This fault code keeps popping up after few fixing attempts:

17524 - Oxygen (Lambda) Sensor Heating: B1 S1: Open Circuit

P1116 - 35-00 - -

This is the pre catalysator wide band lambda (5-wire Bosch) which controls the fuel mixture.

First I found one cut wire from the cable harness side of the lambda sensor connector. However this was not in the the heater circuit, grey and white pin number 3 & 4.

I soldered this loose cable and reset fault codes. It did not help as I excpected. This loose cable had propably caused bad idling and inconsistent acceleration earlier this year. Those problems cleared out after few months. I assume that this faulty cable snapped completely off when those idle and acceleration problems vanished. Engine propably got into some kind of fall back mode ignoring missing lambda readings. It is odd that there was not any fault code related to that cut off cable, only this one about open heating circuit..

Now this snapped off wire is fixed. I continued to pursue that problem related to fault code. I got replacing lambda sensor from my friend and tried that. It didn't solve the problem still that code popping up right after clearing. Usually that dashboard warning light won't come up until second time I start engine again. It's odd that it won't lid up after first start up..

I checked resistance of my friend's lambda before installing it. It gave 3.2 ohms which indicates that it is ok. After I replaced the lambda I cheched this presumably faulty one and it was also 3.2 ohms. So the problems is not in the heater element.

Next step is still open and now I would need some precious knowledge of yours. I should go through the cable harness and see if there is some discontinuations between lambda and control unit. Problem is that I haven't found any information about where are those cables coming out of that cable harness.

Is it usual that this kind of fault is in the control unit? If it is in there I guess there won't be cheap fix for that? Where is this control unit located? Is it in that aluminum ECU box fixed in the firewall? If you have any thoughts about how to tackle this problem I would appreciate it a much.

Thanks, that helps.

T121 is probably some big connector going to ECU? At least now I can try to find that big connector somewhere in the engine bay.. and now I know which pin it is in that connector.

Can you see from that software were is the E29 harness side wire coming out of the harness?

What is that software and is it easily obtainable somewhere? Looks quite handy..

T 121 is the main connector on the ECU

Here are the earth points

Close up of #29

This info is from a program called EslaWin, it's basically a copy of the dealer manuals, available on ebay around £20~30

Very big thanks,

Of course that E stands for earth.. I thought that wire would go to chassis ground but I just didn't figure out that E marking. Now it seems very easy to check those cables as I can see from the pictures exactly where they are going.

I really hope that fault is in the cables, not in the ECU. Does anybody know how common it is to have fault of this type in the ECU?

Is there any chance that it could be some blown fuse? My number plate lights started to work again just removing and inserting the fuse. After that I removed and inserted every fuse in hope of that heater circuit would start to work like number plate lights did. It didn't work. I would like to check that specific fuse again if there is any. I guess it is some of those general engine control fuses that are not specified in details. At least in the fuse card..

I should really buy some of those more detailed manuals for my car -> No need for guessing how things work.

Very big thanks,

Of course that E stands for earth.. I thought that wire would go to chassis ground but I just didn't figure out that E marking. Now it seems very easy to check those cables as I can see from the pictures exactly where they are going.

Is there any chance that it could be some blown fuse? My number plate lights started to work again just removing and inserting the fuse. After that I removed and inserted every fuse in hope of that heater circuit would start to work like number plate lights did. It didn't work. I would like to check that specific fuse again if there is any. I guess it is some of those general engine control fuses that are not specified in details. At least in the fuse card..

I fell into that assumption that E was the earth connection, it's not it's a power supply connection from fuse #9 10amp in the fusebox inside the car, via a 6 pin Brown connector pin 6( C on the first diag) located @ 10 on the second pic, the earth path for the heater elements is through the ECU thats how it is able to detect the current flow through the heating element and hence put up the faultcode, The main earth connection for the ECU is #29 as in the previous posting

If I understood right this bl/ro wire going to the E29 in the schematics is actually high side wire which is routed through fuse number 9. So now I'm trying to put all of this together and it goes like this: 12V goes via fuse #9 which is wired to socket C pin #6. mating connector of the socket C leads that current from the pin #6 to the bl/ro wire which ends to lambda sensor socket pin #3. Low side of the heater element goes through pin #4 of the lambda sensor socket and eventually to the pin #121 of the ECU unit. From the ECU current sinks to common ground point E29.

This would make sense as the heater current is PWM regulated and the most common way to design PWM switching is to use low side N-FET or NPN transistor as they have better characteristics than high side counter parts.

I still found very confusing that E29 is earth point and still it is somehow pundled together with the heater pl/ro wire which is a high side wire.

BTW as you mentioned this Elsawin I look for it from Ebay and found it under 10£ P&P included. 1+3 DVDs, most of the VW production from 1994 to 2012.

So now I'm trying to put all of this together and it goes like this: 12V goes via fuse #9 which is wired to socket C pin #6. mating connector of the socket C leads that current from the pin #6 to the bl/ro wire which ends to lambda sensor socket pin #3. Low side of the heater element goes through pin #4 of the lambda sensor socket and eventually to the pin #121 of the ECU unit. From the ECU current sinks to common ground point E29.

That's the way I also read it, the low side of the heater going to pin 4 of the 121 pin ECU connector

As you see the ECU's main ground is made at Earth point 29 by the battery, as seen in the previous location pictures

I still found very confusing that E29 is earth point and still it is somehow pundled together with the heater pl/ro wire which is a high side wire.

Yes that also confused me! to be honnest these diagrams do make me wonder at VW logic!! Like the box with the number 103 coming from the heaters never appears on any other diagram, but by tying up the diagram for the fuses and what fuse is for what, along with the wire gauge and colours, then it makes sense

As you can see box 103 now becomes box 63 ????

That elsawin sounds correct, one disc is for the basic operating program & VW the others are for the other makes, Audi, Seat & Skoda