Bank 1 vs bank 2 fuel trim: what a split between the banks means
When one bank's fuel trims move and the other bank's stay near zero, the cause belongs to that bank only: an injector, an exhaust leak ahead of that bank's upstream sensor, that bank's sensor, a misfire on one of its cylinders, or an intake leak close to that bank's runners. Anything both banks share, such as the MAF, the fuel pressure, the purge valve, the coolant sensor or a leak into the common plenum, moves both banks together. So the first question on a V engine is not how lean or how rich, but whether both banks agree.
What bank 1 and bank 2 mean
Bank 1 is the side of the engine that holds cylinder 1; bank 2 is the other side. Sensor 1 is the upstream sensor ahead of the converter and sensor 2 is downstream, so B1S1 is bank 1's upstream sensor. Which side cylinder 1 is on, and so which physical side bank 1 is, is set by the manufacturer and varies, so check the service information before touching a sensor. An inline engine has only bank 1.
The lean codes follow the same numbering: P0171 is bank 1 too lean and P0174 is bank 2 too lean, with P0172 and P0175 for rich. One lean code alone on a V engine already says the problem may belong to one bank.
Shared causes and one bank causes
- Both banks move together: a leak into the common plenum, a MAF reading low or high, fuel pressure too high or too low, a purge valve stuck open, a coolant sensor reading cold.
- One bank moves, positive: an exhaust leak ahead of that bank's sensor, a sensor biased lean, a misfire on one of its cylinders, or an intake leak at that bank's runners.
- One bank moves, negative: a leaking injector on that bank.
In the TrimCase model a difference of 5 points or more between the banks' totals in any cell counts as a split. On a real engine, small differences between banks are normal, so a split matters most when one bank is near zero and the other is well out.
Reading a bank split, step by step
- Confirm the bank numbering. Find which side holds cylinder 1 in the service information before naming a side.
- Add the trims per bank per cell. Short plus long term trim for each bank at idle, 2,500 rpm with no load and cruise.
- Decide shared or one bank. Both banks together points to something they share. One bank alone points to something on that bank.
- Check that bank's misfire counters. Counts on one of its cylinders make that bank's trims positive and its downstream sensor read rich.
- Compare the downstream sensors. A downstream sensor reading rich on the bank that is adding fuel means its upstream sensor is being fooled.
- Confirm on the car. Inspect for exhaust leaks ahead of the sensor, test the injectors on that bank, or swap parts across banks where the service information allows, and see whether the problem follows.
Worked example: three bank 2 faults on a V6
Three faults on bank 2 of the model's 3.5 L V6, each at reference size with no noise. Bank 1 is healthy in all three, which is the first clue. The rest of the screen names the fault.
| Fault on bank 2 | Bank 2 idle | Bank 2 cruise | Bank 1 idle | Bank 2 downstream | Misfires |
|---|---|---|---|---|---|
| Exhaust leak | +12.0 | +3.8 | 0.0 | 0.66 V | none |
| Sensor biased lean | +7.0 | +7.0 | 0.0 | 0.90 V | none |
| Misfire, cylinder 4 | +8.0 | +6.0 | 0.0 | 0.90 V | cyl 4: 120 |
| Leaking injector | -12.0 | -2.7 | 0.0 | 0.66 V | none |
All four leave bank 1 at 0.0. The exhaust leak is positive at idle and fades at cruise with a normal downstream sensor. The biased sensor is positive by the same +7.0 everywhere, with bank 2's downstream sensor at 0.90 V, rich. The misfire also drives the downstream sensor rich, but the counter on cylinder 4 names it. The leaking injector is the only negative one.
Common mistakes
- Assuming bank 1 is the driver's side. It is the side with cylinder 1, and that varies.
- Replacing both upstream sensors when only one bank moved.
- Blaming the sensor on the moving bank without reading its misfire counters and downstream sensor.
- Looking for a one bank cause when both banks moved together. A shared fault, such as a plenum leak or fuel pressure, is more likely.
- Swapping a sensor to the other bank without checking the service information for connector and wiring differences.
Practise it
- The daily drill uses V6 and V8 engines as well as inline fours, so bank splits come up every week.
- Read the misfire and exhaust leak fingerprints, the two most confused one bank faults.
- Follow the diagnostic order, where comparing the banks is step nine.
Sources
Last reviewed . Standards and regulations are named so you can look them up; the service information for the vehicle always governs.
- SAE J1979, the OBD II diagnostic services standard, and its successor SAE J1979-2 for OBD on UDS. They define the fuel trim parameters (PIDs 06 to 09: short and long term trim for bank 1 and bank 2, scaled from minus 100 to plus 99.2 percent, where positive means fuel is being added) and the mass airflow parameter (PID 10, in grams per second).
- SAE J2012, which defines the standard trouble code meanings, including P0171 and P0174 (system too lean, bank 1 and bank 2) and P0172 and P0175 (system too rich, bank 1 and bank 2).
- Manufacturer service information for the vehicle in front of you. It sets the real trim limits, when a lean or rich code is allowed to set, which side is bank 1, and the confirmation tests. These vary by make, engine and model year, so no number on this page replaces them.
- The TrimCase engine model, which computes every number in the worked example on this page.
TrimCase is a training aid built on a simulated engine model. It is not a diagnostic instruction for any real vehicle.
Frequently asked questions
Which side of the engine is bank 1?
The side that holds cylinder 1. Which physical side that is depends on the manufacturer and the engine, so check the service information rather than assuming the driver's or passenger's side.
Why is only bank 1 lean?
Because the cause belongs to bank 1: an exhaust leak ahead of its upstream sensor, a lean biased sensor, a misfire on one of its cylinders, or an intake leak at its runners. Anything the banks share would move both.
Can a vacuum leak affect only one bank?
A leak into the common plenum affects both banks about equally. A leak at one bank's intake runners or its gasket can affect that bank more than the other.
How much difference between banks is normal?
A few points. TrimCase counts a difference of 5 points or more in any cell as a split. On a real engine, look hardest when one bank is near zero and the other is well away from it.