What does it mean?
OBD II fault code P0471 is a generic code that is defined as “Exhaust Pressure Sensor “A” Circuit Range/Performance”, and is set when the PCM (Powertrain Control Module) detects an implausible signal from the exhaust pressure sensor that is labeled “A”, or a signal from this sensor that does not match the desired value when the ignition is switched on, and is therefore out of the expected or possible range.
In the context of “key on” values, this refers to the fact that the signal voltage from this sensor must match the signal voltages from the manifold pressure and intake air temperature sensors, even though there are no actual pressures to measure in the intake manifold or exhaust system beyond the current ambient atmospheric pressure.
Note that this code applies to all applications (both diesel and gasoline) that are fitted with variable nozzle/geometry turbochargers, since exhaust backpressure has major influence on how well (or otherwise) these types of turbochargers work. Note also that the exhaust backpressure sensor should not be confused with the DPFE sensor, which performs entirely different function.
While a certain amount of exhaust backpressure is desirable because it assists with extracting exhaust gas from the engine, excessive backpressure causes a plethora of issues that range from excessive fuel consumption, loss of power under some conditions, engine overheating, and even damage to turbochargers.
To prevent these and other complications, exhaust backpressure sensors are used to monitor the actual exhaust backpressure, which must agree closely with the desired backpressure value for a signal to be valid. In terms of operation, most exhaust backpressure sensors are supplied with a 5-volt reference voltage that changes as the resistance of the sensor changes in response to changes in the actual backpressure in the exhaust system.
As a practical matter, the PCM interprets the changing signal voltage values as pressure values that serve several purposes. In the case of diesel applications, the backpressure value serves as a measure of the efficiency of the diesel particulate filter, while on other applications, the actual exhaust backpressure is one of the reference points the PCM uses to calculate appropriate boost pressures, as well as adaptations to fuel and ignition timing strategies.
When the PCM detects an abnormal or out-of-range signal from the exhaust backpressure sensor, it will recognize that it cannot control one or more critical engine and/or boost control functions effectively, and it will set code P0471, and illuminate a warning light as a result.
Note that along with P0471, the PCM may also set one or more (or several) turbocharger, DPF ( D iesel P articulate F ilter), catalytic converter, and exhaust emission related codes, some of which may illuminate dedicated warning lights.