P0420 = Catalyst System Efficiency Below Threshold

Some suggested steps for troubleshooting and fixing a 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

P0102 = Mass Air Flow MAF Circuit Low

Potential causes for this trouble code may include: Dirty or contaminated mass air flow sensor Failed MAF sensor Intake air leaks MAF sensor electrical harness or wiring problem (open, shorted, frayed, poor connection, etc.)

Visually inspect all MAF sensor wiring and connectors to make sure they are intact, not frayed, broken, routed too close to ignition wires/coils, relays, motors, etc.
Visually inspect for any obvious air leaks in the air intake system Visually *closely* inspect the MAF sensor wires or film to see if you can see contamination such as dirt, dust, oil, etc.
If the air filter is dirty, replace it with a new original equipment filter.
Carefully clean the MAF using MAF cleaner spray is generally a good DIY friendly diagnostic/repair step If the air intake system has a mesh in it, make sure that is also clean.
Loss of vacuum to the MAP sensor can trigger this DTC
A low minimum air rate through the sensor bore may cause this DTC to set at idle or during deceleration. Inspect for any vacuum leaks downstream of the MAF sensor.
Use a scan tool to monitor real-time sensor values from the MAF sensor, O2 sensors, etc.
Check for Technical Service Bulletins (TSBs) for your particular make/model in case of known issues on your vehicle.
The barometric pressure (BARO) that is used in order to calculate the predicted MAF value is initially based on the MAP sensor at key ON. A high resistance on the ground circuit of the MAP sensor can cause this DTC to set

P0113 = IAT Sensor 1 Circuit High Input

A code P0113 may mean that one or more of the following has happened:
Internally failed IAT sensor Faulty connection at IAT sensor.
Open in IAT ground circuit or signal circuit.
Short to voltage in IAT signal circuit or reference circuit.
IAT harness and/or wiring routed too close to high-voltage wiring (e.g. alternator, spark plug cables, etc.) Faulty PCM (less likely but not impossible)