P1156

Code

CADILLAC P — Powertrain

HO2S Rich Average Bank 2 Sensor 1

Brand: CADILLAC

Type: P — Powertrain

Views: UK: 28 EN: 50 RU: 41

AI status

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Page language: EN

Causes

Faulty or contaminated Bank 2 Sensor 1 (HO2S)
Wiring or connector fault (short to voltage, open, high resistance, poor ground)
Fuel system over-fueling (high fuel pressure, leaking injector(s), stuck fuel pressure regulator)
Faulty mass airflow (MAF) or manifold absolute pressure (MAP) sensor or incorrect intake air readings
Incorrect engine coolant temperature (ECT) sensor reading causing enrichment
Exhaust leak downstream/upstream causing abnormal sensor readings (less common for rich average)

Symptoms

Check Engine Light (MIL) illuminated
Poor fuel economy (higher fuel consumption)
Black smoke from tailpipe or rich-smelling exhaust
Rough idle or hesitation under acceleration (may be intermittent)
Failed emissions test (elevated HC/CO/CO2)
Possible reduced catalyst efficiency over time

What to check

Scan for all stored/pending codes and freeze frame data; note related codes (fuel, air metering, other O2 sensors)
Check live data: Bank 2 Sensor 1 voltage and compare to Bank 1 Sensor 1 and downstream sensors
Inspect wiring harness and connector for Bank 2 Sensor 1 for corrosion, damage, or pin push-out
Measure sensor heater supply and ground (12V+/control and continuity to ground) and heater resistance per spec
Check short-term and long-term fuel trims (STFT/LTFT) for Bank 2 and overall fuel trim behavior
Verify fuel pressure and regulator operation under static and dynamic conditions

Signal parameters

Typical narrowband HO2S voltage range: ~0.1 V (lean) to ~0.9 V (rich) under closed-loop
Sensor should switch regularly (approx 1 Hz switching under stable closed-loop conditions) between low and high voltages
A 'rich average' will show the Bank 2 Sensor 1 spending more time at higher voltages or an elevated average voltage
Heater circuit: continuity expected (typical resistance roughly a few ohms to a few tens of ohms depending on sensor; check OEM spec) and relay/fuse/PCM should provide heater voltage when commanded
Short-term fuel trim (STFT) typically oscillates around 0%; sustained large negative trims on Bank 2 suggest the ECU is pulling fuel back from a perceived rich condition

Diagnostic algorithm

Retrieve codes and live data; record Bank 2 Sensor 1 voltage waveform, STFT/LTFT for Bank 2, and compare to Bank 1.
Visually inspect sensor and connector; repair any damaged wiring or corroded pins. Wiggle test wiring while watching live sensor voltage for intermittent faults.
Measure sensor heater resistance and verify heater supply voltage/ground when engine is cold and key-on/engine-running per OEM procedure.
If wiring and heater are good, monitor sensor voltage: if stuck high or slow to switch, consider replacing the sensor. Confirm replacement resolves signal behavior.
Check fuel pressure at the rail at idle and under load; compare to specification. Repair fuel pressure regulator or leaking injectors as needed.
Inspect and test MAF, MAP, and ECT sensors: clean or replace MAF if contaminated; verify ECT reads correctly to ECM.
Check for intake or exhaust leaks; use smoke test for intake leaks and inspect exhaust near the upstream sensor for leaks.
After repairs, clear codes and perform a drive cycle; verify Bank 2 Sensor 1 switches normally and fuel trims return to expected range. Re-scan to ensure no further P1156 or related codes.

Likely causes

Bank 2 upstream O2 sensor failing or contaminated with oil/coolant/fuel
Sensor signal wire shorted to battery voltage or heater supply
Fuel rail pressure too high (weak return, stuck regulator) or one or more injectors leaking
MAF giving low airflow signal (aftermarket or contaminated sensor) causing overfuelling
Faulty coolant temp sensor commanding a cold-rich mixture
Poor ground at sensor or ECM causing biased voltage readings

Fault status

⚠️ Status

Bank 2 Sensor 1 (upstream HO2S) reporting a persistently rich average signal (high voltage bias) — possible sensor, wiring, or fuel/air system fault causing over-rich condition on Bank 2.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1-3 hours

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P1156 — HO2S Rich Average Bank 2 Sensor 1