|Trouble Code||Fault Location||Probable Cause|
|P0171||Fuel trim (FT) system too lean, bank 1||Intake/exhaust leak, AIR system, MAFNAF sensor, fuel pressure/pump, injector(s), HO2S|
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What Does Code P0171 Mean?
Special note on trouble code P0171 and Chrysler vehicles: While DTC P0171 is a generic code that affects all OBD II compliant vehicles, the most common causes of this code sometimes vary between most vehicle makes. This article will therefore deal with code P0171 as it applies specifically to Chrysler vehicles.
OBD II fault code P0171 is a generic trouble code that is defined as “System too lean – Bank 1” and is set when the PCM (Powertrain Control Module) detects an air/fuel mixture that is too lean on bank 1. Note that in the context of this code, “system” refers to faults in the air/fuel metering system that cause, or contribute to an imbalance in the ideal ratio between air and fuel in the air/fuel mixture, while “Bank 1” refers to the bank of cylinders on V-type engines that contains cylinder #1. Note also that this code is often accompanied by code P0174 – “System too lean – Bank 2” on V-type engines.
During normal engine operation under closed-loop operation, the PCM uses information about the exhaust gas composition obtained from the upstream oxygen sensor(s) to regulate and maintain the air/fuel mixture at a ratio of 14.7 parts of air to one part of fuel. During this process, the oxygen sensor(s) compares the concentration of oxygen in the exhaust stream with a sample of oxygen in ambient (atmospheric) air and then converts this comparison into an electrical signal that the PCM interprets as the level of oxygen in the exhaust stream.
During steady cruising conditions, the level of oxygen in the exhaust stream typically remains relatively constant, since the demand for fuel does not change much. However, during driving conditions that involve many throttle inputs, the engines’ demand for fuel changes with every throttle input, and since the level of oxygen in the exhaust gas is a direct function of the combustion process, the PCM needs to “know” how this level changes to regulate the air/fuel mixture effectively under all operating conditions.
Thus, as a practical matter, the PCM switches the signal from the upstream oxygen sensors to alternately read rich (too little air in the mixture) to reading lean (too much air in the mixture) multiple times per second to average out the measured oxygen values in the exhaust gas. This has the practical advantage that the PCM does not have to make large adaptations to the fuel trims*, which if it did, would produce large amounts of exhaust emissions since the engine would necessarily run rich for longer periods under some driving conditions. By making small adaptations more often, the ideal air/fuel ratio is maintained more easily, which improves combustion, and reduces fuel consumption and exhaust emissions, at the same time.
* Fuel trims are the adaptations the PCM makes to the injectors’ pulse widths, to increase or reduce the volume of fuel injected into the engine. These changes are based entirely on input data obtained from the upstream oxygen sensors when the fuel delivery system is operating in closed-loop operation, which brings us to-
What causes code P0171 on Chrysler applications?
While there is no single cause of code P0171 on Chrysler vehicles, engine vacuum leaks seem to be the most common. See the section on “Common causes” for more details.
Thus, if a fault, defect, or malfunction occurs that prevents the PCM from adequately compensating for a lean running condition, it will recognize that it cannot control or regulate the air/fuel mixture effectively, and it will set code P0171, and illuminate a warning light as a result.
Where is the P0171 sensor located?
This image shows an example (circled) of the type of engine vacuum leak that can be extremely challenging to find and repair. In this case, the site of the vacuum leak only became apparent after the intake manifold had been removed to inspect the manifold gaskets, which turned out not to be defective.
Thus, to avoid the unnecessary disassembly of major engine components when tracing suspected engine vacuum leaks, we recommend that you employ a smoke machine to detect vacuum leaks before dismantling engine components, which could create more problems if the reassembly is not done to industry standards.
What are the common causes of code P0171?
There are too many Chrysler models produced in different markets globally to list all possible causes of code P0171 here. However, long experience in the automotive repair industry has taught that some common causes of this code on Chrysler applications appear more often than others do, so the common causes of this code are listed here in their order of prevalence, with the causes with the highest prevalence listed at the top of the list-
- Engine vacuum leaks, with the most common leak sites being defective, faulty, or failing PCV (Positive Crankcase Ventilation) valves and/or hoses, leaking of failing intake manifold gaskets/seals, leaking brake booster diaphragms, and EVAP charcoal canister purge valves stuck in the open position
- Failed, failing, or defective upstream oxygen sensors
- Failed or clogged fuel injectors
- Exhaust system leaks
- Insufficient fuel pressure
- Failed, defective, or malfunctioning MAF/MAP sensors
- Failed or failing PCM, but note that since this is a rare event, the fault must be sought elsewhere before any control module is reprogrammed or replaced
What are the symptoms of code P0171?
- Stored trouble code and illuminated warning light
- Depending on the application and the nature of the problem, multiple additional codes may be present along with P0171
- Misfires on one or more cylinders may be present
- The engine may stall unexpectedly at low engine speeds
- The engine may hesitate or stumble upon acceleration
- Fuel consumption may increase substantially
- Idling may be rough, or the idling speed may fluctuate wildly
- Varying degrees of power loss may be present
- Brake operation will be affected in cases where the brake booster diaphragm is leaking
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