Error Code P1273 is described as Air Fuel Ratio Sensor 1 Bank 1 Lean Shift Monitoring. This refers to a lean condition due to too much oxygen, usually caused by a vacuum leak, fuel pressure, dirty MAF (Mass Airflow sensor), or defective or dirty injectors.

Unlike most error codes, this code is NOT a generic trouble code, meaning it does NOT apply to all vehicles with OBD-II system or vehicles made since 1996 up to present. Rather, it is a vehicle specific code, which applies to certain makes of vehicles, most commonly among Nissan cars.


The A/F or Air Fuel Ratio sensor is a planar dual-cell limit current sensor. It comes with a sensor element that combines with the Nernst concentration cell (sensor cell) with an Oxygen-pump cell, which job is to transport ions. It also has a heater element.

The sensor in this component is capable of precise measurement λ = 1, and also in lean and rich range. Along with its control electronics, the sensor outputs a clear and continuous signal through a wide range of λ range (0.7 < λ < air).

The gas components of the exhaust diffuse through the diffusion gap of the electrode of the oxygen pump and Nernst concentration well. This is where they are brought to thermodynamic balance. There’s an electronic circuit to control the pump current through the oxygen pump cell, to make sure the exhaust gas composition in the diffusion gap remains at a λ = 1 constant. Thus, the A/F sensor can indicate the air/fuel ration through pumping of the current. Aside from that, the integrated heater in the sensor ensures the required operating temperature is at 700 – 800 C (1,292 – 1,472 F).

Common Symptoms

As with other error codes, this code activates the Check Engine light. This kind of lean condition error code doesn’t usually cause drivability issues, other than rough engine idle.

Possible Causes

There are many possible causes that for this problem to arise, some of the most common causes are:

  • Faulty A/F sensor 1 bank 1
  • Open or shorted A/F sensor 1 Bank 1 harness
  • Poor electrical connection in A/F sensor 1 bank 1 circuit
  • Fuel pressure
  • Defective Air Flow Meter
  • Defective Fuel Injector

How to Check

This code suggests lean condition, which means too much oxygen caused by a variety of factors, such as vacuum leaks, fuel pressure, dirty MAF, or bad or dirty fuel injectors. Thus, diagnosis usually starts by checking these components to make sure if they’re the ones causing the lean condition.

How to Fix

The easiest and usually the most common fix for this error code is cleaning the MAF sensor and connector (Using the appropriate cleaner of course).

The next step is cleaning the Oxygen connections.

For cases where the O2 sensor is reading a lean condition, it is a case of either the sensor is correct, or it could be stuck lean (lazy O2 sensor). If this is the case, you can spray a carb spray into the intake with the scanner hooked up for live data for sensor 1. The carb spray will spike up the O2 sensor to a rich condition. If this doesn’t fix the problem, then replacing the O2 sensor could.

Sometimes the problem could be a leak in the flex pipe. In which case, replacing the flex pipe can quickly solve the problem.