The lambda sensor, or oxygen sensor, sits in the exhaust before or after the catalytic converter. It measures the oxygen content of the exhaust gases and reports to the engine control unit, which adjusts the air-fuel mixture in real time.
Over time the sensitive tip collects soot, oil deposits, fuel residue and moisture. The sensor responds more slowly: fuel consumption rises, misfires appear, power drops and the Check Engine light comes on with fault codes. Cleaning does not always revive a sensor, but with light contamination it often helps.
What you will need
Important: Only work on a cold engine: the exhaust system reaches several hundred degrees. Never sand or scrape the sensor tip and never wash it in petrol or acids, as this will ruin the sensing element.
Step 1
Locate the lambda sensor
The sensor is screwed into the exhaust pipe near the manifold or catalytic converter, with a wire and connector leading to it. Many cars have two sensors, one before and one after the catalyst. The OBD scanner code usually tells you which one is at fault.
Step 2
Let the engine cool down
Switch off the engine and wait at least 30 minutes, preferably longer. For convenience and safety, disconnect the negative battery terminal.
Step 3
Remove the sensor carefully
Unplug the wiring connector without pulling on the wire. Exhaust threads often seize from heat and road salt, so first apply WD-40 Specialist Penetrant to the base of the sensor and wait 10-15 minutes. The penetrant works into the thread and breaks down rust. Then undo it smoothly with the spanner, without jerking, to avoid stripping the thread.
Step 4
Inspect the tip
Look at the colour and type of deposit. Light grey or brown soot can be cleaned. White or glossy deposits, melting or cracks mean the sensor is poisoned (coolant, additives, lead) or physically damaged, and only replacement will help.
Step 5
Clean the sensor
Spray WD-40 Specialist Contact Cleaner generously onto the tip and give it a few minutes to dissolve the deposits. Brush gently with a soft brush, without pushing any tool into the holes of the protective cap. Repeat 2-3 times while dirty liquid runs off. Flush the connector contacts separately, as they often show corrosion.
Step 6
Dry and refit
Let the sensor air-dry completely; the cleaner evaporates without residue. Screw the sensor in by hand until it seats, then tighten with the spanner without excessive force. Reconnect the plug and battery terminal, clear the codes with a scanner and take a test drive.
Tip: If the sensor soots up again quickly, look for the cause in the engine: high oil consumption, faulty injectors or an air leak. Cleaning the sensor treats the symptom, not the cause.
What next
Inspect the lambda sensor whenever a mixture fault code appears or every 30,000-40,000 km. Check the connector too: moisture and salt corrode the contacts, which WD-40 Specialist Contact Cleaner removes easily. For seized exhaust bolts, keep WD-40 Specialist Penetrant to hand.
These uses come from the experience of WD-40 users in many countries. Always follow the instructions and warnings on the product label.
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