What causes the air fuel ratio to be lean

• DTC P0171 Fuel Trim System Lean Bank 1
• DTC P0174 Fuel Trim System Lean Bank 2
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The engine control module (ECM) controls the air/fuel metering system in order to provide the best possible combination of driveability, fuel economy, and emission control. Fuel delivery is controlled differently during Open and Closed Loop. During Open Loop, the ECM determines fuel delivery based on sensor signals without oxygen sensor (O2S) input. During Closed Loop, the O2S inputs are added and used by the ECM to calculate short and long term fuel trim (FT) fuel delivery adjustments. If the O2S indicates a lean condition, FT values will be above 0 percent. If the O2S indicate a rich condition, FT values will be below 0 percent. Short term FT values change rapidly in response to the heated oxygen sensor (HO2S) voltage signals. Long term FT makes coarse adjustments in order to maintain an air/fuel ratio of 14.7:1. If the ECM detects an excessively lean condition, DTC P0171 or P0174 sets.
Diagnostic Aids
• Excessive resistance in the fuel injector ignition 1 voltage circuit or the fuel injector control circuit may cause the following symptoms:
- Lean condition
- Misfire
- Rough idle
• The system will go lean if an injector is not suppling enough fuel.
• A lean condition could be present during high fuel demand.
• A malfunctioning MAF sensor can cause this DTC to set. Refer to DTC P0101 .
• Review the Failure Records with a scan. If an intermittent condition is suspected, refer to Intermittent Conditions .

DTC Descriptors
This diagnostic procedure supports the following DTCs:
• DTC P0420 Catalyst System Low Efficiency Bank 1
• DTC P0430 Catalyst System Low Efficiency Bank 2

This code could mean the catalytic converter on that bank has failed .

Causes of lean condition trouble codes P0171 P0174 GM 5 3L

P0420 P0430 catalytic converter catalyst inefficiency code diagnosis

1.6 GLi COROLLA

DTC NUMBER TROUBLE AREA 12 RPM signal 13 RPM signal 14 IGN signal 16 ECT Signal 21 Oxygen sensor signal 22 Water temperature signal 24 Intake air temperature sensor 25 Air / Fuel ratio lean 31 Vacuum sensor signal 41 Throttle sensor 42 Vehicle speed sensor 43 Starter signal 52 Knock sensor 51 Switch COND

Engine Code 25, Air-Fuel Ratio Lean Malfunction.

it could be ther are many causes for this code and you must check for vacumm leaks.

I will try to paste an article for you. Several sources indicate this is a Bank 2 O2 sensor problem and that it is failing to switch and adjust for the fuel mix. But as you can see, many defects can lead up to this O2 sensor Code 1150.

I would try to use a spray bottle of water and go around the Intake manifold and spray the water while the engine is running. An intake leak will draw water into the engine and change RPM.

If no success then try changing the O2 bank 2 sensor. You may need to wipe the computer and clear the Code. H40 DTCs P1131, P1132, P1151 AND P1152: UPSTREAM HO2S NOT SWITCHING. DTC P1130: FUEL SYSTEM NOT SWITCHING AT FUEL TRIM (RICH OR LEAN)
Diagnostic Trouble Codes (DTCs) P1131 bank 1 (Cylinder 1) and P1151 bank 2 indicate the fuel/air ratio is correcting rich for an overly lean condition. The HO2S voltage is less than 0.45 volt. DTCs P1132 bank 1 (Cylinder 1) and P1152 bank 2 indicate the fuel/air ratio is correcting lean for an overly rich condition. The HO2S voltage is greater than 0.45 volt.
DTC P1130 and P1150 indicate the fuel control system has reached maximum compensation for a lean or rich condition and the HO2S is not switching.
DTC/HO2S Reference List
•HO2S-11 = DTCs P1131, P1132 and P1130
•HO2S-21 = DTCs P1151, P1152 and P1150
Possible causes:
Fuel system
•Excessive fuel pressure.
•Leaking fuel injector(s).
•Leaking fuel pressure regulator.
•Low fuel pressure or running out of fuel.
•Contaminated fuel injector(s)
•Vapor recovery system
Induction system
•Air leaks after the MAF.
•Vacuum leaks (vacuum lines and gaskets).
•Restricted air inlet.
•PCV system.
•Improperly seated engine oil dipstick.
EGR System
•Leaking gasket.
•Stuck open EGR valve.
•Leaking diaphragm.
Base engine
•Oil overfill.
•Cam timing.
•Cylinder compression.
•Exhaust leaks before or near the HO2Ss.
•Check intake air system for leaks, obstructions and damage.
•Check air cleaner element, air cleaner housing for blockage.
•Verify integrity of the PCV system.
•Check for vacuum leaks. )

Dont clean your throttle body yet..........Theres an idle air control valve{ black round top about the size of a 50cent piece} on the back of the throttle body, cleaners will ruin it. It mounts with 2 screws facing the back of the motor. This is what controls your idle.........

DTC P0133Circuit Description


After achieving closed loop fuel control, a commanded air fuel ratio of 14.7:1, RPM between 1,500-3,200, and air flow between 7-25 grams per second, the PCM will continuously monitor the front oxygen sensor signal for 100 seconds. During this time, the PCM counts the number of rich to lean and lean to rich switches. To be considered a switch, the voltage must go below 300 mv and above 600 mv. The PCM also keeps track of the time it takes to make the switch. The number of switches along with the time to achieve these switches are used to calculate the average response time to perform each switch. Also this information is used to calculate if the proper ratio of the switches is correct. The 3 diagnostic trouble codes listed below are used to identify different types of faults that can occur with oxygen sensors.

Conditions for Setting the DTC

DTC P0133
This DTC is set if the average response time of the switches from the sensor is too slow after the 100 second test is performed. If the lean to rich time is greater than 125 ms or the rich to lean time is greater than 156 ms, this code will set.

Diagnostic Aids
Repair Procedure
The oxygen sensor wiring and connections should be checked if any of these DTCs are set because corrosion or loose connections could cause these DTCs to set.
The most likely cause of setting any of these DTCs is contamination. This contamination could be caused by fuel, improper use of RTV sealant, engine oil, or coolant contamination.
The source of contamination should be identified and corrected before replacing the sensor.

You may have poor plug wires, or even a low compression cylinder. Also, make sure the air filter is clean. Your oxygen sensor (upstream) the one closest to the engine has detected a rich condition.

H40 DTCs P1131, P1132, P1151 AND P1152: UPSTREAM HO2S NOT SWITCHING. DTC P1130: FUEL SYSTEM NOT SWITCHING AT FUEL TRIM (RICH OR LEAN)

Diagnostic Trouble Codes (DTCs) P1131 bank 1 (Cylinder 1) and P1151 bank 2 indicate the fuel/air ratio is correcting rich for an overly lean condition. The HO2S voltage is less than 0.45 volt. DTCs P1132 bank 1 (Cylinder 1) and P1152 bank 2 indicate the fuel/air ratio is correcting lean for an overly rich condition. The HO2S voltage is greater than 0.45 volt.
DTC P1130 and P1150 indicate the fuel control system has reached maximum compensation for a lean or rich condition and the HO2S is not switching.
DTC/HO2S Reference List

• HO2S-11 = DTCs P1131, P1132 and P1130
• HO2S-21 = DTCs P1151, P1152 and P1150
Possible causes:
Fuel system

• Excessive fuel pressure.
• Leaking fuel injector(s).
• Leaking fuel pressure regulator.
• Low fuel pressure or running out of fuel.
• Contaminated fuel injector(s)
• Vapor recovery system
Induction system

• Air leaks after the MAF.
• Vacuum leaks (vacuum lines and gaskets).
• Restricted air inlet.
• PCV system.
• Improperly seated engine oil dipstick.
EGR System

• Leaking gasket.
• Stuck open EGR valve.
• Leaking diaphragm.
Base engine

• Oil overfill.
• Cam timing.
• Cylinder compression.
• Exhaust leaks before or near the HO2Ss.
• Check intake air system for leaks, obstructions and damage.
• Check air cleaner element, air cleaner housing for blockage.
• Verify integrity of the PCV system.
• Check for vacuum leaks.

P0171 - Fuel Injection System Too Lean Bank 1
Possible Causes:
- Intake air leaks
- Front Heated oxygen sensor may be faulty
- Injectors may be faulty
- Exhaust gas leaks
- Incorrect fuel pressure
- Lack of fuel
- Mass air flow sensor may be faulty
- Incorrect PCV hose connection

P0174 - Fuel Injection System Too Lean Bank 2
Possible Causes:
- Intake air leaks
- Front Heated oxygen sensor may be faulty
- Injectors may be faulty
- Exhaust gas leaks
- Incorrect fuel pressure
- Lack of fuel
- Mass air flow sensor may be faulty
- Incorrect PCV hose connection

What Causes These Codes: Fuel injection system does not operate properly. The amount of mixture ratio compensation is too large. (The mixture ratio is too rich.) The Powertrain Control Module (PCM) monitors the engine Air/Fuel mixture ratio, the actual mixture ratio can be brought closely to the theoretical mixture ratio based on the mixture ratio feedback signal from the heated oxygen sensors 1. The PCM calculates the necessary compensation to correct the offset between the actual and the theoretical ratios. In case the amount of the compensation value is extremely large (The actual mixture ratio is too lean.), the PCM judges the condition as the fuel injection system malfunction and light up the engine light. The codes is usually set when there is an intake leak after the air flow meter, before replacing any parts check for loose or broken hoses and intake gasket leak. On Ford some models dirty air filter, and dirty or faulty air flow sensor are the cause of the problem.

P0051 DTC trouble code may be caused by one or more of the following: A short in the heater circuit in the sensor, A failed O2 sensor heater, Wiring/connectors broken/frayed leading to sensor and/or relay, or a failed PCM/ECM Most likely fix is to replace the air/fuel (O2 oxygen) sensor #1 on bank 2 (the side of the engine that doesn't contain cylinder #1).

this code sets when air fuel ratio feedback is stable after engine warm up,and fuel trim is considerably in error on lean side;The signal from air fuel ratio sensor is approximatelyproportional to existing air-fuel ratio.

for this code suspect excessive lean condition,normal rangefor total trim is between -35% to +35%,DTC uses "two-trip detection Logic".

Check system for intake leaks,incorrect fuel pressure,plugged fuel injectors,O2 sensor or coolant sensor operation.

check air-fuel ratio sensors
to do it you need to check them with a good scanner tool.

also,check MAF sensor operation.

personally i would clean with the special spray for MAF sensor (you can get it from you autoparts store) and try a fuel injector cleaner before start spending money in shops.

hope this info can help you.