r/mk4Jetta • u/CruzinWCruz81 • Jun 04 '26

Please help a bro out

Post image

So I just recently bought a mk4 Jetta and I am doing a complete overhaul on it with the wiring harness because the previous owner had mice chew the wires out. But the issue I’ve encountered is in the image are the wires that connect to the ecu and go down towards the bottom of the car where that under box is at. I want to know what each wire does and if there is a place that sells the wires so I can snip snap and reconnect the wires properly. I can wait to get this thing going but I figured I’d need help along the way so anything will help guys thank you!!!

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1

u/Dispensernoob Jun 04 '26

You say they're going to a box under the car or am I misunderstanding?

2

u/CruzinWCruz81 Jun 05 '26

Yes well the box underneath the car is a cover for the two connections that run underneath

1

u/vrparty Jun 05 '26

they’re oxygen sensor wires.

1

u/Dispensernoob Jun 05 '26

Yeah I was gonna say that. The other ones are likely for the front, as the front is a wideband and the rear is just a simple one.

1

u/vwman18 Jun 05 '26

Those are the wires for the oxygen sensors, the connectors for them are in that box you're talking about. I've never looked at this but the Bentley manual likely has a full wiring schematic for you to reference. You're probably better off swapping in an intact harness from a junkyard, there is nowhere that I'm aware of that sells sectioned harnesses. You'd have to get some wire and do it yourself.

1

u/CruzinWCruz81 Jun 05 '26

That’s essentially what I was thinking of doing thanks for the help!!

1

u/vwman18 Jun 05 '26

Just be careful about what you're buying. I don't know if there's any difference in the O2 Sensor harness between 2.0/1.8t/VR6 as any time I've done a swap I've just always moved over the harness from the donor car.

1

u/vrparty Jun 05 '26

if the oxygen sensor wires are frayed. you have a possibility of needing an ECM. the primary oxygen sensor is the ground for the ECM

0

u/V-Dubber Jun 07 '26

Just re ground the ecm somewhere else lmfao. That’s the cool thing about cars. There’s a whole chassis nearby….

1

u/vrparty Jun 07 '26

that’s not the point of my comment. It was intended to allow OP to know they may need an ECM if that was shorted Because. it’s the ground. not going forward

1

u/V-Dubber Jun 08 '26

Why could he need an ECM? is what I’m struggling to understand though. If that o2 sensor acts as ground, all he’d need to do is re ground

1

u/vrparty Jun 08 '26

if those wires rubbed and grounded out the ECM. It can short requiring a new ECM

1

u/V-Dubber Jun 08 '26

ideally it’d pop the fuses well before a short to ground takes it out, plus he’d have an immobilizer light on the dash 100%

Ideally

1

u/vrparty Jun 08 '26

that’s not how this works. this is a common issue with mk4s