I ran fault codes on my 2002 Mazda Protege. It says fuel too lean bank 1. I have had my fuel pump check, and that's not the problem.... Any ideas?

This code means 1 banks is running to lean on fuel, might be caused by a air leak or a faulty MAF sensor.

code p0171 refers to system to lean bank 1==causes intake /exhaust leak--AIR system--MAP/MAF sensor--fuel pressure/pump--injectors--HO2s Have a fuel pressure test done on the fuel manifold to check for blocked filter/faulty pump/fuel pressure regulator faults
code p0174 refers to system to lean bank 2 --same causes including hose connection/s
code p0420 refers to catalytic converter system -bank 1 efficiency below threshold==causes catalytic converter--wiring--HO23. Check the cat converter is not cracked inside and the oxygen sensor is working.

Hi Paul, Have you had a fault code relating to the knock sensor? That sensor controls ignition timing which can have a relationship to the fault codes you report? Regards John

Here's a link to your P0171 code.

Not the fuel filter

More like a vacuum leak

It would help if you could get the car scanned with vag-com, it will show the full fault code.

P0171/ Fuel Trim; Bank 1: System Too Lean
Possible Causes

* Mass Air Flow Sensor defective

Possible Solutions

* Check Mass Air Flow Sensor
* Check Fuel Pressure Regulation
* Check Fuel Pump
* Check Injectors
* Check Intake System for Leaks
* Check Exhaust System for Leaks
* Check Secondary Air System for Leaks
* Check Vacuum Pipes for Leaks

YOU COULD HAVE AIR LEAKS IN THE INTAKE MANIFOLD OR EXHAUST MANIFOLD.RESTRICTED FUEL FILTER AND FUEL INJECTORS.CHECK FOR BROKEN OR LOOSE VACUUM HOSES.FUEL PRESSURE REGULATOR FAILED.

it means your fuel and air mixture is not correct could be a oxygen sensor or a semi clogged fuel injector lots different things could be try running fuel injector cleaner through see if it helps but really neeed more symptoms

I AM LOOKING AT STOP UP FUEL FILTER OR FAULT IN THE PRESSURE REGULATOR SOLENOID VALVE.

Trouble Code: P0421


Catalyst Efficiency Below Normal (Bank 1)

Possible Causes:

Air leaks at the exhaust manifold or in the exhaust pipes
Catalytic converter is contaminated, damaged or has failed

Front HO2S and/or the rear HO2S is loose in the mounting hole

Front HO2S older (aged) than the rear HO2S (HO2S is lazy)

Front HO2S or rear HO2S is contaminated with fuel or moisture


Trouble Code: P1173

HO2S-21 (Bank 2 Sensor 1) Circuit Malfunction

Possible Causes:

Base engine mechanical fault affecting one or more cylinders

EVAP Purge valve stuck or the hoses to the valve are reversed

Fuel metering fault (fuel injector sticking open or leaking)

Fuel pressure too low or too high, fuel supply contaminated

HO2S element is contaminated or the HO2S heater has failed

PCV system has air leaks at the valve or related hoses



Trouble Code: P0174
Adaptive Fuel Trim Too Lean (Bank 2)

Possible Causes:

Vehicle driven low on fuel or until it ran out of fuel

One or more injectors restricted or pressure regulator has failed

Fuel delivery system supplying too much or too little fuel during cruise or idle periods (e.g., faulty fuel pump, or dirty fuel filter)

Air leaks after the MAF sensor, or air leaks in the PCV system

Air leaks at the EGR gasket, or at the EGR valve diaphragm

Exhaust leaks before or near where the front HO2S is mounted

HO2S is contaminated, deteriorated or it has failed

Fuel control sensor is out of calibration (i.e., ECT, IAT or MAP)

Base engine -mechanical- fault affecting one or more cylinders