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My Corolla 2006 had the engine light come up, and the code PO 171. I changed the mass air flow sensor, and a mechanic tried to change the oxygen sensor but he told me that it was welded to the the manifold. (This must have happened years ago after I took the car to a mechanic for repairs after an accident.) I know it's not supposed to be that way, but here is my question: When I park the car on an incline, nose down, overnight for a couple of days, the light comes up. When I park the car on an incline the opposite way, tail down, the light disappears after 2 nights, and stays that way unless the car is either nose down again for a while, or leans to the right side for at least a few minutes at a light. The engine light also disappeared after he tried to detach the oxygen sensor from the manifold (to replace it with a new one) and banged on it a few times, but it came back after I hit a bump on the road. I asked a repair shop to check if anything was loose, they said everything looks fine, and it would have nothing to do with the oxygen sensor anyway, but that's thinking of the way a regular oxygen sensor is attached, not when it's glued to the manifold. Any suggestion please? I know there is a risk detaching the oxygen sensor from the manifold, as the manifold might break, but I wonder if the problem has anything to do with the sensor detaching itself either from the manifold or from the exhaust system. (knocking on the sensor had the light disappear)

Can someone also tell me if a Corolla 2006 has one or two oxygen sensors please? I thought it only had one, but the last shop told me it has two.

Thank you very much.

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  • Welcome to Motor Vehicle Maintenance & Repair! Dec 6, 2023 at 17:17

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As far as it being "glued" to the manifold, O2 sensors are made to be replaced, so it being glued to the manifold seems weird to me. It should thread into the manifold so it can be removed and a replacement put in its place.

Your 2006 Corolla would have two O2 sensors, one prior to the catalytic converter (or "cat") and one after. Cars in the States started having a setup like this after OBDII was mandated in 1996, so any car after that would have two per bank. If there are two cats on a car, like for a V6 or V8 engine, there'd be a total of four O2s. For most vehicles today, the pre-cat O2 is to keep the engine running correctly, while the post-cat O2 is for checking the efficiency of the cat.

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  • Sorry, I meant "welded" onto the manifold. Apparently, it's not unusual for the oxygen sensor to be welded into the manifold and the exhaust system when parts were broken while fixing a car.
    – Cynthia
    Dec 6, 2023 at 18:14
  • @Cynthia - No, that would be quite unusual. There is no good reason to weld it into the manifold or exhaust. The only reason that'd be done is because someone was trying to do a cheap/quick repair. That is DEFINITELY not the right way to repair it. Dec 6, 2023 at 18:15
  • I understand. Now, with the engine light going off or on depending on the car's front headed up or down, would that have anything to do with the welding coming loose maybe?
    – Cynthia
    Dec 6, 2023 at 18:19
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    Cynthia, when your mechanic says the O2 sensor was "welded" to the manifold, I'm sure that he doesn't mean actually welded, such as arc welded. He means that it's badly stuck as if it were welded. The manifold is cast iron and the O2 sensor is almost certainly stainless steel. These two materials don't play well together in all common welding techniques, so I'd say that it's impossible that a collision repair shop literally welded these two parts together. They are simply stuck and beyond the capability of your mechanic to get them apart.
    – MTA
    Dec 6, 2023 at 20:09
  • I actually remember the mechanic telling me he had "tack welded" a few spots from the O2 sensor to the manifold. Another mechanic confirmed that it's what he saw years later, but let's say that maybe I misunderstood. What could possibly cause the engine light to come on when it's parked "front down?" on an incline, and then go off when it's parked "front up?" The engine light also went off after a mechanic tackled the O2 sensor trying to replace it with a new one. (and came back after I rode over a bump)
    – Cynthia
    Dec 6, 2023 at 20:45

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