPRESSION.

check power steering pump .check fluid level.