Did a bunch of searching online for this when mine broke and couldn’t really find anything that related to the 4 cylinder E46 BMWs. Got a good steer from the good folk over at Bimmerforums so thought I’d share the full process in the hope it saves you the same journey.
I have a 2000 BMW 316i SE 4 cylinder. The engine light came on and I checked the error codes using my Peake R5/FCX-3 reader. This plugs in under the dash in the ODB2 port (or in the engine bay on pre 2001 cars with an adapter) and came up with a single code ‘A6’ which reads in German as “Periodendauer Lambdasonde vor Kat” … and translates as “PreCat oxygen sensor”. In normal speak, the exhaust sensor just before your catalytic convertor is faulty. Thinking about it, the engine did seem a bit sluggish and sometimes stuttered at low revs when parking and would sometimes ‘dip’ during initial acceleration, especially from low revs or up a hill. Some form or fuelling / management problem would explain all of that.
The oxygen sensor (also known as a lambda sensor) tells the engine management how to alter the fuelling based on what gases are currently coming out of the engine. It can sense too much unburnt fuel and then reduce the intake mixture and vice versa.
4 cylinder cars (316 and 318 models)
There is a sensor before the catalytic converter (pre cat) and after (post cat) on the 4 cylinder BMWs. They are located under the car and can be clearly seen slicking out of the exhaust pipe either side of the catalytic converter. This is what the rest of this article is about.
6 cylinder cars – Bonus description!
Just as a side note, on the 6 cylinder engines, there are two pre cat sensors. They are located directly on the exhaust manifolds as they exit the engine block. There is 1 sensor for each bank of 3 cylinders. These banks are either referred to as Bank 1 and Bank 2 or Cyl 1-3 and Cyl 4-6. The sensors are tucked down the side of the engine and need a special tool to remove them. There are plenty of YouTube videos about replacing these sensors though.
You can find cheaper versions for around £45 but I went for the OEM Bosch part number 11781247406 at around £85 off Ebay. You want to be searching for something like “BMW e46 318 pre cat sensor” but then check the description for compatible models and years. If possible, contact the seller to make sure as these are notoriously easy to get the slightly wrong part / connector. Click here to search for the sensor via Google.
Jack the car up on the front right corner jacking point. The standard BMW jack will do. Go as high as you can then place a secondary support under the car to act as a backup if anything goes wrong. I used a pile of bricks with a small piece of wood on top. You’ll be working under the car, so being super-safe is critical.
Locate the sensor. Looking under the car from the left side, it looks like this. Note the jack on the far side of the car. This is BEFORE I added the backup support.
Give the sensor a good spray of WD40 and leave for a few mins. Then just unscrew the sensor. You’ll be doing this from below so think about the direction a little before. With the spanner on the sensor, pull towards the front of the car. In the image from step 2, the unscrew direction is anti-clockwise – as normal. It may take a bit of a tug though as it’s been expanding and shrinking a whole load over time.
Unclip the wire and carefully pull the connector out of its retaining clip too. You will probably need two hands to disconnect the connector. I needed a small flat-headed screwdriver to initially prise the connector tabs apart so I could then pull the two halves apart.
Not really a step but here’s the old and new sensor next to each other. Note the grease on the thread of the new sensor. It came like this but if yours didn’t, but a dab of copper grease or similar on the threads before you install.
Screw in the new sensor. I had a bit of a mare locating the thread when trying to do it one handed and from an angle. After some huffing and puffing, I crawled further under the car and used both hands. Went in straight away. Strange. Tighten it up and reconnect the connector, seating the cable in the clips as you go.
When I started the car, the engine light was still on and the Peake reader reported the same fault. Most modern cars need to be run through 3 full cycles for it to confirm that the fix has cleared the fault. Read more here. I’m currently doing this but the initial pickup when accelerating was noticeably stronger, so I’m happy it has done the job.
Hope that helps!