System Too Lean – Bank 1
How does a P0171 code trigger the check engine light? This code will trigger the check engine light as follows:
The adaptive fuel strategy in the vehicle’s computer constantly monitors the fuel delivery system to make sure the engine is running at an optimum air to fuel ratio, which is 14.7:1. The computer adjusts injector pulse width to regulate the amount of fuel going into the engine. The oxygen sensors relay information to the Powertrain Control Module (computer), informing it of the oxygen content in the exhaust. This information is translated by the computer, and used to determine if more or less fuel is needed. The computer will then adjust fuel flow (and possibly other related engine operating characteristics), to keep the correct air fuel mixture. This loop continues as long as the engine is running. Please refer to our article Oxygen Sensor Code Diagnosis for more info on this system. A P0171 check engine light code is set when the computer has reached a rich calibration limit and can not add enough fuel to maintain the correct mixture.
Refer to our Oxygen Sensor Code Diagnosis article for a list of all possible causes. The following is an overview of P0171 check engine light code possible causes.
Leaking or faulty fuel pressure regulator
Plugged or dirty fuel filter or lines
Fuel pump weak or defective check valve
Injectors leaking or faulty
Low fuel pressure or running out of fuel
Leaking EVAP system components
Faulty FRP (Fuel Rail Pressure Sensor)
Air Intake System
Contaminated, damaged or faulty Mass Air Flow sensor
PCV valve leak or stuck open
Air induction turbulance caused by wrong filter
Oil dipstick not seated
Air leaks after the Mass Air Flow Sensor (MAF)
Oil coated aftermarket air filter
Any exhaust leak before or near the oxygen sensors
Vacuum line disconnected from EGR System Module (ESM)
EGR valve, tube or gasket leak
EGR vacuum regulator valve leaking
Secondary Air Injection System
Mechanically stuck secondary air injection valve
Intake Cam Position Actuator Open Circuit – Bank 2
How does a P0020 code trigger the check engine light? This code will trigger the check engine light under the following condition:
The comprehensive component monitor, referred to as the CCM, monitors the Variable Camshaft Timing (VCT) circuit to the computer for high and low voltages. If the voltage falls below a set limit for a specific amount of time during this test, the check engine light will come on and set this trouble code.
Open or short in the VCT circuit
Open in the Vehicle Power circuit
Open or short in the VCT solenoid valve itself
Damaged computer (PCM)
This check engine light code is caused by a circuit fault of the VCT system. Testing should include all related wiring circuits, the VCT solenoid and PCM itself. Please refer to out article on Automotive Circuit Testing for help with this. You may also use the “Get Help” link if you need specifications or have any other related questions. Remember to refer to an appropriate manual for specific instructions.