How to change your Subaru Impreza knock sensor

The tyres been fixed (turned out it was a little screw causing all the headaches – thanks to Jax Albury for sorting it out in lightning time) and while I was on the way to the tire shop, Repco called me to tell me the knock sensor was in a day early! Time to pop this into the Bax the Subaru Impreza, clear the fault codes and enjoy some normal motoring for a time!


Think of this Subaru knock sensor as a small microphone, bolted to the engine block listening intently for anything going wrong and sending possible warnings to the ECU if it picks up any unwanted noises. The ECU in turn reduces performance (and sadly fuel economy) until whatever is causing the issue isn’t heard again. However this happens when your knock sensor is on the way out too – it can send the wrong information (or no information at all which the ecu questions) and when that happens, it’s replacement time.

My diagnostic test gave me error 2-2 (22), Google tells me it’s the knock sensor. The sad state of the one in there also suggests it’s definitely time to go, hence this new one.

Subaru knock sensor

Now I’ve gone with an aftermarket version from Goss here for two reasons – one it was half the price of the unit at the dealership and two, while Subaru’s can be a bit finicky when using aftermarket sensors apparently knock sensors aren’t an issue. At $101 at the time of writing I’m sure there’s eBay specials far cheaper from lesser known brands but as long as this works, I’ll be happy (and so will my fuel tank.)

(Shopping in the US? Here’s a handy Amazon link to find something that fits –Subaru Knock Sensors Affiliate link)

Goss Subaru Knock Sensor


-Firstly take the negative cable off your battery. This is to reset your ECU and clear any old fault codes. Not much point in driving around with old fault codes still stored in the car. Also take a photo of the old one, especially the way it’s positioned.

-The way it sits, you’re going to need a socket (12mm here) and an extension. Luckily you can fit it through the throttle cables easily.

  • Unbolt the old one, make sure the bolt doesn’t go off anywhere it shouldn’t, then disconnect the electrical plug (push down on the tab and pull the other half out)

Subaru knock sensor

  • When installing the new one, it’s a lot easier to put the bolt in and hand tighten (see below) and then plug in the electrical part. It’s a little fiddly and hopefully your hands are small enough.

  • The important bits Make sure it’s aligned the same way the old one was, it needs to be set up at correct angle which is why you took a pic of it before you removed the old Subaru knock sensor. This is how they work, stick to what’s there. Equally as important, if not more, don’t tighten it more than hand tight. Or you run the risk of cracking it and it looking like my old one and then you’re back ordering another one..

  • All done? Angled in the original position and clipped into the electrical connector? Time to pop the negative cable back on the battery and test things out.



After you reset the ECU (either negative cable off battery and go and do something else for at least five minutes or negative cable off battery, depress brake pedal to clear excess energy, cable back on) expect your Subi to want to stall when you start things up again. This is the car trying to relearn the idle and is completely normal. It’ll take a little time to remember factory settings so keep the revs up for a bit (over 2k) and then go for an uninterrupted drive around a few corners. I did 2.5kms around a few blocks just to make certain and get things back to operating temp. Before long you’ll have your idle back where it should be and your Subaru rumbling like it usually does.

Subaru ecu reset

(Don’t do what I did and that was reset the ECU over night and then drive to work without it set properly the next morning, in the dark. Give yourself some time to get it back to where it was) 



After the test drive and the ECU no longer playing funny buggers, turn the car off, plug the two black plugs under your dash together, ignition to on and watch what your check engine light is doing.  What you’re hoping to see is a series of constant flashes on and off and on and off (with no long flashes). Or 1-1-1-1-1-1. That’s the all clear you’re looking for and your Subaru knock sensor issues all sorted!

(Turn the car off, unplug the plugs and away you go!)

Impreza check engine light

So no more errors, air in the tire and the coolant seems to be staying where it should. Chat to you next time I discover/fix something on this wagon!

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