Ford B-Max Changing the lambda Sensors

My car has been showing an intermittent P0420 error code for a while now. This indicates poor performance of lambda sensors, catalytic converter or both. The sensors are due for change around 160k km which is about 100k miles. I've only done 75k but given the message I thought Id better investigate. The sensor have never been changed to my knowledge so these seem like the best place to start. I bought lambda sensors from Bosch as both pre-cat (0 258 017 290) and post-cat (258 986 602) sensor versions.

Luckily access to the sensors is pretty straight forward from under the bonnet (pre cat) and from Under the car (raise on ramps) post cat. 

I used "shock and unlock" to ease the sensors and this worked well. 

Both sensors could be loosened with lambda electrode sockets and these worked well. I changed the post cat sensor first, it was badly soiled with carbon and had clearly had a hard life. The new sensor simply screwed in. The guidance is to treat this like a sparking plug so screwed in tight and then nip up 1/4 turn.

Old sensor- wiped off but was initially very black

New  sensor simply screws in

Clip the wire from the new sensor to this clip and pass it upwards to the front of the motor.

Pull the old sensor up to compare lengths. Its necessary to cut the old sensor off the connector (hidden somewhere deeper in the engine bay) and connect the new one using the fancy connector supplied by Bosch. There is a table to guide you with the 4 wire connections, but luckily in my case the new sensor (2 white wires, a blue and a grey) matched the existing wiring perfectly so it was simply a case of connecting like to like.

Wiring from new sensor- before cutting off the old.

I dont know if the sensor wires are always live or not- but I didnt want to risk it so disconnected the battery earth strap before cutting. Note put the key in the ignition and open the doors and wind down a window before disconnecting the battery in case you lock yourself out of the car. The key must be in the ign to prevent the alarm from sounding.

Disconnect and isolate the earthing strap

Then cut the old sensor off and separate the strands inside the heat resistant sheath

Bosch supply a new connector to join the old wiring to the new. Its extremely fiddly and almost impossible to use. I have no idea whether resistance in this wire is critical or not so although I'd prefer to use a crimped multiplug, I thought I'd better stick to the connector supplied. Even so I didnt measure the length of wire which might also be critical- I'll need to check that. I just lined up the old and new wires and snipped through both at a convenient point before separating the strands. The connector isnt intended to come apart again and simply jams the wires into a plastic screw cylinder and then encases all 4 cylinders into  a junction block. Its a dreadful design and makes fitting it neatly almost impossible. I have learned since that as its impossible to cut and slide the heat resistant sleeve to just the correct length, its a good idea to slip a length of heat shrink over the cables each side of the junction. Then make the connections and fit as best you can, before sliding the heat shrink up flush against the connection block and shrinking it to make a neat join... I'll do that next time!

However despite the raggedness of the join, the new sensor did work and I noticed an immediate smoothing of the motor tone. In particular, and to my great relief, the loud tapping that was occurring at revs below 1k disappeared and the motor was quieter and seems to have more power. 

I reset the engine warning light although sadly this re-illuminated after some 50 miles so although an improvement, the post cat sensor alone is clearly not the only problem.

Precat Sensor

Unlike the postcatalyst sensor the precat comes with a plug already connected. Although its not visible immediately, I can say now that this matches the plug on the car so swapping should be a simple problem. However, as usual Ford have got there before you and decided to make all the wires just an inch or two too short and thus a job that would be simple if you could see it becomes a 20 minute fumbling task to be achieved by touch alone!

To have any hope at all you must remove the air intake pipe... fortunately a simple job with 2 "rotate-to-lock" jubilee clips.

The new sensor comes prefitted with a connection clip. This one is complicated it has 2 physical anchor prongs as well as the electrical connections.

Note the the lug on the same side (bottom) of the conector as the anchor studs and furthest from my hand in this pic. Most/all modern car electrical multi-pin connectors feature some form of latch or lock to keep them fastened once connected. These are the bane of my life, and its frequently impossible to see what needs to be pushed, pulled, pressed or twisted to release the lock. All clips are different and there is NEVER any explanation as to how to disconnect them. This lug is a clue. Tracing the lead from the front sensor to the connection block shows that connection is made somewhere down here... you can feel it but you can barely see it. Run your hand down to the connector and whilst gripping the plastic block, pull it upwards to pull the 2 physical latch prongs out of their housing on the motor. The connector then has a little more freedom but is still impossible to see. The latch identified above is on the same side as the anchors so working by feel twist the connector until these are uppermost. Use a longish flat-bladed screwdriver to feed down to the connector, align it on the tab and press inwards on the motor side of the connector whilst pulling the plug apart. I couldn't photo this in position, but once the plug was diconnected I could show the engine-side connector to which the new sensor must attach. Note the broad plastic tab next to the wires entering the connection.

This (just above the thumb in the picture) is what you need to press inwards in order to detach the plug.

Carefully feed the new sensor connecting wire through the clips along the same path as the old. Use cable ties to secure it where necessary. Align the anchor plugs on the new sensor with the side of the engine connector bearing the latch as above and press together until they click. Then if necessary rotate the connector so that the anchors point downwards towards the road, and press them firmly into their locations on  the side of the motor. I could never see these locating holes and did the job by feel.

You can now remove the old sensor. 

Count the turns as it unscrews... I found that to be 6 turns anticlockwise. Wind the new connector 6 turns anticlock so that it unwinds the flex as you screw it in 6 turns clockwise

Tighten with a lambda sensor socket and nip up an extra 1/4 of a turn. Refit the air intake hose. I once more reset the error codes and we will see if two matched sensors are sufficient to avoid future error code recurrence.