Bank 1 vs Bank 2: how to tell which side of the engine the code points at
Bank 1 and Bank 2 refer to the two cylinder rows on a V or flat engine. Here is how to identify them on common V6, V8, and boxer engines.
Bank 1 is the side of the engine that contains cylinder number 1. Bank 2 is the other side. That sounds obvious, and then you realize cylinder 1 is in different places on a Ford 5.0, a Chevy 5.3, a Toyota 3.5 V6, and a Subaru boxer. Pick the wrong bank and you replace the wrong O2 sensor, the wrong VVT solenoid, or the wrong cam sensor. Money wasted, fault unchanged.
Why banks exist as a concept
OBD-II uses banks to describe which cylinder row a sensor or system belongs to on engines with more than one row of cylinders. Inline 4 and inline 6 engines have only one bank and codes do not need to specify. V6, V8, V10, V12, and flat (boxer) engines have two banks, and codes will say “Bank 1” or “Bank 2” to point you at one side.
Bank 1 by engine family
| Engine family | Bank 1 location |
|---|---|
| Ford 4.6 / 5.0 / 5.4 / 5.0 Coyote (V8) | Passenger side (right) |
| Ford 3.5 EcoBoost / 3.7 Cyclone (V6) | Passenger side rear |
| GM LS / LT (5.3, 6.0, 6.2 V8) | Driver side (left) |
| GM 3.6 LFX / LGX (V6) | Rear bank |
| Chrysler 5.7 / 6.4 HEMI | Passenger side |
| Chrysler 3.6 Pentastar | Rear bank |
| Toyota 2GR / 3GR / 1GR (V6) | Rear bank |
| Toyota 2UR / 1UR (V8) | Passenger side |
| Toyota 1UZ / 2UZ / 3UZ (older V8) | Driver side |
| Nissan VQ35 / VQ37 (V6) | Passenger side |
| Subaru FA / FB / EJ (boxer) | Passenger side |
| BMW N52 / N54 / B58 (inline 6) | Single bank, no bank 2 |
| BMW N62 / N63 / S63 (V8) | Driver side |
| Mercedes M156 / M157 / M177 (V8) | Driver side |
| Honda J35 (V6) | Front bank (closest to firewall on transverse) |
| Honda J37 (V6) | Front bank |
A V6 mounted transversely (Camry, Pilot, Sienna, Pacifica) has Bank 1 on the firewall side. A V6 mounted longitudinally (4Runner, Tacoma, Mustang V6) has Bank 1 typically on one specific side per manufacturer.
How to identify bank 1 yourself
The reliable answer for any specific vehicle is the OEM service manual. If you do not have one:
- Find the engine block stamping or look at where cylinder 1 lives on a diagram. Cylinder 1 is on Bank 1.
- On most V engines, cylinder 1 is the front-most cylinder on one bank. The other bank starts with the next cylinder in the firing order.
- For Ford and Mopar, Bank 1 is typically the right side (passenger side in US-spec).
- For GM small-block V8, Bank 1 is the left side (driver side in US-spec). Cylinder 1 is the front cylinder on the left bank.
Where O2 sensor codes show up
| Code | Means |
|---|---|
| P0130 | O2 sensor 1, bank 1, circuit malfunction |
| P0140 | O2 sensor 2, bank 1, circuit |
| P0150 | O2 sensor 1, bank 2 |
| P0160 | O2 sensor 2, bank 2 |
| P0171 | Bank 1 system too lean |
| P0174 | Bank 2 system too lean |
| P0420 | Catalyst efficiency bank 1 |
| P0430 | Catalyst efficiency bank 2 |
| P0014 / P0024 | Camshaft position B timing, bank 1 / bank 2 |
| P0301 to P0312 | Misfire on cylinder X (use the cylinder, then the bank diagram) |
Sensor 1 is upstream of the cat. Sensor 2 is downstream. So P0140 (O2 sensor 2 bank 1 circuit) points at the downstream sensor on the side of the engine that contains cylinder 1.
A practical fix-finding sequence
- Identify the code and which sensor it refers to.
- Identify which bank contains cylinder 1 for your specific engine.
- Locate that sensor under the car or in the engine bay.
- Verify with a scan tool: live data for O2 sensors will be labeled B1S1, B1S2, B2S1, B2S2.
- Replace the correct sensor. Verify the code clears after a drive cycle.
A note on flex-fuel and rebadged engines
Some engines are shared across brands but rotated. A 2GR-FKS in a Tacoma is mounted differently than a 2GR-FE in a Highlander, and bank orientation changes. Always confirm against the specific platform manual, not a generic engine family reference.
Inline engines and banks
An inline 4, inline 5, or inline 6 has only one bank. A code that mentions “Bank 2” on an inline engine is almost always a misreading, a generic code definition pulled from a database, or a fault that does not apply. If you get a Bank 2 code on an inline engine, look at the actual fault circuit, not the bank.