Will a disconnected O2 sensor set off Check Engine?

[CobraSix]

I was test driving a 98 Mounty 5.0 the other night and found this:

The engine idled really smooth, best I've seen to date.

However, as soon as we started to drive, it was stumbling really bad. Getting on the highway was painful. Drove it for about 10 minutes (just due to where exits were located). I told the owner (since he worked at a dealership) the stumbling was a no go. Some of the other smaller issues aren't a show stopper.

So, he had it checked out and he said it was an O2 sensor that was disconnected. I'm sure that could cause the stumble, however wouldn't it also set of the Check Engine light?

THe car was idling for 15 minutes prior to even driving, plus 10 minutes of driving. Maybe that isn't enough time. I know my Volvo takes like 2 hours to reset lights, so maybe it didn't have enough time.

The only really annoying thing left on the car is the Door Ajar light won't turn off and the blower motor is a little loud.

I figure it's worth another test drive to see if that cleared up the problem.

 

[Dan Whitaker]

Yes a disconnected o2 will cause the CEL to set. Could be it was cleared to hide the fact that there is a problem.

 

[CobraSix]

That was my thought, but I figured 30 minutes of the engine running would be enough time to set off codes.

 

[Dan Whitaker]

Not necessarily. My CEL was on earlier this year and I thought I had it fixed & cleared the code. I put over 100 miles straight on it before the CEL came on again with the same code.

 

[CobraSix]

Yeah, I've seen that with my Volvo, and that's why I wanted to ask.

So, really, if the car runs better now it shouldn't be a large concern.

I do remember now that when he pulled up he said the battery was completely dead and he had to jump it. It was sitting for a few weeks apparently.

Slade

 

[rileyrs]

an unplugged o2 should set if off faster.
if the engine is cold, it will immediately sense an o2 heater problem. and very quickly after detect a bad signal from that o2 bank.
maybe this was an odd case, but i would think it would happen faster.

 

[jremington59]

Tell him to put it in writing that there's no check engine light for the first hundred miles. You'll find out if he's lying.

 

[ahodges]

I think the check engine light would be set off faster than that.

With that model car i would say that is the mass-air-flow sensor, GM vehicles from around that year are known for that.
Also that type of car is known for electrical flukes with the door warning lights and set belt lights.

The loud blower probably means the bearings/ bushings are going bad in the motor and it will have to be replaced. I've done that before in a similar model car, its not hard.

 

[jremington59]

What GM vehicle are you talking about? I thought it was a Mountaineer.

 

[ahodges]

The abbreviation threw me off.. Mounty, i've never heard a mountaineer refered to as that. I was thinking he was refering to a monte carlo (i missed the "u" in mounty).

It still could be the mass-air-flow sensor but i'm not as sure now..

The door ajar light is a common problem on that year explorer's and rangers.. Best advice for that is to lubricate the door locks, key holes, and latches with some WD-40. In a lot of cases that fixes the problem.

The blower advice stays the same...

 

[rileyrs]

ya check tbars's thread about the door ajar light, i did it on the ranger and it fixed all my door ajar problems.

maf is possible too.

are you test driving from a dealer, or private seller?
if its a dealer, ask to demo it, then you can get a real feel for it for a few days.
if its a private seller, ask him to clean the maf, and fix the o2 problem that he claims is causing it, and then tell him you will consider it if it runs correctly after that.
any motivated seller should be willing to plug in an o2, and take 10 mins to clean a maf sensor.

 

[CobraSix]

Private or dealer...honestly hard to say. The owner bought it through his brother's dealership at auction but never registered it as a private car. It is a large dealership (mostly new cars).

He fixed the O2 so I may take it for another drive this weekend and see.