P1273 – Heated oxygen sensor (H02S), bank 1 – lean mixture (Nissan)

Avatar photo
By Reinier (Contact Me)
Last Updated 2023-11-21
Automobile Repair Shop Owner
CodeFault LocationProbable Cause
P1273 Heated oxygen sensor (H02S), bank 1 – lean mixture
(Buy Part On Amazon)
H025, fuel pressure, injectors, intake air leak

We recommend Torque Pro

Table of Contents

  1. What Does Code P1273 Mean?
  2. Where is the P1273 sensor located?
  3. What are the common causes of code P1273?
  4. Get Help with P1273

What Does Code P1273 Mean?

OBD II fault code P1273 is a manufacturer specific code that is defined by carmaker Nissan as “Air Fuel Ratio Sensor 1 Bank 1 Lean Shift Monitoring”, and is set when the PCM (Powertrain Control Module) detects a lean running condition. Note that “lean” refers to a condition where there is too much air (relative to fuel) in the air/fuel mixture. Also, note that other definitions of code P1273 include “Air / Fuel Ratio Sensor 1”, and “Air fuel (AF) ratio sensor 1 – lean mixture”, without specifically referring to the #1 air/fuel ratio sensor on Bank 1, which is the  bank of cylinders that contains cylinder #1.

NOTE: Regardless of the different wording though, all three definitions of code P1273 mean the same thing, which is the PCM has detected a lean running condition.

While wide-band heated, oxygen sensors are reasonably effective in controlling short-term fuel trims, air/fuel sensors are capable of more precise measurements of the oxygen content in exhaust gas than oxygen sensors are. In practice, this means that the adjustments the PCM makes to fuel trims are smaller and more frequent, which translates into improved fuel economy and reduced emissions.

In terms of operating principles, air/fuel sensors also measure the oxygen content of exhaust gas, but the way in which they do it is vastly different from how oxygen sensors work. Unlike oxygen sensors whose signals vary between about 0.9 volts and about 0.3 volts depending on whether the air fuel mixture is rich or lean, an air/fuel ratio sensor measures the exact oxygen content of the exhaust stream, and then generates a signal that corresponds exactly to that value.

Moreover, an air/fuel ratio sensor’s complex internal electronics cannot only measure air/fuel mixtures that range between extremely rich to extremely lean; it can also cause the signal current to flow in two directions, these being positive and negative. In practice, the PCM supplies the air/fuel ratio sensor with a 2.6-volt reference voltage (3.3 volts in most other cases) that serves as the sensor’s power supply. As the air/fuel mixture changes during normal engine operation, the generated signal voltage changes in the positive direction when the air/fuel mixture becomes leaner and in the negative direction when the air/fuel mixture becomes richer.

However, if the sensor is fully functional and there are no defects in the engine or exhaust system, the current flow stops altogether when the composition of the exhaust gas is “stoichiometric” (also known as Lambda = 1) which means that all of the fuel is combusted using all of the available air in the air/fuel mixture. The PCM recognizes the cessation of current flow as the ideal air/fuel mixture for that application, so when current flow starts to flow in any given direction as a result of changes in the air/fuel mixture, the PCM makes the required adjustments to the fuel and ignition systems to the point where the current flow stops again.

On modern applications, these adjustments can happen several times per second, but unlike oxygen sensors that cause the PCM to make relatively large adjustments, the signal from an air/fuel ratio sensor is linear, in the sense that the signal voltage from an air/fuel ratio sensor changes smoothly. Because air/fuel ratio sensors can react to changes in the exhaust stream much faster than an oxygen sensor can, the practical advantage of an air/fuel ratio sensor oven an oxygen sensor is that changes to fuel trims and ignition timing are smaller, because they are made more frequently than on applications that use even advanced oxygen sensors.

Nonetheless, when the PCM detects a lean running condition, it will set code P1273 and illuminate a warning light.

Where is the P1273 sensor located?

The image above shows the typical location of the #1 air/fuel ratio sensor on a 4-cylinder Nissan application. Note that on V-type Nissan engines, the sensor may be located further down the exhaust system, but in all cases of V-type engines, the #1 air/fuel ratio sensor will be located upstream of the catalytic converter on the part of the exhaust system that serves Bank 1.

WARNING: To avoid confusion and a misdiagnosis, always refer to the manual for the affected application to locate and identify #1 air/fuel ratio sensor correctly, since on many applications there may be other sensors located upstream of the catalytic converter as well.

What are the common causes of code P1273?

Note that there is no single cause of this code on Nissan applications that occurs more frequently than any other given cause, or set of circumstances with the possible exception of loss of sensitivity of the sensor’s sensing element. Nonetheless, some possible causes of code P1273 could include the following-

  • Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors, and particularly in wiring that controls the air/fuel ratio sensor’s heating element
  • Exhaust leaks
  • One or more defective fuel injectors
  • Contaminated or defective MAF (Mass Airflow) sensor
  • Defective air/fuel ratio sensor – NOTE: “Defective” includes loss of sensitivity due to long use, as well as contamination of the sensing element by oil, engine coolant, some aftermarket fuel additives, or silicon-based compounds that are commonly found in aftermarket gasket sealers
  • Inadequate fuel pressure, but note that this condition will almost always be indicated by a code or codes other than P1273
  • Unsuitable, or substandard replacement air/fuel ratio sensor
  • Improper air/fuel ratio sensor replacement procedure(s) – NOTE: Because no two air/fuel ratio sensors can ever be identical in all respects, the PCM needs to “learn” to interpret the signal from a replacement air/fuel ratio sensor. Therefore, it is necessary to follow the manufacturer’s prescribed procedures to delete what the PCM has learned from the old sensor, and to allow the PCM to relearn the signals from the replacement sensor. Failure to do this will result in code P1273 setting again, even in the absence of any defect(s).
  • Failed or failing PCM or other control module. Note though that control module failure is rare, so the fault must be sought elsewhere before any control module is replaced

NOTE: Code P1273 can be accompanied by one or more generic codes, with the most common being P0036, P0037, P0038, P0042, P0043, P0044, P0050, P0051, P0052, P0056, P0057, P0058, P0062, P0063 and P0064, all of which relate to faults in the air/fuel ratio sensor’s heating element’s control circuit(s). Thus, if one or more of the above codes are present, these codes must be resolved in the order in which they were stored; failure to do this will result in a misdiagnosis and the unnecessary replacement of parts.

Help Us Help You

Please comment below describing your issue as well as the specifics of your vehicle (make, model, year, miles, and engine). To get a detailed, expedited response from a mechanic, please make a $9.99 donation via the payment button below.