Bad O2 Sensor but NO Check Engine Light

[OLDMNSX]

I recently replaced my complete 91 exhaust system. I had an emission test done prior to any work. See "First Test - Pass".

After replacing the entire exhaust system I started the car and almost instantly got a Check Engine Light (CEL). The code flagged the forward O2 sensor. I swapped the O2 sensors, front to rear, reset the ECU and ran the engine. This time the CEL code flagged the rear O2 sensor. This confirmed that the problem followed the bad sensor. I replaced the bad sensor with a new NTK 24172. I reset the ECU, started the engine and NO CEL. I thought that all was well. I drove the car for 479 miles and had another emission test done. See "Second Test - FAIL"

Knowing that the cats were good, since John had used my cats for an emission test on his car and it PASSED, the only thing that could be a problem is the remaining OEM O2 sensor. I replaced that sensor with another new NTK 24172, drove the car for 128 miles and had another emission test completed. See "Final Test - Pass".

The moral to the story is that you can have a bad O2 sensor and never get the CEL to notify you that there is a problem. Had I continued to drive the car with the bad sensor I would have experienced reduced fuel mileage and in time could have burned out a catalytic converter from the over rich mixture. So I spent $28 on emission tests but I know that the engine is running as efficiently as possible.

Brad

 

[MANIACura]

That's weird, Brad. Glad you got it fixed. I've been trying to figure out what it was but I was leaning toward bad Cats. Not that I'm an expert but I wouldn't have guessed it was the O2 sensor especially with no CEL. Did you ever try resetting the ECU? You didn't get anything on the O2 sensor during the swap did you? It's just weird that they both failed just from swapping them to another manifold.

I'm guessing that an NA2 system would have caught the rich mixture and thrown a CEL with the downstream O2 sensor? Anybody have any theories?

 

[OLDMNSX]

Matt, Glad to hear that you didn't start the header swap till you got home, also glad to hear that you like the new complete exhaust system. I have to agree on the increased pull on acceleration. I am looking forward to hearing your car next time I'm down.

Did you ever try resetting the ECU? ?

Each time I made a change of an O2 sensor I reset the ECU.

You didn't get anything on the O2 sensor during the swap did you? ?

I didn't use anything to help get the O2 sensors out and tried very hard to keep them clean and not bang them around. Obviously I didn't succeed here.

It's just weird that they both failed just from swapping them to another manifold.?

Yes indeed.

I'm guessing that an NA2 system would have caught the rich mixture and thrown a CEL with the downstream O2 sensor?

Matt, remember that I didn't change any of the electrical, so I still have the OBD 1 system. I just plugged the bung hole in the cat where the second O2 sensor would go if it was used on a OBD 2 vehicle.

 

[MANIACura]

Matt, remember that I didn't change any of the electrical, so I still have the OBD 1 system. I just plugged the bung hole in the cat where the second O2 sensor would go if it was used on a OBD 2 vehicle.

Yeah, I know. I was just wondering if an OBD2 system would be smart enough to throw a CEL if both upstream O2 sensors where acting like yours were. Of course in your case all four probably would have gone bad :wink:

 

[OLDMNSX]

Yeah, I know. I was just wondering if an OBD2 system would be smart enough to throw a CEL if both upstream O2 sensors where acting like yours were.

Yes, I believe that it would. When John used my cats to do his emission test he said that the OBD 2 flagged both of his post-cat sensors. This is all that I have to go on. After he replaced both of the post-cat sensors the next thing the OBD 2 flagged was one of the cats. Sure a good thing that Acura is warranting the emission systems on the 97+ cars. I think that you have to have less than 100k on the miles too, but not sure. I think that you have access to a 97+ with less than 100K?????????????

Brad