Hey Guys,
Gd3 2008 169k
Got a p420 a day the starter cranked kinda weak so I was looking at the voltage and this happened.... Really odd.
Let off the gas and the voltage goes up. Rpm down and volts go up.

https://youtu.be/E_Fs4IJTbWg

As for the cat, the temp on it is good and 15 miles after resetting the code it's been within tolerances (shrug)

 

Nobody else seen this or nobody else looked lol

 

2008 GD3 5MT - 197,XXX miles

Hey man, I'm a newbie to the GD3 myself and I'm dealing with voltage issues too. I've got an aftermarket Gauge that plugs into my OBD2 port and I've been noticing that voltages can run from as low as 12.43v and go all the way up to like 14.80v. Mine behaves similar to your on the gas, off the gas fluctuation, but sometimes mine just goes wonky on a wet day or when my lights are on.

Makes me feel like my alternator is shitting the bed and that my battery may not be getting properly charged/discharged either. I've even got a reduced pulley load as I've ran into some AC clutch issues and routed it out using a BMW AC belt lol. My car is older and wasn't very well taken care of so it's not out of the question I could be dealing with shot components here.

I've got a P0420 code as well and I've been putting off buying the sensor myself as I don't really want to chuck a $150+ sensor at it and my cat foul it. I know that when I try and drive mine before it gets to it's "warmed up temp" like 129F or so it just bogs heavily and if I stop, the car stalls altogether
​​​​​​​I hope we can compare notes or have some more senior members chime in.

 

I'm a full tank of gas through and I'm getting no more codes I don't know if something just hiccup or what. Unfortunately I don't think that that's the shape that you're in. I do wonder if this battery is not getting properly charged I hear the alternator on these is kind of a pain to do


​​​​​ if you look around I don't think you need to spend $150 on a replacement oxygen sensor.

 

Dual charging system almost cuts the alternator out if there is no demand (idling in drive with lights and wipers off)

 

Shouldn't that result in the opposite of what we're seeing?
No demand, alternator cuts, voltage drops.

Were seeing no demand voltage increases, rpm climbs voltage decreases.