P0406 Code – What Does It Mean & How To Fix It

This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to fix it, and what other codes may show related to it.

Definition

OBDII code P0406 is defined as a vehicle having an issue with Exhaust Gas Recirculation Sensor A Circuit High.

Meaning

A P0406 trouble code means that there is something wrong with the exhaust gas recirculation system (also known as ‘EGR’). This particular system is designed to lower combustion temperatures under certain conditions, thereby reducing NOx emissions.

This system is able to do this by recirculating inert exhaust gasses back into the cylinder to be burned with a mixture of air and fuel. Inert gas burns slower than other types of gas and lowers the temperature needed for combustion. The EGR valve is controlled by a computer and opens so that the exhaust gas can flow into the cylinders.

Causes

There are a few primary causes in any vehicle that can create a P0406 trouble code. Most of these issues are related to errors with or involving the EGR. The EGR sensor signal circuit could be shorted to a B+ battery voltage.

This can also be caused by an internal failure on the EGR sensor or solenoid. It is also possible that debris is caught in the valve in a way that is holding it open or closed.

Symptoms

There are some common symptoms that you should pay attention to if you suspect a P0406 trouble code may be present. Any of these may be indicative of the P0406 trouble code.

There could be an increase in combustion temperatures and/NOx emissions. The malfunction indicator lamp (MIL) may also be illuminated. You may feel that the vehicle is surging while you are driving it, and the vehicle may stall intermittently.

Diagnosis

There is a set of steps that you should follow in properly diagnosing this issue. First, you should check to see if there are any other codes that could be related to the P0406 trouble code. Make sure that you clear your check engine light with FIXD. Take a look at the freeze frame data for the associated temperature of your vehicle and load at the time the code is being set.

Make sure you conduct a visual inspection of the EGR sensor circuit for disconnected and/or shorted wiring. You may also need to remove any carbon buildup within the EGR valve. This is what may be preventing it from closing or opening properly. If the problem seems to persist, you may need to consider replacing the EGR valve.

Common mistakes

The most common mistake includes prematurely replacing the EGR valve without visually inspecting to make sure the EGR sensor circuit isn’t disconnected.

How serious is this?

Fortunately, this is not a trouble code that will prevent you from operating your vehicle. Though, code P0406 may include a variety of vehicle symptoms that could prevent your vehicle from running normally or make your vehicle more difficult to drive. If you notice these symptoms or this code should persist, you should be able to drive it to your nearest dealership.

What repairs can fix the code?

First, you need to perform a general check. Check the wire harness for obvious damage, like chafing and burns, as these both can cause circuit problems. Check for bent or broken pins, corrosion, or evidence of water entry. Repair as necessary and make sure your connectors are properly seated.

Perform an EGR Position Sensor Check. Most EGR position sensors are the three-pin type: reference voltage (Vref), ground, and signal. In the case of our Toyota EWD, these are VC, E2, and EGLS, respectively.

If the EGR valve is vacuum-actuated, you can bench-test the position sensor with a DMM and vacuum-pump. Disconnect the connector and measure across the entire circuit (VC-E2), and you should measure something like 6 kΩ (6,000 Ω). On the signal wire, measuring between VC and EGLS, you should measure somewhere between 0.1 kΩ and 5.5 kΩ (100 Ω and 5,500 Ω), depending on how much vacuum you apply to the actuator port.

If the EGR is electronically-actuated or you want to check output voltages, back probe the connector with your DMM and measure the voltage with KOEO (key on engine off). VC should measure somewhere between 4.5 V and 5.5 V and E2 should measure 0 V. ELGS should measure somewhere between 0.3 V and 4.2 V, which may vary, depending on the valve’s actual position. You can test the voltage output of vacuum-operated valves by varying how much vacuum you apply with the hand pump.

If you measure 5 V on E2, you most likely also measure 5 V on EGLS, which means you have an open circuit back to the ECM on the E2 ground circuit. Skip to Circuit Check to diagnose and repair.

If you measure 5 V on VC and EGLS, but 0 V on E2, you may have an internal or external short circuit. Disconnect the sensor and check for resistance between VC and EGLS on the sensor itself. If you show less than 100 Ω, then you have a short-circuited sensor. In that event, you need to replace it.

If you show between 100 Ω and 5,500 Ω, you may have a short circuit in the wire harness. You can skip to the circuit check to verify and repair it.

If you measure 0 V on E2 and over 4.5 V on EGLS, then you have a fault in the sensor itself. Replace the sensor.

Make sure to check the circuits. Disconnect the ECM and EGR position sensor and measure the resistance on the three wires. Each wire, VC, EGLS, and E2, should measure 0 Ω end to end. Additionally, there should be no connection between any of the three wires or to ground. You should measure at least 10 kΩ between VC, EGLS, E2, and body ground. Repair as necessary.

Related codes

  • P0403: Exhaust Gas Recirculation (EGR) Control Circuit
  • P0404: Exhaust Gas Recirculation (EGR) Control Circuit Range/Performance
  • P0405: Exhaust Gas Recirculation (EGR) Sensor “A” Circuit Low
  • P0407: Exhaust Gas Recirculation (EGR) Sensor “B” Circuit Low
  • P0408: Exhaust Gas Recirculation (EGR) Sensor “B” Circuit High
  • P0409: Exhaust Gas Recirculation (EGR) Sensor “A” Circuit
  • P0486: Exhaust Gas Recirculation (EGR) Sensor “B” Circuit
  • P0489: Exhaust Gas Recirculation (EGR) Control Circuit Low
  • P0490: Exhaust Gas Recirculation (EGR) Control Circuit High

Conclusion

Although this code should not prevent you from driving and operating your vehicle, we recommend that you verify this code and have the error fixed by getting your vehicle to a qualified mechanic as soon as possible.