SL55 amg, Replaced both o2 sensors before cats with exact fit parts from parts geek, black connector fit right in no problem.

Car runs like crap with codes of no activity from both sensors, replaced them both again and same exact thing.

Checked actual values via star diag and o2 sensors are putting out voltages that change from .4V up and down, last i remembered the o2 sensors were putting out .6V and the fuel trim went to -25% then car stalled.
Checked voltages on all 4 o2 sensor pins with multimeter,

signal ground is 0.4V
signal hot measured 0V but no continuity to vehicle ground (so it's not shorted)
O2 heater is 12V
o2 Heater ground is approx 4V

So now i don't know what to do..... I am assuming wiring is good, all the voltages check out.
Assuming ECU is good because it reads and displays the o2 sensor voltages no problem, no other codes besides o2 sensor inactivity

Any ideas??

Currently driving with sensors unplugged, when i first plug them in it idles rough, so i tell star to re-set mixture adaptation values, it works well for 30 seconds or so then runs rough again.

I know it's an old thread but I have the same symptoms on a 2004 SL 55 on both pre-cat cat o2 sensors. Changed multiple times with no impact.

What are the fuel trim values?

Per freeze frame:
Short term : B1 +3.9%, B2 -15.6%Long term: B1 +0.8, B2 +1.6%

​​​

if both sides are the same you should probably try the OEM bosch sensors
give me your vin and I'll tell you the correct parts
don't assume that part geeks know exactly what they are doing 100% of the time.

Thank you. My VIN is WDBSK74F74F069733. My understanding is that Bosch makes the Mwecedes-branded O2 sensor, so I replaced both pre-cat sensors with Bosch Part No. 16167. It looks like you might be suggesting that the Mercedes-branded version may make a difference.

Thank you, however, isn't 16328 the dowstream o2? I need the upstream O2 sensors. Appreciate it.
https://shop.advanceautoparts.com/p/...28/15470383-P#

yes it is I misread you
sorry, I will look again in one hour

I found 16167 also
so your problem is somewhere else

Thank you for your help. I will try to see if it is a wiring problem. I was also wondering whether an exhaust leak would cause both oxygen sensors to switch slowly or would it affect the heating element inside the oxygen sensor?

Be cognizant of the fact that below a coolant temperature of c. 50 degrees Celsius the fuel injection is operating "open loop" and ignores feedback from the oxygen sensors. So if the engine runs poorly at all temperatures, then the oxygen sensors surely are not an issue.