Understanding Error Code P0490: EGR Control Circuit “A” – High Value
Introduction
- What does the P0490 code mean? (General definition: high voltage value detected in the EGR valve “A” control circuit).
- This is a generic powertrain code (OBD-II), applicable to all makes and models from 1996 onwards.
- Important: Specific troubleshooting steps vary depending on the vehicle.
- These engine fault codes indicate an electrical malfunction of the EGR (Exhaust Gas Recirculation) system.
- Essential role of EGR: reduce the formation of harmful nitrogen oxides (Nox) in the cylinders.
- EGR operation: controlled by the engine management computer (PCM/ECU) which opens or closes the valve based on load, speed, and temperature to maintain an optimal cylinder head temperature.
- Key components:
- Electric solenoid (usually two wires) for valve activation.
- Potentiometer integrated into the solenoid that signals the computer the position of the EGR pintle (opening/closing of the passage).
- Analogy with a light dimmer: the computer attempts to modulate the voltage to control the EGR opening/closing.
- The P0490 code indicates that the computer does not detect a corresponding high voltage change when attempting to open or close the EGR, suggesting the valve is potentially stuck in a position.
- Distinction from code P0489: Fundamentally identical, but indicates a low circuit instead of a high circuit.
- Importance of EGR for:
- Reducing Nox emissions.
- More advanced ignition timing for more power without knocking.
- A leaner air-fuel mixture for better fuel economy.
Possible Symptoms of Code P0490
- Symptoms vary depending on the position of the EGR pintle at the time of failure:
- Engine running extremely rough
- Check Engine light illuminated
- Decreased fuel economy
- Loss of power
- No start or very hard start followed by rough idle
Potential Causes of Code P0490
- List of possible causes:
- Short to ground
- Short to battery voltage
- Bad connector with pushed-out pins
- Corrosion in the connector
- Clogged EGR pintle
- Faulty EGR solenoid
- Defective EGR valve
- Faulty ECU or computer
Repair Procedures for Code P0490
- Step 1: Warranty and TSB Check:
- If the vehicle has less than 160,000 km (100,000 miles), check the warranty (often 130,000 to 160,000 km for emission controls).
- Search online for relevant Technical Service Bulletins (TSB) for this code and your vehicle.
- Tools needed for diagnosis:
- Voltmeter / Ohmmeter (multimeter)
- EGR system wiring diagram specific to your vehicle
- Jumper wire
- Two paperclips or thin sewing needles
- Step 2: EGR Mechanical Blockage Test (Stall Test):
- Open the hood and start the engine.
- If the engine is idling, disconnect the EGR valve connector.
- If the engine idle improves, the EGR pintle is likely stuck in the open position.
- Stop the engine and replace the EGR valve.
- Step 3: Visual Inspection of Connector and Wiring:
- Carefully examine the EGR valve connector (usually 5 wires).
- Identify the power (battery voltage) and ground wires (usually the two outer wires).
- The three center wires are related to the EGR position potentiometer (including a 5-volt reference terminal).
- Look for pushed-out pins, corrosion, or bent pins in the connector.
- Inspect the wiring harness for any damaged insulation or potential short circuit.
- Look for broken wires that could cause an open circuit.
- Step 4: Testing EGR Valve Power Supply:
- With the voltmeter, probe one of the end terminals of the EGR connector (red wire) and place the black wire to the vehicle ground.
- Turn the ignition on (without starting the engine) and check for the presence of 12 volts on both end terminals (power and ground).
- If no voltage is displayed, there is a break in the power wire between the EGR and the ignition circuit.
- If 12 volts are present on only one side (the power), the EGR valve may have an internal open circuit. Replace the EGR.
- Step 5: Testing EGR Valve Activation with a Jumper:
- Remove the EGR valve connector.
- Turn the ignition on (engine off). Probe the two outer terminals to identify the power supply (12 volts).
- Place a paperclip in the end terminal that did not have power (this is the ground terminal).
- Attach the jumper wire to the paperclip and place the other end of the jumper to the vehicle ground.
- You should hear an audible “click” when the EGR valve is activated.
- Disconnect the jumper from ground and start the engine.
- Reconnect the jumper to ground. The engine idle should become rough when the EGR is powered and stabilize when the ground is removed.
- If the EGR clicked and affected the engine idle, the EGR valve is likely functional and the problem is electrical. Otherwise, stop the engine and replace the EGR.
- Step 6: Testing the EGR Potentiometer Reference Circuit:
- Probe the center terminal of the EGR connector with the voltmeter (red wire). Place the black wire to the vehicle ground.
- Turn the ignition on (engine off). You should measure approximately 5.0 volts if the computer is working correctly. Turn the ignition off.
- Step 7: Testing the Reference Circuit at the ECU/PCM:
- Use the EGR wiring diagram to locate the “EGR Reference Voltage” terminal on the computer (ECU/PCM) connector.
- Insert a fine pin or paperclip into the computer connector at that terminal to probe it.
- Turn the ignition on. If 5 volts are present, the computer is likely good and the problem is in the wiring harness between the computer and the EGR.
- If no voltage is present, the computer could be faulty.
- Alternative Solution (Workaround) for a Faulty Reference Circuit at the ECU/PCM:
- Consult the wiring diagram and locate the reference voltage terminal for the engine coolant temperature sensor.
- Probe this terminal with the ignition on. If a 5-volt reference voltage is present, turn the ignition off and mark the two reference terminals used (EGR and coolant temperature).
- Disconnect the computer connector and solder a jumper wire between these two reference terminals.
- Reinstall the connector. The EGR should function normally without replacing the computer. Note: This is a temporary repair and it is recommended to diagnose and repair the root cause of the ECU problem if possible.