Upstream 02 sensor after cats? - Camaro5 Chevy Camaro Forum / Camaro ZL1, SS and V6 Forums

tlamb2 · 10-29-2025, 06:37 AM

So I threw a P0134 Code. 02 sensor bank one sensor 1. So I picked up an Upstream 02 sensor, and when I went to replace it, the downstream sensor was in the Header bung, and the upstream was behind the cat, so I just replaced it and left it in the rear. And the CEL went away, and both banks are the same. I have headers, so I knew the rear were deleted in the tune, but why are they in the header? I contacted the tuner who did the header tune, and he said it goes first in line. I had the headers installed 14 years ago, and since then, have added Maggie, Meth, and a stage 2 blower cam, three tunes total. Does it make any difference to have the upstream sensor behind the cats?

CamaroCracka · 10-29-2025, 07:54 AM

I don't see how your ECM can control fuel delivery if it is behind the cat, which would be catalyzing unspent fuel. In other words, a working cat will make the sensor behind it unable to sense a rich condition. Your car has probably just been running rich for 14 years, or maybe it just runs in open loop because the ECM sees feedback that makes no sense to it. Surprised you don't have a CEL.

tlamb2 · 10-29-2025, 07:58 AM

You would think the tuners would see it right? Now, when I move them up, will it mess with the tune?

FasNuf · 10-29-2025, 07:59 AM

Can you send me your tune? I'm curious. The O2 sensors that are controlling fueling are best positioned closest to the exhaust valve. If the secondary's are actually off in the tune then he's just using them as a plug for the hole, but why the active ones are downstream of the cats is a mystery to me. Moving them would most likely upset the tune so I wouldnt do that without talking to your tuner.

tlamb2 · 10-29-2025, 02:29 PM

FasNuf, I will get a copy of the tune from the tuner and send it to you when I get it. It will be a week till I get out there. Thanks for your input. It was funny when I was reading your reply; I was also watching your YouTube video on installing your wideband gauge. You have a great channel on YouTube, keep up the good work.

FasNuf · 10-29-2025, 02:40 PM

Thanks!
I sent you a PM

1JEWLDSSRS · 10-30-2025, 12:40 PM

Agreed with both of the guys above on the upstream 02's, they need to be "before" the cats to work properly with the ECM..

10-29-2025, 06:37 AM	#1
tlamb2 WHPLSH Drives: 2010 SS Maggie,ARH,Meth, BTR stg2 Join Date: Mar 2010 Location: San Clemente ,Ca Posts: 135 View My Garage	Upstream 02 sensor after cats? So I threw a P0134 Code. 02 sensor bank one sensor 1. So I picked up an Upstream 02 sensor, and when I went to replace it, the downstream sensor was in the Header bung, and the upstream was behind the cat, so I just replaced it and left it in the rear. And the CEL went away, and both banks are the same. I have headers, so I knew the rear were deleted in the tune, but why are they in the header? I contacted the tuner who did the header tune, and he said it goes first in line. I had the headers installed 14 years ago, and since then, have added Maggie, Meth, and a stage 2 blower cam, three tunes total. Does it make any difference to have the upstream sensor behind the cats?

10-29-2025, 07:54 AM	#2
CamaroCracka Drives: 2010 Turbo LS3 Join Date: Dec 2009 Location: Palm Bay, FL Posts: 3,041	I don't see how your ECM can control fuel delivery if it is behind the cat, which would be catalyzing unspent fuel. In other words, a working cat will make the sensor behind it unable to sense a rich condition. Your car has probably just been running rich for 14 years, or maybe it just runs in open loop because the ECM sees feedback that makes no sense to it. Surprised you don't have a CEL. __________________ 2010 SS - Kind of an On3 kit, CTR 78/80, CompCam 239/251 620/632 122+4, E85, Z28 suspension and ZL1 diff with Outlaw axles. Gen6 ZL1 brakes. 2011 Vert - 416/w 230/236 .612/.602 115lsa, 1LE suspension w/32mm rear bar. Z28 diff. Gen6 ZL1 brakes.

10-29-2025, 07:59 AM	#4
FasNuf Drives: 2014 Camaro ZL1 Join Date: Jul 2019 Location: Oregon Posts: 1,703 View My Garage	Can you send me your tune? I'm curious. The O2 sensors that are controlling fueling are best positioned closest to the exhaust valve. If the secondary's are actually off in the tune then he's just using them as a plug for the hole, but why the active ones are downstream of the cats is a mystery to me. Moving them would most likely upset the tune so I wouldnt do that without talking to your tuner. __________________ . . . . https://www.youtube.com/channel/UCgN...etl17hA/videos

10-29-2025, 02:40 PM	#6
FasNuf Drives: 2014 Camaro ZL1 Join Date: Jul 2019 Location: Oregon Posts: 1,703 View My Garage	Thanks! I sent you a PM __________________ . . . . https://www.youtube.com/channel/UCgN...etl17hA/videos

10-30-2025, 12:40 PM	#7
1JEWLDSSRS Drives: 2011 SS-RS Join Date: May 2011 Location: FLORIDA Posts: 14,043	Agreed with both of the guys above on the upstream 02's, they need to be "before" the cats to work properly with the ECM.. __________________ TSP STAGE 3 CAM,231/246 @112 LSA 614/641 LIFT, CIRCLE D 5C-4200-4500 CONVERTER,LOD INTAKE.CORSA EXHAUST. DSS 1 PC DS. PEGASUS SOLID CRADLE MOUNTS. 486RWHP/475RWTQ.LOTS MORE.

10-29-2025, 07:58 AM	#3
tlamb2 WHPLSH Drives: 2010 SS Maggie,ARH,Meth, BTR stg2 Join Date: Mar 2010 Location: San Clemente ,Ca Posts: 135 View My Garage	You would think the tuners would see it right? Now, when I move them up, will it mess with the tune?

10-29-2025, 02:29 PM	#5
tlamb2 WHPLSH Drives: 2010 SS Maggie,ARH,Meth, BTR stg2 Join Date: Mar 2010 Location: San Clemente ,Ca Posts: 135 View My Garage	FasNuf, I will get a copy of the tune from the tuner and send it to you when I get it. It will be a week till I get out there. Thanks for your input. It was funny when I was reading your reply; I was also watching your YouTube video on installing your wideband gauge. You have a great channel on YouTube, keep up the good work.