Exhaust gas recirculation - (EGR) solenoid valve

The purpose of this test is to evaluate the correct operation of the Exhaust Gas Recirculation solenoid valve (EGR valve) based on the switching voltage and frequency during engine hot run conditions.

How to perform the test

Plug a BNC test lead into Channel A on the PicoScope, place a black clip on the test lead with the black moulding (negative) and a Back-pinning Probe or multimeter probe onto the test lead with the red moulding (positive). Place the black clip onto the battery negative terminal and probe the exhaust gas recirculation solenoid valve's connections with the Back-pinning Probe or multimeter probe as illustrated in Figure 1. If you cannot reach the terminal or plug with a probe, then you may be able to use a breakout box or lead if you have one available.

The valve will have two electrical connections:

  1. 12 V supply
  2. switched earth

(Note that there will be 12 V on both terminals until the right conditions are meet to switch the valve.)
The valve will also have a vacuum supply and a vacuum connection to the EGR valve.

The electronic solenoid is activated by the switching the earth path to ground under specified conditions. This is controlled by the Engine Control Module (ECM). The vehicle may have to be roadtested in order to simulate the correct conditions.

Example waveform

Waveform notes

The purpose of Exhaust Gas Recirculation (EGR) is to recycle a small amount of the exhaust gas back into the induction process to reduce oxides of nitrogen (NOx). NOx is produced when combustion temperatures are high, often associated with lean burn engines. By recycling a small amount of exhaust gas, the combustion charge temperature is lowered and there is a reduction in NOx. The EGR solenoid is controlled by the Electronic Control Module (ECM) and also works in association with other devices that monitor the amount of gas that is recycled. This set up is often different for each manufacturer and there is normally a combination of vacuum and electrical solenoid valves.

EGR operation takes place under very specific conditions. The ECM controls the earth path to the solenoid valve. The information that the ECM requires for this operation is: engine temperature, vehicle speed and engine load. With such precise data required it will only be possible to see the activation of the EGR solenoid valve whilst the vehicle is on road test.

Technical information

The function of Exhaust Gas Recirculation (EGR) is to lower the oxides of nitrogen (NOx) under certain circumstances. As the internal combustion temperature rises, the nitrogen within the air/fuel mixture starts to oxidise causing NOx to be produced. While this burning is less than desired, it is inevitable as the air/fuel ratio is increased and a weaker mixture is ignited.

The NOx output is at a maximum when the engine has reached its normal operating temperature and the vehicle is subjected to light throttle or light load conditions.

The catalytic converter is designed to eradicate the majority of the NOx by neutralising it when it comes into contact with the precious metal rhodium, but by reducing the NOx before it reaches the catalytic converter ensures even lower outputs. The EGR valve allows a small amount of the exhaust gases to 'bleed' back into the inlet manifold to lower the combustion temperature and reduce the chances of the nitrogen burning. The EGR valve is a small mechanical device that allows the passage of exhaust gas when it receives a vacuum supply.

This supply is governed by a vacuum switch which in turn is activated by a signal from the Electronic Control Module (ECM). NOx, like hydrocarbons, is measured in parts per million and the reading recorded in a workshop environment is significantly lower to that recorded when the vehicle is at cruise.

Figure 2 show a diagram of an typical EGR set up, with the EGR valve in the closed position.

EGR taken to excess can affect combustion and increase hydrocarbons. It is therefore necessary to monitor the amount of exhaust gas that enters the inlet manifold. Different manufacturers perform this task in different ways, and some general examples of this are described below.

Honda use an ECM containing a programmed map. The map contains information on the correct amount of EGR according to factors such as: engine speed, road speed, temperature and load.

Under the right conditions for EGR to take place, the ECM earths the path of the solenoid valve and this allows a vacuum source to operate the EGR valve. The EGR valve also includes a lift sensor, a similar device to a throttle potentiometer. It has a 5-volt supply, earth and variable signal back to the ECM depending on the position of the EGR valve. If the amount of exhaust gas passing through the valve exceeds the parameters in the ECM's map, the ECM shuts the solenoid valve by removing its earth path. This making and breaking, or 'pulsing', of the earth path allows fine adjustments to be made ensuring precise control of EGR.

GM/Vauxhall/Opel have a similar system, but the solenoid valve, lift sensor and the EGR valve are all one unit (as shown in Figure 3). Fault-finding is also made harder by the fact that EGR takes places inside the cylinder head through a passage connecting the exhaust and inlet manifolds.

Ford, as always, have some interesting names and acronyms for the components within their EGR system. To start with, the control solenoid is referred to as an Electronic Vacuum Regulator (EVR) and their method of monitoring the amount of EGR is by a Differential Pressure Feedback Electronic System (DPFE). The DPFE (as shown in Figure 4) sensor measures the pressure inside the EGR tube either side of a restriction (venturi). This pressure difference is then converted into a voltage and sent to the ECM for reference. Again the ECM contains a map for the correct amount of EGR and if this differs the ECM adjusts the control of the EVR to trim the amount of gas passing to the inlet manifold.

Circuit diagrams for the Ford and GM/Vauxhall/Opel systems can be found in Figure 5 and Figure 6.


This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.

Suitable accessories

  • Back-pinning Probe Set


  • Flexible Back-pinning Probe


  • Multimeter Probes


  • PicoScope Battery Clip


  • Large Dolphin/Gator Clips


  • Premium Test Lead: BNC to 4 mm, 3 m


  • Premium Test Leads: Set of four leads 3 m (TA125 - TA128)


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Guided test: EGR Solenoid Valve