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How to Locate the Oxygen Sensor in Question

By Ralph Kalal

Your code tool tells you that there’s a problem with “Bank 1 H02S 2.”

OK, how do you find “Bank 1?”

No problem. “Bank 1” is the cylinder bank with the number-1 cylinder.

Well, which cylinder is that?

If your car has an in-line engine (all the cylinders in a single row), the answer’s easy: “bank 1” is the only cylinder bank you’ve got. The number-1 cylinder is the cylinder closest to the front of the car.

But if the car is a V-6, a V-8, a V-10, or a V-12, locating the number-1 cylinder isn’t so easy. There is no accepted rule for designating the number one cylinder in a V configuration engine.   
As a general rule, the cylinder furthest forward is the number-1 cylinder. In a V configuration engine, one cylinder will be slightly ahead of the cylinder on the opposite side, as the opposing cylinders are slightly offset to allow clearance between the connecting rods on the same crankshaft throw. If the engine is mounted transversely, the number-1 cylinder usually is the one furthest from the flywheel.

In practical application, on most GM and Chrysler V-8 engines, the number-1 cylinder is the one closest to the front of the car on the driver’s side. The cylinders are then numbered in alternating sequence to the back of the car: number-2 is the furthest forward on the passenger’s side, etc. Ford V-8s use an entirely different numbering. Number-1 is the furthest to the front on the passenger’s side, and the cylinders then number serially back on that bank before returning to the front of the opposite bank, with number-5 cylinder.

An exception to the customary GM numbering sequence is the Northstar V-8 used in Cadillac’s and some Buicks and Pontiacs. The number-1 cylinder on a Northstar is opposite the conventional GM system. However, because the Northstar is usually installed transversely, even figuring out which end of the engine is the “front” is difficult. The number-1 cylinder in a transverse Northstar is the cylinder furthest to the passenger’s side on the rearmost cylinder bank.

Even by consulting a factory manual, it can be difficult to ascertain the cylinder-numbering system for the engine. However, it is normally cast into the intake manifold, or stamped into the valve covers, or both. You may have to spend some time removing a few concealing parts to find it, but the designation will be there.    
Now that you know which is “Bank 1,” which is HO2S 2?

Sensors are numbered front to back, so HO2S 1 is the sensor in the exhaust manifold. H02S 2, however, can be a reference to either a downstream sensor or to a pre-catalyst sensor, depending on the configuration of the exhaust system in the car. If the car has dual exhausts with a catalyst in each, Bank 1 HO2S 2 will be the sensor located after the catalyst fed by the bank 1 cylinders. However, if the engine feeds both cylinder banks into a single catalyst, Bank 1 HO2S 2 could refer to either a pre-catalyst sensor—in that case there will also be a downstream sensor: Bank 1, H02S 3—or it could refer to the downstream sensor.  

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