Faulty Bank 2 Sensor 1 heated oxygen sensor (most common); Contaminated sensor from coolant or oil entering the exhaust stream; Wiring damage or corrosion in the sensor circuit, including the heater circuit; Exhaust leak upstream of the sensor allowing outside air to skew readings; Failed sensor heater element causing the sensor to stay cold and inactive; PCM software or calibration issue (less common)

DTC codes / Mazda codes / P1173

P1173 · MazdaHO2S Bank 2 Sensor 1 Circuit Fixed Bank 2 Sensor 1

Q: What does code P1173 mean?

HO2S Bank 2 Sensor 1 Circuit Fixed Bank 2 Sensor 1. P1173 on Mazda vehicles means the powertrain control module has detected that the Bank 2 Sensor 1 heated oxygen sensor (HO2S) signal is stuck or fixed within a narrow range instead of switching normally. The computer expects the upstream sensor on the bank opposite cylinder one to oscillate rapidly between rich and lean as it corrects fuel delivery, and a fixed signal indicates the sensor or its circuit is not responding to changing exhaust oxygen content.

P1173 on Mazda vehicles means the powertrain control module has detected that the Bank 2 Sensor 1 heated oxygen sensor (HO2S) signal is stuck or fixed within a narrow range instead of switching normally. The computer expects the upstream sensor on the bank opposite cylinder one to oscillate rapidly between rich and lean as it corrects fuel delivery, and a fixed signal indicates the sensor or its circuit is not responding to changing exhaust oxygen content.

SystemPowertrain

Code typeMazda 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 as the system defaults to a richer mixture
Occasional rough idle or hesitation during acceleration
Failed emissions test due to incorrect air-fuel ratio
In rare cases, a sulfur or rotten egg smell from the exhaust
Poor engine performance under load, especially when climbing hills

Common causes, most likely first

Faulty Bank 2 Sensor 1 heated oxygen sensor (most common)
Contaminated sensor from coolant or oil entering the exhaust stream
Wiring damage or corrosion in the sensor circuit, including the heater circuit
Exhaust leak upstream of the sensor allowing outside air to skew readings
Failed sensor heater element causing the sensor to stay cold and inactive
PCM software or calibration issue (less common)

How to diagnose and fix it

Scan all modules for stored codes and freeze-frame data to confirm P1173 is the only oxygen sensor code
Visually inspect the Bank 2 Sensor 1 wiring and connector for damage, corrosion, or loose pins
Check the sensor heater resistance and supply voltage with a multimeter to rule out a failed heater
Perform a smoke test on the exhaust system upstream of the sensor to detect leaks
Use a scan tool to watch the Bank 2 Sensor 1 voltage in real time; a fixed reading confirms the sensor is stuck
Replace the Bank 2 Sensor 1 heated oxygen sensor with a quality OEM or equivalent part
Clear codes and verify the sensor signal switches normally during a test drive

Why a fixed O2 signal is not always the sensor

A stuck signal can be caused by a rich-running condition from a leaking injector or high fuel pressure, which contaminates the sensor and makes it read fixed; replacing the sensor without fixing the root cause leads to repeat failure.
Exhaust leaks between the cylinder head and the sensor pull in outside oxygen and can make a good sensor appear lazy or fixed, so always smoke-test the exhaust before condemning the sensor.
On some Mazda engines, a failing ignition coil or spark plug on Bank 2 can send unburned fuel into the exhaust and skew the sensor reading, setting P1173 as a secondary code.
The sensor heater circuit is often overlooked; a heater that is weak but not open can keep the sensor from reaching operating temperature, causing a fixed signal at idle and low speeds.
After replacing the sensor, the PCM may need a drive cycle or relearn procedure to reset fuel trims; simply clearing the code may not be enough for the monitor to run and confirm the repair.

Can I keep driving with P1173?

You can usually drive the vehicle short distances to a repair shop, but avoid heavy acceleration, towing, or long trips because the fixed sensor signal can cause poor fuel economy and increased emissions. If the check engine light starts flashing or the engine runs rough, stop driving and have it towed to prevent catalytic converter damage.

Tools that help

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