still having lean bank 1 problems - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 02-03-2008, 06:36 PM   #1
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recently i replaced the o2 simulator with a standard rear o2 sensor i also have a highflow cat and it is the one that is 66 dollars on zzperformance i have the flowmaster f body crossover muffler on it and no other exhaust mods i have heard from a few members that that cat is not a good choice for our cars.

all the codes i get are lean bank 1, and catalyst failure P0420 code i think

i am so lost on this i tried everything and i am at a total lost
my thought is it might be back pressure because as soon as the TCL turns on on the dash i can rev it up and it turns back off. i am curious if anyone with the minor mods i have has ever had issues like this?
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Old 02-04-2008, 01:22 AM   #2
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Old 02-04-2008, 01:25 AM   #3
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You replaced the REAR O2 simulator with a sensor? It has absolutely zero effect on lean bank codes or anything else other than catalyst efficiency.

Check for vacuum leaks, clean both ends of every battery cable, and make sure you don't have a spark plug wire touching the O2 harness in the rear manifold.
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Old 02-04-2008, 02:36 AM   #4
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You'll need a scantool and knowledge of acceptable sensor voltage ranges to diagnose a lean code. Do you have a K&N or other oilable air filter? If you do, the oil may be getting on the mass airflow sensor'* hot wire and causing it to read out. Very common. You can use throttle body cleaner to clean the sensor. The PCM uses a lot of sensors to influence primary fuel delivery, mostly the MAF but also other sensors like the throttle position sensor and coolant temperature sensors, etc. Again, scantool for them.

One thing worth ruling out is misfire. Does the car run smoothly with no "pops" or wavering sounds coming from the tailpipe? Misfires dump raw unburned air and fuel into the exhaust, and the O2 sensor sees this as a lean condition.
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Old 02-05-2008, 12:25 AM   #5
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sorry for the poor spelling and confusion created with the other post. to clear things up willwren, and others... i replaced the o2 simulator becuase, i used to get frequent lean bank 1 codes, since replacement of the simulator with a standard rear o2 sensor i have gotten the lean bank 1 code less frequently but i am now experiencing code P0420 catalyst efficiency below threshold. i observed with my buddies snap on determinator scanner that the upstream is nearly twice as high in millivolts as the downstream. my concern is that the magnaflow high flow catalytic convertor i purchased at ZZP in conjunction with my other mods (F-body crossover flowmaster muffler, and cold air intake) may be the cause of my anger and frustration. i have been battling this for a long time and i'm looking to you guys for cures since you are great with these cars

i hope this cleared up the confusion

p.*. i just did a tune up with TR55'* and AC DELCO cables and replaced the LIM gasket and re-did all of my vacuum lines then when i had them disconnected
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Old 02-05-2008, 02:44 AM   #6
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You WANT the downstream sensor not to read as high or as low as the upstream. When you graph the sensor voltages on the scan tool, you should see the downstream sensor follow the upstream sensor, but the downstream sensor shouldn't move as far up and down from centre (0.45V) as the upstream. This is because a working converter will burn up excess air and fuel in the exhaust and "mask" the signal of the upstream sensor. Here is a crappy MS paint drawing I made to show you what things SHOULD look like:



Notice how the downsteam sensor follows the upstream, but it'* curve is much less pronouced? This is good. When scanning a vehicle with a non-functioning converter, you'll see that the two curves are almost exactly identical. In other words, the converter is doing nothing to mask the signal of the upstream sensor. It'* dead and nothing is going on in there. The PCM watches the O2 sensors for this similarity in the curves, and this is when it throws a catalyst efficiency code.

The catalyst efficiency code can also come up in some other situations. One is when you've got a misfire that'* dumping raw fuel (and oxygen) into the exhaust and confusing the O2 sensors. Another is when you've got a bad sensor. Watch their voltages on the scantool. The upstream should cycle back and forth between a high value (above 0.45V, ex 0.7V) to a low value (below 0.45V, ex 0.3V) and back again. It should do it quickly and constantly. A sensor that takes a long time to cycle is often called a "lazy" sensor. The downstream sensor should cycle too, but more slowly and as mentioned before, not as far from centre. You can make sure the sensors are reading properly with a *sort-of* quick test. Keep the car idling for this. First give the throttle a quick jab, right to the floor. This will cause the PCM to dump fuel and the A/F ratio will go rich for a moment. Both sensors should read high as soon as you do this, then go back to their normal cycling after a second. Once things have stabilized, do another test. Pull a vacuum line off to induce a lean condition and watch your voltages. Both sensors should read low. If one of your sensors doesn't respond as expected, or can't reach high voltages of over 0.8/low voltages of under 0.2, replace it.

Worn out sensors usually lose their high voltage capability first. Hence they loose the ability to tell the PCM it'* rich. This is why changing old O2 sensors sometimes makes a big improvement to fuel economy.
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Old 02-24-2008, 07:46 AM   #7
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This is an awesome detailed explanation of what is happening with the O2 sensors.
I plan on going to a Magnaflow Spun Metallic Cat next month. I am going to put new O2 sensor in also. Is there one or 2 sensors?(I have not looked yet.)

Will any certain brand be better? I know to avoid Bosch.
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