What does it mean?
A stored code P0488 means that the powertrain control module (PCM) has detected a discrepancy between the actual position of the exhaust gas recirculation (EGR) valve and the desired throttle position. The PCM uses input signals from the throttle position sensor (as well as the vehicle speed sensor and other engine sensors) to determine the desired percentage of EGR valve activation at any given time and under a particular circumstance.
There are two basic types of EGR valves; linear and vacuum diaphragm. Both types are controlled by the PCM in different ways. The linear EGR valve is opened using one or more servo motors that are directly attached to the EGR pendulum. The PCM not only controls EGR valve function, it also monitors manifold absolute pressure (MAP) and exhaust oxygen content to determine whether or not the EGR is functioning properly.
Some EGR valves are also equipped with valve position sensors which reflect (to the PCM) the percentage in which the valve is actually opened. The vacuum diaphragm EGR valve is also controlled by the PCM but engine vacuum is used for opening. Typically, an in-line (intake vacuum) vacuum control solenoid is provided a voltage signal when the engine is running.
When the PCM deems that EGR operation is feasible, a fluctuating ground is applied to the solenoid; causing it to open and allow a percentage of engine vacuum to reach the EGR valve. The EGR valve should open accordingly and the PCM looks for MAP changes that reflect such. The typical TPS is a potentiometer that is moved along with the throttle plate. A five-volt reference circuit is completed by the TPS.
As the throttle plate is opened, sensor resistance decreases, allowing circuit voltage to increase. The PCM recognizes these variations in TPS voltage as changes in throttle position. TPS signal voltage normally ranges from .5-volts at idle to 4.5-volts at wide-open throttle. As the PCM monitors changes in MAP and exhaust oxygen content (in relation to the EGR system), data is compared to the throttle position sensor.
Variations between perceived EGR system performance and TPS position, over a set period of time, will result in a stored code P0488 and possibly an illuminated malfunction indicator lamp.