How to replace a lambda sensor

So your car has just failed the MOT due to a high Lambda reading and you need to replace the sensor. You could pay a garage a lump sum to do the job for you but it’s a lot more cost effective (and satisfying) to do the job yourself. This is something I had to do recently and I’m going to teach you to do the same.


You will need the following:

  • Jack
  • Axle stands
  • 22mm spanner (size may vary)
  • Hammer
  • Protective eye ware
  • Copper grease
  • Replacement lambda sensor (I used universal 4 wire, see below for more info)

Optional equipment:

  • Penetrating spray
  • Soldering iron
  • Lighter (for heat shrink)

Lambda Sensor

Some cars have just one lambda, some have two. If your car has two lambdas, the front sensor (pre-cat) will be the one affecting emissions, the rear is to ensure the catalytic converter is doing it’s job.

You have two options here, you can either go for an OEM lambda sensor which will be a like for like replacement of your faulty one, or spend half the money going for a universal sensor. Being a hands-on kind of guy, and also struggling to find a replacement sensor in time, I went with the universal option which requires a little more work.

If you are following my footsteps and also using a universal lambda, make sure you buy the correct sensor. Lambda sensors come in various forms, 2 wire, 4 wire, 5 wire etc. A similar one to mine can be found over on eBay.

Remove the old lambda

To get started, jack your car up from the front and secure it with axle stands then locate the lambda sensor. Before getting under the car, make sure you find some protective eye ware, I’ve lost count of the number of times I’ve had crap fall into my eyes whilst being under the car!

Due to the amount of salt and rain that hits our roads in the winter months, there’s a good chance you’ll be fighting your fair share of corrosion. To make life easier, spray the knackered sensor with penetrating fluid and leave it to soak for a few minutes.

The first thing to do is unplug the sensor, in most cases you can simply follow the wire until you find the plug. If the wire goes through the floor pan, be prepared to pull up the carpet to get to the connector.

Now take your 22mm spanner (sizes may vary, mine was 22mm) and hammer, and do your best to loosen the old sensor without rounding off the nut. I was lucky, a heavy hit and the sensor came off with ease, cross your fingers and hope it’s the same for you.

If you’ve found a like for like replacement, you can skip to the final fitting stage. If you’ve bought a universal sensor, please read on.

Prepare the new sensor

My replacement universal sensor had 4 wires, two for the heater, signal to ECU and a ground, pictured below.

Next you need to cut the wires to length. On my sensor, I decided to keep the plug and floor bung together and cut the old sensor off halfway down the wire. Your sensor may differ and instead you could just cut the plug off leaving just enough wire free to solder. Before cutting, make sure your new cable has enough length to reach where it needs to go.

You’ll find your old sensor cable is covered in a protective rubber sheath, cut a length of this off, this will be used to protect the bare wires you’ve just exposed. Ensure the rubber sheath is long enough before cutting.

Solder the plug back on

Your next step is to prepare the wires ready for soldering. Firstly you need to put the sheath in place. Grease up one wire (either the sensor or plug end) and slide the sheath over the top until the wires are exposed.

Strip the ends off each wire and apply heat shrink to one set of wires.

Now you need to work out which wire goes where. On the universal sensor, they were as follows:

  • Black – Signal to ECU
  • White x 2 – Heater (these are not polarity sensitive and can be connected either way round)
  • Grey – Ground

If the colours of your old sensor match, then that’s most likely how they’ll pair. On my original lambda, I had the following wires:

  • Blue – Signal to ECU
  • White – Ground
  • Black x 2 – Heater

Therefore I connected:

  • Blue to Black
  • White to Grey
  • Black to White
  • Black to White

If you are unsure, the best way to work out which wire goes where is to use a multimeter.

Firstly, discard the two matching colours from your plug, these will be the heater wires. Next, check for continuity between each wire and a chassis ground. Whichever has continuity is the ground, the other is the signal.

If you are still unsure, browse around an owners club for your car, the topic has most likely been covered before.

Next, solder up the wires. I won’t go in to how to solder, that is way beyond the scope of this how-to.

Cover the joins with heat shrink and use a lighter to shrink into place.

Now you need to slide the rubber sheath you prepared earlier over the bare wires. This will add some protection to the newly made up cable.

That’s all there is to it. You’ve made your own lambda sensor for a fraction of the price of buying a new one. All that’s left to do is fit it to your exhaust.

Fit the new sensor

Fitting is simply a case of reversing what you’ve already done. Before doing so, copper grease is a must, this way when you need to take the sensor off next, you won’t find it seized. Grease the thread of the sensor, don’t be afraid to use too much, just try not to get any on the actual sensor.

Screw the sensor into the exhaust and tighten with the spanner. Jack the car back down and you’re done.

I bet that was easier than you expected, wasn’t it? Now get back to the MOT garage and pass your retest!

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There are 6 comments

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  1. khizar_07

    The original sensors can last for over 10 years doing over 100k miles. The after market sensors will probably only last 50-60K miles. If your car is over 10 years old and only has another 5 to go then its worth saving several hounded pounds for the two sensors!

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