[OverRunWithSons]

More info on those codes:

A code P0172 may mean that one or more of the following has happened: (Code PO175 said the same basically)...

The MAF (Mass Air Flow) Sensor is dirty or faulty. Note: The use of "oiled" air filters can cause the MAF to become dirty if the filter is over-oiled. There is also an issue with some vehicles where the MAF sensors leak the silicone potting material used to protect the circuitry.
There could be a vacuum leak.
There could be a fuel pressure or delivery problem
Possible Solutions
Possible solutions include:

Inspect all vacuum and PCV hoses, replace if necessary
Cleanthe MAF sensor. Consult your service manual for it's location if you need help. I find it's best to take it off and spray it with electronics cleaner or brake cleaner. Make sure you are careful not to damage the MAF sensor, and make sure it's dry before reinstalling
Inspect fuel lines for cracks, leaks, or pinches
Check the fuel pressure at the fuel rail
Check the fuel injectors, they may be dirty. Use fuel injector cleaner or get them professionally cleaned/replaced.
Check for an exhaust leak before the first oxygen sensor (this is unlikely to cause the problem, but it is possible)

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P0420 is one of the more common trouble codes that we see.

This is indicative of (among other things) a converter that is not working as efficiently as it should be (according to specs). Catalytic converters are not normally a "wear" type item, that is they are not designed to wear out and need replacement. If they have failed, it is likely due to something else that caused it to fail. This is what a P0420 means in simplified terms.

Symptoms
The main symptom to the driver is the illumination of the MIL (malfunction indicator lamp). You will likely not notice any drivability problems, although there may be symptoms. For example, if the substance inside the catalytic converter is broken or failed, it may be restricting the exhaust which will result in a feeling of reduced power output from the vehicle.

P0420 DTC Code

Causes
A code [/p0420]P0420 may mean that one or more of the following has happened:

Leaded fuel was used where unleaded was called for (unlikely)
A damaged or failed oxygen / O2 sensor
Downstream oxygen sensor (HO2S) wiring damaged or connected improperly
The engine coolant temperature sensor is not working properly
Damaged or leaking exhaust manifold / catalytic converter / muffler / exhaust pipe
Failed or underperforming catalytic converter (likely)
Retarded spark timing
The oxygen sensors in front and behind the converter are reporting too similar of readings
Leaking fuel injector or high fuel pressure
Cylinder misfire
Oil contamination
Possible Solutions
Some suggested steps for troubleshooting and fixing a [/p0420]P0420 error code include:

Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
Inspect the downstream heated oxygen sensor, replace if necessary
Replace the catalytic converter
Diagnostic Tip
Generally speaking you can look at the temperature of the exhaust just before and just after the converter using an infrared temperature gun. The output should be about 100 degrees Fahrenheit hotter when the engine is fully warmed up.

Overall probably the biggest mistake vehicles owners make when they have a P0420 code is to simply replace an oxygen sensor (02 sensor). It is important to do proper diagnosis so you're not wasting money replacing parts unnecessarily.