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So I have a P1133 ‘HO2S Insufficient Switching Sensor 1’ code and P0300 misfire code but I’ve replaced the following items: ACDELCO O2 sensor bank 1 sensor 1, MAP Sensor, camshaft sensor, new spark plugs and spark plug wires and fuel injectors, also did a smoke test for vacuum and exhaust leaks (none leaks) and the P1133 and misfire codes just keep coming back after drive cycle completes.. current tuner says he deleted those O2 sensor codes from my tune file as I have LT Headers.. it continues to not run great mostly when cold and it also smells of gas through the exhaust. It’s at my mechanic shop right now and even he is at a loss and also racking up a large bill trying to solve it and pass inspection…

Things I havent checked, battery, fuel pump, fuel lines.

Anyone experienced this before? appreciate any advice you have.

Mods list:
Gpi ss2 vvt cam
Longtube headers
Br pushrods
Btr trunion kit
Ls7 lifters
Afm/dod delete
New timing chain and tensioner
New plugs
Spark plug wire covers
X pipe & spintech 9000 mufflers
Cold air intake
Moroso oil catch can
 

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P0300 is random misfires. When you have a P030X (X = 1 to 8), then you know what cylinders are effected. That's when you need to look at plugs, wires, and injectors. Bad fuel and air mixtures will cause the random misfires.

Bank 1 is the passenger side, or the general rule is the side with Cylinder #1.

Upstream (forward) sensors are the ones that determine the A:F ratios. Downstream only monitor the cats, and do not effect A:F ratios, and are the only ones you can "delete" with a tune. Deleting the forward ones, if possible, would cause a ton more issues than what you have now. The code can be tuned out though, because of the long-tube headers.

The ECU monitors the oxygen sensors in closed-loop operation for 90 seconds to determine if there is anything wrong with them. It does this by counting the number of times the oxygen sensors switch back and forth between rich and lean (it should look like a heart monitor). When there is an insufficient number of switches between rich/lean and lean/rich, the computer will throw P1133.

If your O2's are now farther downstream with the long tube headers, bank 1 may not be getting up to normal working temps where "switching" tests occur. The tuner needs to adjust the time in which the test starts, so the sensor can heat up in time for the test, if that is the case.

If you happen to have access to a scan tool, you can use it to capture the short and long-term fuel trim values. You’ll need to make sure that the engine is warmed up before you do this. Doing this will help you determine if the O2 sensor is operating within spec. You’ll need to compare the values that you capture with your scan tool with the values the manufacturer specifies,or with the other side.

People would swap the two upstream sensors to see if the problem follows the sensor. Since you replaced it, that takes care of that question. The new one still shows the same issues. Since it is only one side, check the O2 sensor wiring; for chaffed and exposed wires. Long tube headers usually means you need extensions on the O2 harnesses. They can rub and melt if not secured properly.

There is a thread here on this, but both sides:
 
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