A worn or contaminated bank 2 downstream oxygen sensor that no longer switches cleanly; An exhaust leak between the bank 2 catalytic converter and the downstream sensor that lets outside air skew the reading; A failing bank 2 catalytic converter that has lost its oxygen storage capacity; Damage or corrosion in the sensor wiring harness or connector, especially near the exhaust and road splash areas; An engine misfire or rich running condition on bank 2 that has contaminated the sensor or converter; Oil or coolant consumption entering the exhaust stream on bank 2 and coating the sensor element; A restricted exhaust or a leaking injector on bank 2 that alters the exhaust gas composition

DTC codes / GM codes / P1165

P1165 · GMHO2S Cross Counts Bank 2 Sensor 2

Q: What does code P1165 mean?

HO2S Cross Counts Bank 2 Sensor 2. P1165 on a GM vehicle indicates that the powertrain control module has detected an abnormal number of voltage cross counts from the downstream heated oxygen sensor on bank 2, which monitors the catalytic converter on the side of the engine that does not contain cylinder number one. The computer uses cross counts to judge whether the sensor is switching actively and whether the converter is storing and releasing oxygen as designed, so a low or erratic count tells the module that the sensor signal is lazy or the exhaust stream is not behaving as expected.

P1165 on a GM vehicle indicates that the powertrain control module has detected an abnormal number of voltage cross counts from the downstream heated oxygen sensor on bank 2, which monitors the catalytic converter on the side of the engine that does not contain cylinder number one. The computer uses cross counts to judge whether the sensor is switching actively and whether the converter is storing and releasing oxygen as designed, so a low or erratic count tells the module that the sensor signal is lazy or the exhaust stream is not behaving as expected.

SystemPowertrain

Code typeGM specific

SeverityMedium

Fix difficultyIntermediate

AreaFuel and air metering

Symptoms you may notice

Illumination of the check engine light, often with no other noticeable symptoms
A slight decrease in fuel economy over time as the system runs less efficiently
Occasional rough idle or hesitation that is more noticeable when the engine is cold
A faint sulfur or rotten egg smell from the exhaust if the converter is being overworked
Failed emissions testing because the readiness monitor for the oxygen sensor or catalyst will not complete
In rare cases, a stumble or surge under light acceleration when the module adjusts fuel trim based on the faulty signal

Common causes, most likely first

A worn or contaminated bank 2 downstream oxygen sensor that no longer switches cleanly
An exhaust leak between the bank 2 catalytic converter and the downstream sensor that lets outside air skew the reading
A failing bank 2 catalytic converter that has lost its oxygen storage capacity
Damage or corrosion in the sensor wiring harness or connector, especially near the exhaust and road splash areas
An engine misfire or rich running condition on bank 2 that has contaminated the sensor or converter
Oil or coolant consumption entering the exhaust stream on bank 2 and coating the sensor element
A restricted exhaust or a leaking injector on bank 2 that alters the exhaust gas composition

How to diagnose and fix it

Connect a scan tool and record the bank 2 downstream oxygen sensor cross counts and voltage activity along with any stored freeze-frame data
Visually inspect the bank 2 downstream sensor, its connector, and the exhaust piping for leaks, corrosion, or physical damage
Perform a propane or smoke test around the bank 2 exhaust joints to confirm there is no unmetered air entering ahead of the sensor
Check the bank 2 sensor heater circuit resistance and supply voltage to rule out a heater fault that is dragging down sensor performance
Compare bank 2 downstream sensor activity with bank 1 downstream sensor activity on a warm engine to see whether the problem is sensor-specific or converter-specific
If the sensor is original and the cross counts are low or flat, replace the bank 2 downstream oxygen sensor with a quality direct-fit unit and recheck the counts
If cross counts remain abnormal after sensor replacement, perform a back-pressure or temperature test on the bank 2 catalytic converter before condemning it

What cross counts reveal about the converter

Cross counts are simply the number of times the oxygen sensor voltage crosses the midpoint threshold, so a sensor that stays rich or lean for long periods will log very few counts and set this code even though the sensor is still producing a voltage signal.
A common misdiagnosis is replacing the bank 2 downstream oxygen sensor when the real problem is an exhaust leak at the converter flange, because a small leak pulls in outside oxygen and makes the sensor read leaner than the exhaust actually is.
The bank 2 downstream sensor on many GM V6 and V8 engines is difficult to reach, and technicians sometimes damage the connector or pigtail during replacement, which creates a new cross-count fault immediately after the repair.
This code does not always mean the catalytic converter is failing, because a lazy upstream sensor on bank 2 can push the fuel mixture around enough to confuse the downstream cross-count logic without any converter fault at all.
On some GM platforms the downstream oxygen sensor cross-count test is part of the catalyst monitor, so a P1165 can appear alongside a P0420 or P0430 even when the converter itself is still functioning within specification.

Can I keep driving with P1165?

You can usually continue driving with P1165 because the fault affects emissions monitoring more than immediate engine safety, but avoid ignoring it for long periods since a rich or misfiring condition on bank 2 can overheat and destroy the catalytic converter. Have the vehicle diagnosed promptly if the check engine light is flashing or if you notice rough running, and do not tow a heavy load until the fault is resolved.

Tools that help

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