What are codes P0720, p1574, p0420? 2002 Nissan Altima

code p 0720 refers to output shaft speed (OSS) sensor ---circuit malfunction===causes --wiring--OSS-- ECM --TCM( transmission control module
If you didn't have the code before , check for wire connections to the transmission after the exhaust work
code p 0420 refers to cat converter system bank 1--efficiency below threshold ===causes --cat converter---wiring HO2s (heated oxygen sensor in the exhaust manifold before the cat converter) code p 1574 refers to the vehicle speed sensor , VSS
again check wiring and connections or replace the VSS

need more info how many miles, whens last tune up, do u here anything like maybe hissing sound, do u smell fuel

HERE IS THE ANSWER.
P0725 - Nissan Type Powertrain - Transmission - ISO/SAE Controlled Description Engine speed (RPM) - incorrect voltage signal ECM to TCM
Cause Wiring, RPM sensor, TCM, ECM
P0720 - Nissan Type Powertrain - Transmission - ISO/SAE Controlled Description AT - vehicle speed sensor (VSS)
Cause Wiring, VSS, TCM
P0710 - Nissan Type Powertrain - Transmission - ISO/SAE Controlled Description Transmission fluid temperature sensor - excessively high or low voltage
Cause Wiring, temperature sensor, TCM, ECM
P0826 - Generic Type Powertrain - Transmission - ISO/SAE Controlled Description Up and Down Shift Switch Circuit

the dtcs of your car are so many, my best suggest is go to garage for repair. or if you would like to repair by yourself, you need to buy a diagnostic tool such as multidiag j2534

P1574 Throttle Position Sensor Disagreement btwn Sensors, obd2 scanner can help to turn off the light.

You may want to check with the dealer on this one.
The code means the computer is getting two different vehicle speeds from the two sensors it uses.

This is way over the head of the home repairman, you will need the dealer to look into these codes.

The problem with the codes yoy have that the real only way to be positive is to have the diagnostic equipment to diagnose it for sure. I can tell you through experience the most possible cause is going to be the transmission control module (TCM) or the revolution sensor but would hate for you to just start replacing parts. It is possible that heat can cause this problem to be worse and if you are in high humidity it is highly possible. Below I have given you some of the information on your codes and the possible causes. I did not give you the testing procedures because you would need the computer equipment to be able to do it. If you want all that information I will be happy to email it to you, just let me know. You can email me at [email protected] with all your information and I will be happy to email you back what I have. This information is directly from the factory service manual for your vehicle. I hope this helps in some way and good luck.

p1574 = "throttle position sensor disagreement btwn sensors"
p0720 = "Vehicle Speed Sensor"

hope this helps you out.

I have a 2002 Nissan Altima. Can anyone tell me what these codes are? And how to fix them? P0420

P1574 is the ASCD Vehicle Speed Sensor so it would be relative to your speedo problem.
P0720 is the out put speed sensor, And the sensor is going to be towards the out put shaft to the axle. 
Good luck and hope this helps.