Code
P1145
HUMMER
P — Powertrain
Heated Oxygen Sensor (HO2S) Cross Counts Bank 1 Sensor 3
AI status
Completed
Completed
100%
Causes
- Faulty or aging HO2S (Bank 1 Sensor 3)
- Contaminated sensor (oil, coolant, silicone, fuel additives)
- Exhaust leaks upstream of the sensor
- Degraded or failing catalytic converter affecting downstream signal
- Wiring damage, short to voltage/ground or poor connector connection
- PCM or sensor ground fault
Symptoms
- Malfunction Indicator Lamp (MIL) / Check Engine Light illuminated
- Failed emissions test
- Possible reduced fuel economy
- Intermittent or subtle drivability complaints (rare)
- No obvious symptoms other than stored DTC
What to check
- Retrieve freeze frame and live sensor data using a scan tool; confirm Bank 1 Sensor 3 is the affected sensor
- Visually inspect sensor harness and connector for corrosion, heat damage, or disconnection
- Check for exhaust leaks upstream of the sensor (gaskets, flanges, cracks)
- Monitor HO2S voltages with scan tool or lab scope while engine is warm and under various loads
- Check sensor heater circuit resistance and supply voltage (with ignition on)
- Check short/opens and continuity between sensor and PCM, including grounds
Signal parameters
- Typical HO2S voltage range: ~0.0–1.0 V (narrowband); downstream sensors often sit near mid-rail ~0.4–0.6 V and do not switch rapidly
- Expected downstream cross counts: low (stable signal). Excessive cross counts = frequent transitions between rich/lean thresholds
- Heater circuit resistance (typical): commonly ~5–20 ohms (manufacturer-specific) — check service data
- Heater supply voltage: near battery voltage with ignition ON; heater current draw measured when cold
- Use a lab scope for timing/shape; upstream sensors show fast switching (~1 Hz or more under closed-loop), downstream should be low-frequency or stable
Diagnostic algorithm
- Verify the DTC and note freeze frame and conditions when fault set (temperature, load, RPM).
- With a scan tool, observe live data for Bank 1 Sensor 3 and compare to upstream sensor(s). Look for excessive switching or abnormal amplitude.
- Visually inspect sensor, wiring, and connector. Repair any obvious damage or corrosion; secure connector and re-test.
- Check heater circuit: measure resistance at the sensor, verify heater supply voltage and ground at connector. Repair wiring or replace sensor if heater is open/shorted.
- Check for exhaust leaks upstream of the sensor (use smoke tester or visual inspection). Repair leaks and re-test.
- Measure sensor voltage with a scope while varying load. If sensor output still switches like an upstream sensor, consider sensor contamination or failure.
- Review fuel trim and upstream O2 behavior. If trims fluctuate widely, address fuel delivery or vacuum issues that may cause rapid AFR changes.
- If wiring and engine systems check good, replace the Bank 1 Sensor 3 with a correct OEM-spec sensor. Clear codes and perform a test drive under the same conditions.
- If the code remains after sensor replacement, evaluate catalytic converter efficiency (temperature differential, backpressure, or exhaust gas composition) and inspect PCM and grounds.
- Document repairs and re-check for recurrence; if intermittent, perform longer road tests or data logging.
Likely causes
- Failed or contaminated HO2S (most common)
- Exhaust leak allowing extra air to reach the sensor
- Wiring or connector corrosion/intermittency at the sensor
- Catalytic converter not performing (less common but possible)
- Heater circuit fault or PCM-related issue (less likely)
Fault status
Status
Heated Oxygen Sensor (HO2S) Bank 1 Sensor 3 showing excessive cross counts: downstream O2 sensor signal switching abnormally or too frequently.
Repair difficulty: Medium
Diagnostic time: 1-3 hours
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