P0171 Diagnostic Theory for Shops and Technicians HCs (Hydrocarbons): Unburned droplets of raw fuel that smell, affect breathing, and contribute to smog NOX (Oxides of Nitrogen): One of the two ingredients that, when exposed to sunlight, cause smog The vehicle may have trouble idling, especially when warm or when sitting at a stoplightĬommon Problems That Trigger the P0171 CodeĬontrol module software needs to be updated Performance problems, such as a lack of power on acceleration and some "coughing" or ![]() ![]() Lean Mixture = Too much air, not enough fuel Rich Mixture = Too much fuel, not enough air When the P0171 code sets, the oxygen sensors are detecting too little oxygen in the exhaust (running "lean") and the control module is adding more fuel than normal to sustain the proper air/fuel mixture. ![]() When these adjustments become too large, a fault code is set. Measures the oxygen content in the exhaust with oxygen sensors and makes adjustments to the mixture by injecting more or less fuel as necessary. Combustion engines function best when they maintain a specific ratio of air to fuel (about 14.7 parts air to 1 part fuel) in the fuel mixture.
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