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DTC codes / GM codes / P1155

P1155 · GMHO2S Lean Average Bank 2 Sensor 1

P1155 on a GM vehicle means the powertrain control module has detected a lean average fuel trim condition from the upstream oxygen sensor on bank 2, which is the side of the engine that does not contain cylinder number one. The computer is not saying the sensor itself has failed; it is reporting that the exhaust stream on that bank contains too much oxygen over a sustained period, so the engine is running lean on that side.

SystemPowertrain
Code typeGM specific
SeverityMedium
Fix difficultyIntermediate
AreaFuel and air metering

Symptoms you may notice

  • Often no symptoms besides the warning light, especially if the lean condition is mild and the long-term fuel trim has not yet maxed out.
  • A noticeable hesitation or stumble on acceleration when the engine is cold or under light load.
  • Rough idle quality that may feel like a slight misfire on one side of the engine.
  • Reduced fuel economy as the computer adds extra fuel trying to correct the lean average.
  • In some cases a sulfur or rotten egg smell from the exhaust if the lean condition overheats the catalytic converter.
  • Intermittent surging or a lean pop through the intake during hard acceleration.

Common causes, most likely first

  1. A vacuum leak on the bank 2 side of the intake manifold, including cracked intake gaskets, deteriorated vacuum hoses, or a leaking brake booster check valve.
  2. A dirty or failing mass airflow sensor that under-reports airflow, causing the computer to command less fuel than the engine actually needs.
  3. A weak or failing fuel pump, restricted fuel filter, or low fuel pressure that affects bank 2 more than bank 1 due to fuel rail design.
  4. Leaking or clogged fuel injectors on bank 2, particularly a partially clogged injector that delivers less fuel than commanded.
  5. An exhaust leak upstream of the bank 2 sensor 1 oxygen sensor, which lets outside air into the exhaust stream and fools the sensor into reporting a false lean condition.
  6. A failing bank 2 sensor 1 oxygen sensor that is slow to respond or has a contaminated sensing element, though this is less common than a true lean condition.
  7. A mechanical engine problem such as a burnt valve or worn camshaft lobe on the bank 2 side that reduces airflow through that bank.

How to diagnose and fix it

  1. Connect a scan tool and record the long-term and short-term fuel trim values for bank 2 sensor 1 at idle and at 2500 rpm to confirm the lean condition and see whether it is worse at idle or under load.
  2. Visually inspect all vacuum hoses, intake gaskets, and the brake booster check valve on the bank 2 side for cracks or loose connections, using a smoke machine if available.
  3. Check fuel pressure at the rail and compare it to the manufacturer's specification, then inspect the fuel filter for restriction if pressure is low.
  4. Perform a propane or carburetor cleaner enrichment test around the intake manifold and vacuum lines on bank 2 to pinpoint a vacuum leak that is not visible.
  5. Inspect the exhaust system upstream of the bank 2 sensor 1 oxygen sensor for leaks at the manifold flange, gasket, or downpipe, and repair any leaks before condemning the sensor.
  6. Swap the bank 2 sensor 1 oxygen sensor with a known-good sensor from bank 1 and see if the lean code follows the sensor, which confirms a faulty sensor rather than a true lean condition.
  7. If all external causes are ruled out, perform a cylinder leak-down and compression test on the bank 2 cylinders to check for mechanical faults such as a burnt valve or worn piston rings.

What a lean bank 2 code really means

  • A P1155 does not automatically mean the oxygen sensor is bad; in fact, a true lean condition from a vacuum leak or low fuel pressure is far more common, and replacing the sensor first often wastes money.
  • Bank 2 is the side of the engine that does not contain cylinder number one, so on a V6 or V8, a vacuum leak that only affects that bank is frequently caused by a failed intake manifold gasket or a cracked vacuum port on that side.
  • An exhaust leak between the engine and the upstream oxygen sensor can pull outside air into the exhaust and create a false lean reading, so always inspect the exhaust before trusting the sensor data.
  • A dirty mass airflow sensor can cause a lean average on both banks, but if bank 2 is worse, the problem is more likely to be a bank-specific vacuum leak or fuel delivery issue.
  • If the long-term fuel trim on bank 2 is maxed out at a positive value and the engine has a rough idle, a mechanical fault such as a burnt valve should be suspected before replacing any sensors.

Can I keep driving with P1155?

You can drive the vehicle short distances to a repair shop, but avoid heavy acceleration, towing, or sustained high-speed driving because a lean condition can overheat the catalytic converter and cause expensive damage. If the check engine light starts flashing or the engine runs rough, stop driving and have it towed.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for GM vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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