Before I start trying to fix this fault code can some confirm that I should be looking at the back of the car when identifying the Rear Mid-Left sensor.

When selecting reverse I simply get the 3 second beep indicating a fault and no display on the radio to indicate which sensor is faulty

As I have the standard factory fit 4 rear parking sensors I am assuming this is the sensor to the left of centre, or second from the left as looking at the rear of the car.

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Thanks to those who responded to original post.

Having now fully investigated the problem and found a source of reasonably priced sensors I have posted a guide to help identify and replace a faulty sensor - see long post below.

Also posted a How To in the Octavia Technical Guides section

The sensor from the dealer will come unpainted, so will need painting before fitting. Only a very thin layer of paint is on the sensor, so its quiet difficult to paint.

The sensor is is clipped in from behind, so you'll need to get access to the rear of the bumper. Unclip, unplug wire and refit in reverse.

You can hear a working sensor emit a clicking noise when reverse is engaged.

For reference my sensors are wired up in reverse, left is right etc.

Further info if anyone else has a failed sensor and wants replace it themselves ........

If a sensor fails you will only get a 3 second squeal when you select reverse and there will normally be no display on the radio. If you have access to VCDS or someone with it then you can run a scan and the fault code should identify the failed sensor. If you do not have access to VCDS then follow the steps below to identify the failed sensor.

For Mk2 Octavias 2005-2009 the replacement part number on the Sensor is '7H0 919 275' they cost about £125 from a dealer but I bought mine from Ebay

http://www.ebay.co.uk/itm/221151213153?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649

Seller is 'goodparts32' in Poland price was just under £33 delivered. Be careful when buying from Ebay as there are several sellers selling sensors with different part numbers and these are not alternative replacements for the Octavia and will not fit the standard sensor housing

1. As stated in the above post working sensors do emit a faint clicking noise - You do not have to start the car just turn the ignition on and select reverse gear. The clicking is very faint so cup your hand around each sensor and place you ear against your cupped hand and wait for a quiet break in any traffic. VCDS had identified my failed sensor correctly but several people have reported that their sensors appear to have been reversed so VCDS identified the wrong sensor.

2. Once you have identified which sensor it is you should obtain a replacement. To replace it you will need to undo the bumper under-tray (the black under trim - 4 plastic screws with 3mm hex heads) and wedge it down so you can get your arm under the inside of the bumper.

3. The Sensors are fitted into an outer carrier or housing (the outer circle when looking at the sensor from the back of the car) The sensor has to be removed with the carrier as you can not release the clip to release the sensor from it with the sensor in place. You will need to wiggle the sensor assemble sideways and up / down until the adhesive holding the assembly on breaks free - this should not damage the bumper even though you may have to use some force. It might help to have a second person pushing the sensor from the outside. The carrier has two side wings which had the adhesive holding the assembly to the bumper.

4. My Sensor wiring harness was clipped tightly into the rear of the bumper - inside the Black Plastic trim - and this meant that the cable was too short to allow the sensor assembly to drop far enough for me to access the connector on the rear of the sensor and disconnect it. I could not pull the clips off to get more slack in the harness but noticed that the harness itself was simply tied to the clips with black tape, so very carefully I cut the tape to free the harness - do this very carefully as you do not want to cut into the harness itself. This gave me enough slack to drop the sensor to a point where I could unclip the connector.

5. With the sensor and its housing removed simply unclip the sensor from its carrier and fit the new sensor into it

6. It is a good idea to check that the new sensor emits the clicks before attaching it to the bumper, though the clicks are very quiet and it may not be possible to get your ear close enough. In that case insert the sensor assembly into the bumper but do not apply any adhesive tape or glue (see below) to the housing, then listen for the clicks from the rear of the bumper. If all sounds OK ........

7. To permanently reattach the housing into the bumper you will need to apply some thin but very sticky double sided tape to the the wings on the sensor housing or you could use some epoxy type glue to hold the housing in the bumper. I would avoid the epoxy approach as it will make it very difficult to replace the sensor should it fail in the future.