Does the downstream O2 Sensor affect fuel/air mixture? (5.2, 1997) - JeepForum.com
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post #1 of 14 Old 08-13-2020, 07:57 AM Thread Starter
demagogue
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Does the downstream O2 Sensor affect fuel/air mixture? (5.2, 1997)

I have a '97 ZJ with the 5.2. Some jerk thief stole my cat, cutting my exhaust with a sawzall, cutting the downstream O2 sensor wire. I've driven the jeep a little bit (< 10 miles, sounds AWFUL) but I was expecting it to throw a code or something.

My question is, am I going to be running extremely rich or anything? Does the downstream sensor do anything other than throw a code in the event of a missing/bad cat? I'm also a little bit perplexed on why I haven't seen a code yet.

I have a new Y pipe/OE cat/catback exhaust on the way but it won't be here for awhile and I will need to drive the ZJ some in the meantime. I just want to make sure I won't be damaging the engine.

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post #2 of 14 Old 08-13-2020, 08:53 AM
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No


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post #3 of 14 Old 08-13-2020, 11:41 AM
CatSplat
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Nope, D/S sensor is only used to check cat efficiency.

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post #4 of 14 Old 08-13-2020, 12:39 PM Thread Starter
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Thanks!


Still surprised I'm not seeing a check engine. Maybe something else is broken. /shrug

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post #5 of 14 Old 08-13-2020, 01:18 PM
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Not 100% certain on the ZJ what conditions are required for the PCM to check the downstream sensor. On most cars it's a specific set of driving conditions, like more than 10 minutes driving above 50 mph with the coolant temp above 180 degrees. Or something like that. If you keep making short trips it might not ever see the right conditions.
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post #6 of 14 Old 08-13-2020, 02:36 PM
Timo_90xj
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In short: JTEC PCMs actually do monitor and slightly adjust A/F ratio based on the downstream sensor as well.

I have a SBEC/JTEC/NGC- booklet as a PDF, and in the O2- sensor segment it mentions that starting in 1996 (OBD2), JTEC- based PCMs use downstream O2- sensors for two things:

1. Monitoring cat converter efficiency by comparing switching rates of upstream and downstream O2- sensors. PCM will store a fault code for cat converter efficiency if switching ratio exceeds a calibrated value.
2. Downstream fuel control = Downstream sensor signal causes PCM to adjust upstream O2- sensor goal voltage. Raising the goal voltage to reduce high O2- content increases overall fuel (= slightly longer pulse width), and lowering the voltage to increase low O2 content by leaning out the mixture.

The effect of the downstream sensor isn't huge, if I'm understanding the graphs in the PDF correctly, it's in the region of around +/- 0.10 volts. O2 sensor voltage varies between ~0.0 volt to ~1.0 volts so we're talking of about +/-10% difference in goal voltage for upstream sensor.
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post #7 of 14 Old 08-13-2020, 02:38 PM
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I'd expect you to at least be getting a downstream 02 sensor heater failure code, since the heater is, uh, missing. As mentioned the actual converter malfunction code may not pop up if you don't have the engine in the right state for long enough.

Also note that the heater and sense lines for the 02 sensors are notorious for frying the PCM if they short in the wrong way, make sure the wires are taped up until you can get things replaced.
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Quote:
Originally Posted by Texas ZJ1 View Post
I heard someone yell "mall crawler" from the back porch. I'll go see who said that. Be right back.
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post #8 of 14 Old 08-13-2020, 08:56 PM
jeepjeepster
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Just dont let those wires short out. Can you get to the plug and just unplug them?

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post #9 of 14 Old 08-14-2020, 12:53 AM
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The OBD II spec states the downstream sensor not only monitors the health of the converter but also _protects_ the health of the converter. As such, the downstream sensor can countermand the upstream sensor in order to protect the converter.

There's numerous instances of people changing the upstream sensor because their fuel economy has dropped, only for there to be no improvement. Changing the downstream sensor remedies the issue.

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post #10 of 14 Old 08-14-2020, 07:37 AM
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Quote:
Originally Posted by jeepjeepster View Post
Just dont let those wires short out. Can you get to the plug and just unplug them?
I'd cut the wires right next to the connector so the connectors stay clean until it gets replaced.

Again, don't short them.

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post #11 of 14 Old 08-17-2020, 09:27 AM Thread Starter
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Thanks...ZJ is currently up on jack stands and replacing everything. PCM seems to be fine so fortunately the wires didn't short. Not looking to replace the PCM again.



Pretty pissed, they cut into the Y pipe in a few places so I have to replace that as well. It's a real pain.

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post #12 of 14 Old 08-19-2020, 02:19 PM Thread Starter
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Turns out the thief ripped the connector the o2 sensor connects to out. I'm having a hell of a time finding pigtail connector to fix this. Anyone have thoughts on the part number potentially?

2002 KJ (Owned 2005-2015, RIP)
1997 ZJ Limited
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post #13 of 14 Old 08-19-2020, 04:04 PM
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Someone will probably jump in after me and prove me wrong but I think those are junkyard parts now. It's not the same connector, but I could not find the harness connector for the oxygen sensor on my 93. And a little searching didn't turn up the one for your 97.
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post #14 of 14 Old 08-22-2020, 10:38 PM
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You can splice in the 02 sensor and remove the connector from the sensor's pigtail.
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5.2 , cat , catalytic converter , o2 sensor

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