P0171 is a lean code for bank one and P0173 is a lean code for bank 2 which is odd since a 4 cylinder only has bank one. So I would start by checking for a leaky intake. Check boost, clean MAF.
What causes the P0173 code?
A dirty or faulty mass air flow sensor.
Leaking intake manifold or torn intake boot.
Vacuum leak
Faulty fuel injector
Faulty fuel pressure regulator
Pinched fuel line
What are the symptoms of the P0173 code?
A Check Engine Light
Increased smog or soot from the exhaust
Loss in acceleration
Hesitation or misfire during idle
Increased fuel consumption
How does a mechanic diagnose the P0173 code?
Visually inspects the mass air flow sensor
Tests the resistance of the mass air flow sensor and circuitry with a digital multimeter
Performs a fuel pressure test
Inspects the wiring of the fuel injectors for fraying
Inspects the intake manifold and vacuum lines for leaks
Uses a scan tool to specify the fuel trim out of range (short-term or long-term)
Common mistakes when diagnosing the P0173 code
Replacing the Mass Air Flow Sensor without cleaning or testing it first
Deeming a component faulty without checking for more common causes, such as vacuum leaks
Not using a scan tool to determine if there is an issue with either the short- or long-term fuel trim
Failing to inspect the wiring of the component thought to be at fault
How serious is the P0173 code?
The life of the catalytic convertor could become shorter because it will have to filter more pollutants in the exhaust gases
Gas consumption will increase
