What does it mean?
OBD II fault code P0490 is a generic code that is defined as “Exhaust gas recirculation (EGR) system –“A” circuit high” and is set when the PCM (Powertrain Control Module) detects an abnormally high voltage in the EGR (Exhaust Gas Recirculation) valve’s control system. Note that while failure of the EGR valve itself can cause this code to set, it is more common for purely electrical issues to cause this code to set.
The purpose of the EGR system is to introduce a precisely controlled volume of exhaust gas into the cylinders to quench the detonation temperature of the air/fuel mixture to below the temperature at which harmful oxides of nitrogen (Nox) forms. Control of the introduced exhaust gas is accomplished by means of a valve that is controlled by the PCM.
When conditions such as engine temperature, throttle position, engine speed, and others are met, the PCM commands the EGR valve to open by a set amount, which introduces exhaust gas into the cylinders. However, since the introduction of exhaust gas affects combustion negatively, the PCM uses input data from the EGR valves’ position sensor to monitor the rate of exhaust gas flow.
When the PCM determines (on the basis of input data from various engine sensors), that the flow rate of exhaust gas is affecting power output beyond a set limit, it will close the EGR valve, using input data from the position sensor in the EGR valve to verify that the valve is closing, or is closed. Note that typically, the PCM will not open the EGR valve during idling, low engine speeds or under hard acceleration on gasoline engines in order to maintain peak engine performance at all times.
In practical terms, code P0490 refers to a situation where the EGR valve is either partially open, (hence the high voltage in the control circuit), or if a short circuit to battery positive is present that makes the PCM “think” that the EGR valve is open when it should be closed. Thus, when an unexpected high-voltage condition occurs in the EGR valve’s control circuit, the PCM will set code P0490 and may also illuminate a warning light.
The image below shows a typical EGR valve in which carbon build-up is preventing the valve pintle from closing fully, which can also cause P0490 to set. Note that applications with VVT/VCT (Variable Valve Timing/Variable Camshaft Timing) system do not have EGR systems, since variable valve.cam timing produces the same quenching effect on combustion temperatures.