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  #1  
Old 12-05-2017, 03:13 PM
Witsend Witsend is offline
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Default P2A04 (P0157) HO2S performance Bank 2 Sensor 2

2007 Chevy Express 4.8L. 108K miles No exhaust leaks evident. Fuel trims ok. Both Rear O2s appear to be switching but bank 2 downstream O2 voltage does not seem to cycle as fast or get as high and low as Bank 1 downstream O2 does. Figured O2 with over a 100k just getting tired , so I replaced it with a direct fit Bosch brand. Cleared code and verify new one working, The next day the customer's check engine light is back on again with same code. Data shows fuel trim is ok and O2 switching. No evidence of harness chaffing or voltage switching flat lining while wiggling the harness. I remove the O2 connector and PCM connectors and no sign of water ingress or green death. I cleared codes and tried to swap the 2 downstream o2 sensors , but bank 1 side is frozen in the bung and would require the blue wrench to heat the bung to an orange before I think it would come free , so I didn't want to damage it and render it unusable, so I dug the original O2 and box out of garbage can and reinstalled the original Bank 2 downstream O2 and asked my friend to warranty swap the replacement as defective and if possible even get another brand like a Denso or GM. When Car first goes into closed loop I see both down steam O2 switch about the same level of voltage but I believe the Bank 2 downstream O2 gets more sluggish after the heater circuit is turned off, so my next step I think after he comes back with a replacement downstream O2 sensor is maybe check if the left bank cat is reaching as high of a temperature as Bank 1 right side reaches. I would imagine maintaining a high enough temperature would be required for the O2 sensor to continue working properly. Anybody see this before?

Last edited by Witsend; 12-05-2017 at 04:41 PM..
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Old 12-05-2017, 04:59 PM
Crusty Crusty is offline
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I don't use ANY BOTCH sensors.....NEVER.....
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Old 12-06-2017, 08:56 PM
STURNER STURNER is offline
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GM and Bosch O2 sensors do not get along.
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Old 12-06-2017, 09:22 PM
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There's a remote possibility that the front O2 is bad, but if your fuel trims are good, then I can't see it happening.
Please check your Freeze Frame data to see what the fuel trims are when the code sets.
If the front O2 is bad, it is possible (even though I've never heard of it happening) that during the Intrusive Test that the PCM believe it is forcing the engine Rich or Lean, but not really doing it correctly.
What's your Mode 6 Test results for TID8A, TID8B, and TID8C?

This one is a stumper. I wish I had better suggestions, but maybe this will tell us specifically what test is failing.

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Old 12-07-2017, 09:03 AM
Crusty Crusty is offline
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A couple of things Wit....
First, does this have available the Oxygen Sensor heater relearn in the scan tool..
Make sure you do this or the ecm may not provide the proper heater current.
Too much can damage the new sensor.
Too little and it won't heat up properly.

Second, when it goes closed loop, it looks for voltage to toggle below 349mv and above 710mv

The ecm will passively look for the toggle voltage thresholds, then, if they are not seen, the ecm will force a rich or lean transition.

At normal operating temperature, run it at 1500 rpm for 30 seconds, then give it three quick snap throttles to wide open and see that the voltage changes by more than 200-mv.
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Old 12-07-2017, 11:21 AM
Witsend Witsend is offline
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Joe and Crusty , thanks for the suggestions. I haven't gone into mode 6 in OBDII yet or did a HO2S heater relearn. Who would have figured they would come up with something akin to having to teach a toaster how to make toast.Hopefully doing the test to relearn the new one doesn't take out the other 3 original ones in the relearn process.
Customer is hopefully bringing me an actual GM part and not a spin off of half the former GM aftermarket brand name sold off and made someplace in Timbuktu.
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Old 12-07-2017, 12:29 PM
BRIAN617 BRIAN617 is offline
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Quote:
Originally Posted by Witsend View Post
Joe and Crusty , thanks for the suggestions. I haven't gone into mode 6 in OBDII yet or did a HO2S heater relearn. Who would have figured they would come up with something akin to having to teach a toaster how to make toast.Hopefully doing the test to relearn the new one doesn't take out the other 3 original ones in the relearn process.
Customer is hopefully bringing me an actual GM part and not a spin off of half the former GM aftermarket brand name sold off and made someplace in Timbuktu.

Just clearing the DTCs will take care of the o2 heater resistance relearn.
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Old 12-07-2017, 04:55 PM
Crusty Crusty is offline
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Quote:
Originally Posted by Witsend View Post
Joe and Crusty , thanks for the suggestions. I haven't gone into mode 6 in OBDII yet or did a HO2S heater relearn. Who would have figured they would come up with something akin to having to teach a toaster how to make toast.Hopefully doing the test to relearn the new one doesn't take out the other 3 original ones in the relearn process.
Customer is hopefully bringing me an actual GM part and not a spin off of half the former GM aftermarket brand name sold off and made someplace in Timbuktu.
Clearing the codes alone won't make it relearn the new oxygen sensor characteristics. Clear the codes (even if there are none...command it) then run the heater relearn.
I've seen far too many vehicles come here from other shops after the Botch (cheaper) sensors were stuffed in. I've had reliable results from NGK/NTK.
Chrysler OE are garbage AFAIC (farfignuten stuff....LMAO). GM OE have been pretty good but not 100%. Ford are OK (so far). Asian have been good with OE, or the NTK which are much easier to source locally.
Look for the amplitude changes passively once it goes closed loop. Then if it isn't happy watch the fuel trims as it does it's own intrusive test trying to force the amplitudes.
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Old 12-07-2017, 09:08 PM
Witsend Witsend is offline
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I put a new downstream bank 2 Denso sensor in . Both side monitored Cats read the same temp . Cleared codes, both downstream O2 sensors initially start switching up to about.8V and bank 2 actually can dip to 0V and go back up to .8V, where Bank 1, downstream only gets down to about .2V, but as the car gets warmed completely, bank 2 downstream change seems to peter out and not reach as high a voltage and switch as well as bank 1 , and only seems to waver from .4 to .6 without as much change as bank 1 downstream. (Man, and I thought it was totally normal for a good cat with good efficiency to hold voltage of the downstream o2 sensor close to half a volt, so I don't know why it's a problem.
I checked the freeze frame data of when the P2A04 code set. At operating temperature of 185 degrees at 60mph, 75% load and long term fuel trim on bank 2 was high at 12.5, bank 1 was 9.4, but Short term trims were at 0 or slightly negative. Chinese tool Mode 6 I didn't see a very comprehensive list of test numbers TID8A, B, or C or even a PID in Engine Data that showed if HO2S heater test completed or incomplete, so I decided to clean his MAF sensor and let him go take it and pick up his kid. I have the feeling it's not fixed yet and be back. Guess the next logical thing is to watch Bank 1 and Bank 2 downstream O2 sensors and disconnect the left downstream O2 sensor that I believe to be bank 2 side and see if it is the bank 1 downstream PID that flat lines. Kinda suck if the wiring harness on the back of the trans somehow got twisted around and the harness connectors for the rear O2 sensors are reversed, lol , Stranger things have happened

Last edited by Witsend; 12-07-2017 at 09:45 PM..
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Old 12-08-2017, 01:34 AM
Crusty Crusty is offline
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It IS normal for the post-cat sensors to be lazier than the front sensors. They better be or the cats are not doing their job.
It does however want to be able to monitor the catalyst capability. In order to do that, the sensors have to be able to go high and go low. This is what that OBD self test is looking for.... the capability of the sensors to respond.
If they can't go below the threshold or above the threshold, it can't tell if gasses are lean or rich and it can't tell if the cats are doing their job.
Once it goes closed loop, it watches to see if the thresholds are passed, and if not, it then forces lean and rich to see if the sensors are capable of responding.
Closed loop from the front O2 sensors and the cats still won't be hot enough to do much so the post cat sensors should be able to "see" a lean or a rich from normal driving and throttle ups and downs....at least until the cats get hot enough.

This is the type of thing that makes the SSC program and SnapOn recording so valuable. You can gather ALL the pids and play them back on a PC, sorting pids so they are together as desired.

Front O2-1
Front O2-2
Rear O2-1
Rear O2-2
Fuel trim (sometimes left & right)
Closed loop
Coolant Temp
Cat temp
Engine Run Time (THIS one is very valuable)

Since your China-Man scanner only records what is on screen, put the 4 O2 sensors on the screen.
Cold start, Idle for 10 or 20 seconds, drive normally for about 5 to 8 minutes.

Now you can see if the amplitudes go above and below the thresholds, and you can see immediately when the ECM starts seeing a normal front O2 activity (cycling) which is WHEN it will go closed loop.
At some point, all 4 sensors should have gone above and below the thresholds (even for only a moment)

Bank-1 will be where Cylinder #1 is. Bank-2 will be the other side (obviously)
Sure, the rears MAY have been switched but doubtfull. Ya you can disconnect and create a failure code (you can't rely on a flat lined signal 'cause that's the kind of thing this OBD test is telling us) to identify which is which, but you can also look at the wiring colour codes to verify which is which. They WILL be different. Check a schematic.
Look in Functional Tests, and/or Output Commands, for the O2 relearn.
HTH
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