Unplugged 02 Sensor no CEL?
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Junior Member
Joined: Dec 2024
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Car: 1991 RS 305TBI
Unplugged 02 Sensor no CEL?
I unplugged my 02 sensor to force my 1991 305TBI into open loop mode while diagnosing a rough idle issue. Car runs great with the 02 sensor unplugged and in open loop, but I don't get a CEL from unplugging the sensor... Is this normal?
It's made a few trips with it unplugged over the past 48 hours and no CEL. If there is supposed to be a CEL, could that mean I have a bad ECM?
It's made a few trips with it unplugged over the past 48 hours and no CEL. If there is supposed to be a CEL, could that mean I have a bad ECM?
Supreme Member
Joined: Oct 2000
Posts: 3,168
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From: Park City, UT
Car: '92 Corvette, '89 1/2-a-'Vette
Engine: LT1, L400
Transmission: ZF6, T5
Axle/Gears: 3.45, 3.31
Re: Unplugged 02 Sensor no CEL?
Does the CEL light illuminate during start up (Key on, engine off)?
Joined: Dec 2004
Posts: 16,754
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From: Mile High Country !!!
Car: 1967 Camaro, 91 z28
Engine: Lb9
Transmission: M20
Axle/Gears: J65 pbr on stock posi 10bolt
Re: Unplugged 02 Sensor no CEL?
If there is no o2 to report to the ecm, the ecm
just stays in OL without codes.
just stays in OL without codes.
Supreme Member
Joined: Jan 2009
Posts: 2,313
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From: belle fourche,s.d.
Car: '82 z28
Engine: L83 5.7
Transmission: 700r4-1985
Axle/Gears: 3.42 posi
Re: Unplugged 02 Sensor no CEL?
Knocked the wire off the O2 sensor on my '82 and the CEL would light after a couple min.
Supreme Member
Joined: Sep 1999
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From: Colorado USA
Car: '83 Firebird (T/A Clone)
Engine: 350 with L-69 components
Transmission: 700R-4, 2000 RPM stall converter
Axle/Gears: 10-bolt/3.73 ..
Joined: Apr 2000
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From: South Windsor, CT
Car: '89 GTA
Engine: ZZ6TPI
Transmission: T56
Axle/Gears: Borg Warner 3.70:1
Re: Unplugged 02 Sensor no CEL?
The CEL will not come on right away if the ECM doesn't see the O2 sensor. It will stay in open loop mode and after driving for a while (maybe 30 minutes or so), the ECM will eventually bring on the CEL and store a code. I don't recall which code. Years ago, I had a bad ECM that had an open circuit for the O2 and this is exactly what it did.
Supreme Member
Joined: Oct 2000
Posts: 3,168
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From: Park City, UT
Car: '92 Corvette, '89 1/2-a-'Vette
Engine: LT1, L400
Transmission: ZF6, T5
Axle/Gears: 3.45, 3.31
Re: Unplugged 02 Sensor no CEL?
I'm with the camp that is saying the CEL will come on. I've never seen a car where that would not happen. The whole point of OBD I was to self monitor exhaust emissions, make necessary adjustments, and notify when it's not working or out of range. By definition, if the O2 is open, the system can not monitor, that = a fail. CEL has to come on, owner "has to" address" it. That's not proof, and neither is my 30+ years of observations, but the two align.
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