[Mk2] [Turbo] Oxygen Sensor Differences - Error code 21

[killerferret666]

The Gen 2 / Gen 3 use different oxygen sensors, in terms of 1 is 1 pin and the other 4 pin (2 heating wires)

when I got my engine (gen2) which had a Gen 3 wiring conversion it still only had one wire for the oxygen sensor, I don't know how but it did and confirmed that wire went to the OX pin on the ECU. So used the Sensor already in the exhaust.

Do the Gen 2 / Gen 3 ECU's expect a different input signal?? value and hence I'm getting the above error. I suspect its always been there, its always ran rich.

Or do the oxygen sensors give out different readings?

If I were to switch to a later Oxygen sensor, It looks like I don't need the heater element to it (its just for warm up) and can just ground the 2nd pin and connect the single wire as expected.

[killerferret666]

After a lot of investigation I have the answers.

The sensors work the same...however the ECU is expectign a heating element to the Sensor and will throw the error 21 if it thinks the heating circuit is open or short circuited.

so having a 1 pin Oxygen sensor means its always been running rich
I know why this wasn't picked up before and that's because the engine is a forged one and they were probably running a wideband with the larger turbo etc...

The good news it isn't too hard to fix and change to the 4 wire one

Black wire - already sends signal to ECU

Wire the 3 below new wires if cant be found;
Earth wire - Brown on the normal loom so can splice in to a brown wire
Signal to ECU - Red / White, there is a HT input into the ECU, I will have to trace this wire back and splice on to that if required
Yellow / Black - Splice on to any point of the Yellow and black.

this should then solve this problem

[killerferret666]

Easier solution implemented rather then all that wiring.

I found that my cabling didn't even have a Pin in the ECU connector for the Lambda sensor heater wire, so I got a pin from an old plug and then soldered a 6k Ohm Resister + a 1.5k Ohm resistor in series to it.

I then put this in the correct position in the plug and connected the other end to a Black / Yellow wire via splice.

Reset ECU, took it out for a drive...

Now I no longer get an o2 Sensor error, problem solved and the car runs much better as well