🚨 Diagnostic Profile
- Vehicle/Equipment Make: CADILLAC
- Technical Description: HO2S Lean Average Bank 2 Sensor 1
This Powertrain code is monitored by the Engine Control Module (ECM) or Powertrain Control Module (PCM). P1155 indicates that the Heated Oxygen Sensor (HO2S) located on Bank 2, Sensor 1 (upstream of the catalytic converter, on the bank of the engine that does NOT house cylinder #1) is reporting a lean average voltage signal that falls outside of the expected calibrated range. The PCM continuously monitors the voltage output of this sensor to calculate short and long-term fuel trims. When the sensor’s average signal remains skewed toward the lean side (indicating excess oxygen in the exhaust stream) for an extended period, the PCM sets this code and often illuminates the Check Engine Light (MIL). Common root causes include a vacuum leak on the Bank 2 intake runner, a failing fuel injector, low fuel pressure, an exhaust leak upstream of the sensor allowing outside air to dilute the sample, or a degraded oxygen sensor element itself. In some cases, high resistance in the sensor’s wiring harness or connector can also cause an inaccurate lean reading to be reported to the ECM.
🚨 Repair Difficulty Tier
- Difficulty Rating: Intermediate
Diagnosing this code correctly requires a quality scan tool capable of viewing live Mode 06 data and bi-directional fuel trim graphs, plus a digital multimeter for wiring continuity checks. While sensor replacement itself is straightforward bolt-on work for a DIYer with basic hand tools, pinpointing whether the root cause is the sensor, a vacuum leak, or a fueling issue requires some diagnostic experience to avoid replacing good parts.
🛠️ Resolution Steps
- Step 1: Preliminary Inspection
Visually inspect the Bank 2 side of the engine for any disconnected, cracked, or brittle vacuum hoses and intake boot connections. Check the exhaust manifold and downpipe near the Bank 2 Sensor 1 location for any leaks, rust-through, or loose bolts that could allow fresh air to contaminate the exhaust sample. Inspect the O2 sensor’s electrical connector for corrosion, bent pins, or melted insulation from heat exposure. - Step 2: Diagnostic Verification
Connect a scan tool and monitor live Bank 2 Sensor 1 voltage and fuel trim data at idle and under light throttle. A healthy sensor should fluctuate actively between roughly 0.1V and 0.9V; a code-triggering sensor will often read a flat, low voltage (lean) with minimal switching activity. Use a smoke machine, if available, to pressure-test the intake tract on Bank 2 for hidden vacuum leaks. Check fuel trim data (Short Term and Long Term Fuel Trim) for Bank 2 — persistent high positive trim values confirm an actual lean condition rather than a faulty sensor reading. - Step 3: Resolution & Reset Process
If a vacuum or exhaust leak is found, repair or replace the damaged hose, gasket, or manifold component and re-test. If the wiring is compromised, repair the harness with proper soldering and heat-shrink or replace the damaged section. If the sensor itself tests faulty with no supporting lean conditions found elsewhere, replace the Bank 2 Sensor 1 HO2S with an OE-quality unit, ensuring the threads are treated with anti-seize compound before installation. After repairs, clear the code using the scan tool and perform a drive cycle to confirm the fuel trims normalize and the code does not return.
📦 Parts, Labor, and Budget Estimates
- Required Components/Software: OE-quality Bank 2 Sensor 1 Oxygen Sensor, intake vacuum hose/gasket set (if applicable), OBD-II scan tool with live data/Mode 06 capability, digital multimeter
- Average Component Price: $60 – $150 USD (sensor), $15 – $60 USD (hoses/gaskets if needed)
- Estimated Labor Hours: 0.5 – 1.5 hours
- Total Estimated Budget: $100 – $280 USD