|Code||Fault Location||Probable Cause|
|P0490|| Exhaust gas recirculation (EGA) system -circuit high |
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|Wiring short to positive, EGA valve|
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What Does Code P0490 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.
What are the common causes of code P0490 ?
Some common causes of code P0490 could include the following-
- Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors
- Defective or clogged EGR valve solenoid
- Failed or failing PCM. Note that this is a rare event, and the fault must be sought elsewhere before any control module is replaced
What are the symptoms of code P0490 ?
Some common symptoms of code P0490 could include the following-
- Stored trouble code and possibly an illuminated warning light
- Engine idle may be rough, erratic, or the idling speed may fluctuate wildly
- Fuel consumption may increase dramatically
- Some applications may experience varying degrees of power loss at low engine speeds
- Some applications may experience hard, or no-start conditions
How do you troubleshoot code P0490 ?
NOTE: A pin out chart that clearly indicates the reference voltage terminals on the PCM connector may be of help in diagnosing this code accurately.
Record all fault codes present, as well as all available freeze frame data. This information can be of use should an intermittent fault be diagnosed later on.
Refer to the manual to locate the EGR valve. Also determine the color-coding, and function of each wire in the connector to avoid testing the wrong circuit and possibly causing short circuits later on.
If the engine starts but the idling is rough, erratic, or fluctuates, disconnect the electrical connector on the EGR valve while the engine is running.
If the idling shows no change, the EGR valve pintle is likely stuck in the open position. Remove the EGR from the engine and inspect it for carbon build-up. Note that while it is sometimes possible to clean out carbon deposits, replacing the valve with an OEM replacement is by far the most reliable repair option.
If the idling smoothes out when the connector is removed, there is likely a short circuit to battery positive in the circuit that either keeps the keeps the valve open, or that makes the PCM “think” that the valve is stuck open.
NOTE: if the EGR valve is vacuum operated, inspect all associated vacuum lines for signs of damage, and test all vacuum actuators and check valves for proper operation. Replace any vacuum lines or components that are in a less than perfect condition.
Since design specifics and testing procedures vary between manufacturers, refer to the manual to determine the correct procedure to follow to test the control circuit. Compare all obtained test readings with the values stated in the manual, and make repairs or replace wiring to ensure that all electrical values fall within the ranges specified by the manufacturer.
NOTE: Pay particular attention to the reference voltage and signal circuits, and be sure to test these circuits between the EGR valve connector and the PCM connector, but be sure to disconnect the PCM connector during resistance tests to avoid damaging the PCM. If the reference voltage circuit checks out, refer to the manual or pin-out chart for details on how to check that the PCM actually delivers a 5-volt reference current. If it does not, replace the PCM.
Steps 1 through 4 will resolve code P0490 in nine out of every ten instances. In the unlikely event that the fault persists beyond Step 4, suspect an intermittent fault. However, these types of faults can be extremely challenging to find and repair, so if an intermittent fault is suspected, repeat all steps above until the fault is found and repaired, or refer the vehicle to the dealer or other competent repair facility for professional diagnosis and repair.
Codes Related to P0490
- P0489 – “Exhaust Gas Recirculation “A” Control Circuit Low”