The lambda sensor is mounted in the exhaust of cars with a combustion engine. It continuously measures how much oxygen is left in the exhaust gases so that the correct ratio of fuel and oxygen can be calculated. Like all parts in cars, this one sometimes breaks. Can you fix this yourself? Can you do without it? And how does it work?
The lambda probe
Cars with a combustion engine convert the energy from petrol, diesel or LPG into motion through combustion. Oxygen is always needed during combustion. The ratio between fuel and oxygen must be good to get as much energy as possible from the fuel: too little oxygen can lead to the waste of unburnt fuel and too much oxygen can lead to too high temperatures in the engine. That is why a lambda sensor has been placed in the exhaust right after the engine block. This sensor measures how much oxygen is left in the exhaust gases. It can often be seen quickly under the hood: the metal tube that is mounted in the exhaust manifold with the wire on the other side.
Many petrol engines have a second sensor placed after the catalytic converter. After all, the catalytic converter changes the chemical composition of the exhaust gases, this second sensor functions, among other things, to monitor the catalytic converter’s performance. To spot these you will have to look under the car.
Operation
The lambda probe contains an element of zirconium dioxide, a kind of rusted zirconium. As the name suggests, there are more than enough oxygen atoms in this element. This element is in direct contact with the exhaust gases in the exhaust on one side and with ‘normal’ air on the other. When the car is in operation, the element is heated with an electric heater and by the exhaust gases. From about 300 degrees Celsius, the zirconium dioxide becomes conductive for oxygen ions. This means that oxygen ions can transport from one side to the other. And of course this happens especially when there is little oxygen in the exhaust gas, because then the difference in oxygen between both sides of the element is greatest. Since this concerns an ion current, a voltage can be measured across the two ends of the element (approximately 0.9 V). If there is a lot of oxygen in the exhaust gas, the difference is much smaller and the voltage will also be much lower (about 0.1 V).
Replace yourself?
Step 1: Diagnosis
Is the engine management indicator lamp on the instrument panel on? This can have several causes. Many cars can display an error code through the dashboard. It is worthwhile to find this out for your car so that it can be determined whether the lambda sensor is defective. It is also important which one is broken, which is before or after the catalytic converter.
If you have a reader, diagnosis is even easier, such devices are sometimes available for a few bucks at Chinese electronics wholesalers. The lambda sensors can be read while driving. The intention is that after starting the motor, the voltage varies between 0.1 V and 0.9 V within a few minutes. If the probe stays at 0 V or around 5 V, it is probably faulty. If the probe only comes within this voltage after kilometers of driving, the heater is probably broken, the lambda probe will only work when the exhaust has reached the right temperature.
Step 2: material and tools
The lambda sensor is fairly easy to replace yourself. For this you need the following parts.
- Wrenches (usually key 22), preferably a torque wrench
- Side cutters (to cut some threads / tie wraps)
- Lambda sensor (when purchasing, make sure that the connector and the resistance of the heater match, price varies from 50 euros to 300 euros)
- Possibly a metal tube to extend the arm of the key
- When it comes to the probe after the catalytic converter: axle stands
Step 3: disassembly
Use a wrench to loosen the lambda sensor (anti-clockwise, anti-clockwise). Rust and other influences can cause the lambda sensor to get stuck. Treating with some penetrating oil can help. Also make sure that the wrench fits properly, an adjustable wrench (baco) is not always the best choice. It can also be useful to cut the wire and place a ring spanner around it so that the bolt is not turned over. Running the engine before disassembly is also a useful way to loosen the probe. The metal alloy of the exhaust is different from that of the probe, this ensures that the screw thread is somewhat ‘wider’. Of course, make sure everything is very warm, so be careful!
Follow the wire to the nearest connector and disconnect it. In the case of the lambda sensor after the catalytic converter: jack up the car and fit the axle supports. Make sure the car is jacked up on both sides (left and right) and that the handbrake is on. Always watch out when working under the car!
Step 4: assembly
As with almost all parts of the car: always check the replaced part. With the lambda probe it is useful to measure the resistance of the heater. Do this by going through all the combinations of the four wires with the multimeter. If all connections are high impedance, the heater in the lambda probe was probably broken. A value between 2 and 20 ohms is normal. Check whether the new lambda sensor has a comparable value. Also check the corrosion of the electrodes in the connector, as this can also be the problem.
Screw the new lambda probe into the exhaust (clockwise, clockwise). It is recommended to keep the tightening torque of the manufacturer, usually around 50 Nm. If you do not have a torque wrench, use the following rule of thumb: 50 Nm equals 200 N at 25 cm, which is about 20 kg of force on a 25 cm wrench.
Plug in the connector and tidy up the wire in the same way as with the old probe. It may be necessary to clear the error codes, please consult the internet on how this can be done for the specific car. Start the engine and see if the engine management indicator light has gone out.
Drive on?
Especially if the first lambda sensor, directly in the exhaust manifold, is defective, the engine will go into ’emergency running’. This means that the engine receives a pre-adjusted amount of fuel and air and this is therefore no longer properly regulated. In the long term, this is less good for the engine and the car drives less efficiently. Removing the lambda sensor and then driving is strongly discouraged, as there is a hole in the exhaust.