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O2 sensor confusion

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Old 08-01-2014, 04:48 AM
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O2 sensor confusion

I had test pipes on my g for about almost a year now... All my sensors seem to be gone bad diagnostic reading states bank 1 and bank 2 the question i have is which downstream and upstream ? And also can bad sensors bring up a lot of codes as well im getting like 6 codes mechanic states it could all be related to the sensors im switching to oem cats n buying new sensors but can someone explain the upstream n downstream n which one is bank 1 n bank 2? Thanks
Old 08-01-2014, 06:47 AM
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Upstream sensors are the ones that are before the Catalytic converters, and downstream ones are the ones after the converters. The reason you have two banks is that the motor is a V-configuration so there are two separate sets of pipes coming out of the motor, each containing exhaust from the 3 cylinders on that side of the motor. Each bank has an upstream oxygen sensor before the Catalytic converter that the motor reads real-time to determine how to meter the fuel injection mixture and other parameters and a set of downstream sensors after the converters, which are basically there just to test the exhaust after it has gone through the converters to make sure that the converters are doing their job.

You've probably fouled the downstream sensors since you have test pipes with no cats and are blowing untreated exhaust on them constantly.... especially if you haven't had your car tuned after your mods.

You might luck out and be able to remove your various oxygen sensors, cleaning them with some sensor-safe solvent and then blasting the hot end of them with a torch. Make sure you use sensor-safe cleaning solvents only on them and don't get any of the gas from the torch on the sensor until the thing is lit. Reinstall the sensors after doing this and they may start to pick up clean readings again. It's best if you have an OBD-II scanner to clear the codes and reset the ECU immediately back to a good state without having to disconnect the battery, etc.

If you continue to run the car without cats, the same problem will happen again. Most people with a no-cat setup do some sort of mod to their cars to trick the system into thinking the downstream O2 sensors are pulling clean data, that way they can completely remove the sensors or just ignore the fact that they are fouled.

You may want to just try getting high-flow cats instead of test pipes... you'll have less problems. Of course, a stock setup is the least problematic in general.
Old 08-01-2014, 07:05 AM
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Originally Posted by partyman66 View Post
Upstream sensors are the ones that are before the Catalytic converters, and downstream ones are the ones after the converters. The reason you have two banks is that the motor is a V-configuration so there are two separate sets of pipes coming out of the motor, each containing exhaust from the 3 cylinders on that side of the motor. Each bank has an upstream oxygen sensor before the Catalytic converter that the motor reads real-time to determine how to meter the fuel injection mixture and other parameters and a set of downstream sensors after the converters, which are basically there just to test the exhaust after it has gone through the converters to make sure that the converters are doing their job. You've probably fouled the downstream sensors since you have test pipes with no cats and are blowing untreated exhaust on them constantly.... especially if you haven't had your car tuned after your mods. You might luck out and be able to remove your various oxygen sensors, cleaning them with some sensor-safe solvent and then blasting the hot end of them with a torch. Make sure you use sensor-safe cleaning solvents only on them and don't get any of the gas from the torch on the sensor until the thing is lit. Reinstall the sensors after doing this and they may start to pick up clean readings again. It's best if you have an OBD-II scanner to clear the codes and reset the ECU immediately back to a good state without having to disconnect the battery, etc. If you continue to run the car without cats, the same problem will happen again. Most people with a no-cat setup do some sort of mod to their cars to trick the system into thinking the downstream O2 sensors are pulling clean data, that way they can completely remove the sensors or just ignore the fact that they are fouled. You may want to just try getting high-flow cats instead of test pipes... you'll have less problems. Of course, a stock setup is the least problematic in general.
Thank u for the info, from researching i thought upstream would be after the cats but yes i understand about tricking the ecu i was using non-foulers on the sensors on the testpipes to space out the sensor from the test pipe but it seems that the engine light is still there after this method im just gonna with the oem cats too many problems with test pipes also the cleaning method ill try hopefully it works if not ill have to buy sensors. Also an obd-II scanner can reset my ecu as well instead of using the ecu reset procedure that involves me pressing n holding the accelerator pedal while the car is off ?
Old 08-01-2014, 10:10 AM
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Go to your local auto part store and buy 18mm sparkplug nonfoulers for about $8 and drill them out with a 1/2 inch bit and you should be good
Old 08-01-2014, 11:37 AM
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post up what codes its throwing and we can give a lot better advice...
Old 08-01-2014, 12:32 PM
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Yep. If it's a P2A00 it's probably an intake issue.
Old 08-01-2014, 12:49 PM
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Bank 1 is the Passenger Side (Right side), Bank 2 is Driver's side (Left side).

Upstream would be Sensor 1, Downstream would be Sensor 2.

So if you have a code for "Sens 1 B 2", it would be the upstream driver's side sensor.


Upstream is the first sensor in the exhaust manifold, the car uses this sensor to monitor and adjust A/F ratio. You car will run like garbage if these sensors are not functioning correctly.

Downstream is farther down after the cat. The ECU typically only uses this for emissions purposes for the catalytic converter, this is the one that people are referring to when they reference "non-foulers" or O2 simulators when the factory cats are removed. Since these are secondary sensors, those tricks usually don't affect how the car runs.
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Old 08-01-2014, 02:17 PM
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Yep. If it's a P2A00 it's probably an intake issue.
What makes you say that?

Last edited by seymore4; 08-02-2014 at 12:00 AM.
Old 08-01-2014, 10:59 PM
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post up what codes its throwing and we can give a lot better advice...
Codes are:

P0037
P0057
P1168
P1148
A DTC 1031 im not sure of that code i dont remember

I have non foulers on the downstream sensors but still have a cel light reset ecu as well nn still comes back on i have megan test pipes v2
Old 08-01-2014, 11:59 PM
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Most likely both your primary O2 sensors (the ones closest to the engine, in the header) are shot. You need to replace them ASAP
Old 08-02-2014, 12:22 AM
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Originally Posted by seymore4 View Post
Most likely both your primary O2 sensors (the ones closest to the engine, in the header) are shot. You need to replace them ASAP
Thanks will doing that do u think will clear off my problems also should i leave the test pipes on with the non foulers or switch to oem cats i have new sensors with non foulers i just need to buy the sensors that are close to the headers n switch em out i heard these dont need non foulers tho of course if i go oem route i can forget about the non foulers but whats ur best advice?
Old 08-02-2014, 12:25 AM
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That depends if you want to have test pipes or not. If you want to keep the test pipes then I highly recommend a tune, not just trying to trick the ECU into keeping the CEL off with non foulers
Old 08-02-2014, 12:39 AM
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That depends if you want to have test pipes or not. If you want to keep the test pipes then I highly recommend a tune, not just trying to trick the ECU into keeping the CEL off with non foulers
How does a tune help out ? N im on a budget as well so ill have to think about that one ... I just know test pipes give a lot of problem i have nismo s tune exhaust so i just love my sound but yeah i shouldve been smarter from the beginning perhaps
Old 08-02-2014, 12:44 AM
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Just put the OEM cats back on, lol
Old 08-08-2014, 11:18 AM
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Ok so i received the sensors today hoping it eliminates the problem i will update soon. Car has been sitting there doing nothing.

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