Code
P1165
OLDSMOBILE
P — Powertrain
HO2S Cross Counts Bank 2 Sensor 2
AI status
Completed
Completed
100%
Causes
- Failing or contaminated Bank 2 Sensor 2 (downstream O2 sensor)
- Damaged, corroded, or shorted wiring/connectors to the sensor
- Exhaust leaks upstream of the sensor
- Degraded or inefficient catalytic converter on bank 2
- Fuel mixture or engine management problems causing abnormal switching
- PCM/software fault (rare)
Symptoms
- Check Engine/MIL illuminated
- Failed or marginal emissions test
- Little or no obvious drivability problems in many cases (downstream sensor typically affects catalyst monitoring rather than fuel control)
- Reduced fuel economy (possible)
- Occasional roughness if root cause also affects air/fuel mixture
What to check
- Retrieve freeze-frame and pending data with a scan tool; note engine conditions when the code set
- Compare live O2 sensor voltage traces: Bank 1 vs Bank 2, upstream vs downstream
- Confirm heater circuit operation by checking heater current/resistance and observing warm-up behavior
- Visually inspect sensor and harness for damage, corrosion, or disconnection
- Check for exhaust leaks on bank 2 (manifold, gaskets, pipes)
- Measure sensor voltages with a digital voltmeter or lab scope while engine is in closed loop
Signal parameters
- Typical O2 sensor voltage range: ~0.0–1.0 V (upstream sensors switch frequently in closed loop)
- Downstream sensor (Bank 2 Sensor 2) is expected to show lower switching activity; excessive cross-counts indicate abnormal switching
- Heater resistance: commonly a few ohms to a couple dozen ohms (manufacturer-specific) — verify against service data
- Cross-counts: number of rich/lean switching events recorded over a defined time; thresholds vary by manufacturer — compare bank 1 vs bank 2
Diagnostic algorithm
- Connect a capable scan tool and record live data and freeze-frame information for the fault event
- Compare downstream HO2S behavior: view voltage traces for Bank 2 Sensor 2 and Bank 1 Sensor 2; if Bank 2 shows excessive switching compared with Bank 1, proceed
- Perform a visual inspection of the sensor and connector; repair any obvious damage or corrosion
- Check heater circuit: disconnect sensor, measure heater resistance and compare to spec; verify voltage/power to heater with key on/engine off
- Backprobe the signal wire with the engine running and observe voltage with a scope or DVOM; confirm whether the signal is noisy, stuck, slow, or switching improperly
- Wiggle test wiring while monitoring signal to detect intermittent opens/shorts
- Inspect for exhaust leaks upstream of the downstream sensor; repair leaks and retest
- Check fuel control system and related sensors (MAP/MAF, fuel pressure, injectors) to rule out mixture problems that could affect sensor switching
- If wiring and upstream systems check good but downstream behavior is abnormal, consider catalytic converter condition testing (temperature rise across converter, backpressure, tailgas)
- Replace the Bank 2 Sensor 2 only after confirming wiring and exhaust/catalyst condition, then clear codes and perform a full drive cycle to verify repair
Likely causes
- Aging or contaminated downstream O2 sensor
- Open/short or high-resistance connection in the sensor circuit
- Exhaust manifold/header or gaskets leaking near bank 2
- Catalyst on bank 2 not reducing switching activity (worn or clogged)
- Fuel trim imbalance that alters O2 sensor behavior
Fault status
Status
HO2S Cross Counts Bank 2 Sensor 2 — downstream oxygen sensor switching frequency/cross-counts outside expected range; possible sensor, wiring, exhaust leak, or catalytic converter issue.
Repair difficulty: Medium
Diagnostic time: 1-3 hours
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