P2271 – Heated oxygen sensor (H02S) 2, bank 1 – signal stuck rich


By Reinier (Contact Me)
Last Updated 2020-12-19
Automobile Repair Shop Owner

Trouble CodeFault LocationProbable Cause
P2271 Heated oxygen sensor (H02S) 2, bank 1 - signal stuck rich Wiring, H025, fuel pressure, injectors, air intake restricted

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What Does Code P2271 Mean?

OBD II fault code P2271 is a generic trouble code that is defined as “Heated oxygen sensor (H02S) # 2, Bank 1 – Signal Stuck Rich”, and is set when the PCM (Powertrain Control Module) detects a signal voltage from the downstream oxygen sensor that indicates a constant rich-running condition. Note that “Bank 1” refers to the bank of cylinders on V-type engines that contains cylinder #1, while “oxygen sensor #2” or “downstream sensor” refers to the oxygen sensor that is located after the catalytic converter.

All modern gasoline engines use oxygen sensors located before and after the catalytic converter(s) to monitor the air/fuel mixture by measuring the level of oxygen in the exhaust stream. The oxygen in the exhaust gas reacts with a sensing element in the sensor, which reaction creates fluctuations in a constant 5V reference voltage that is supplied to the sensor by the PCM.

In practice, the changes in the reference voltage are passed back to the PCM in the form of a signal voltage, which the PCM interprets as deviations from an ideal stoichiometric ratio. Therefore, since the changes in the signal voltage are directly proportional to the level of oxygen in the exhaust gas, the PCM either adds or subtracts fuel from the mixture* to establish and maintain an air\fuel mixture that contains 14.7 parts of air, to one part of fuel. This ratio is known as a stoichiometric ratio, which means that when the air/fuel mixture is combusted, all of the air is used to combust all of the fuel in the mixture.

* Note that this process is known as “closed-loop operation”, which means that the PCM uses input data from both sensors to control the air\fuel mixture. Note also that closed-loop operation only starts when both oxygen sensors and the catalytic converter are heated to their operating temperatures by integrated heating elements that are controlled and monitored by the PCM.

As a practical matter, though, only the upstream oxygen sensor is directly involved with active fuel control. The downstream sensor, on the other hand, is typically* used to monitor the efficiency of the catalytic converter, and in a fully functional system, the signal voltage from the downstream sensor should remain constant at between about 0.45 volts, and about 0.5 volts during steady cruising conditions.

* There are a few exceptions to this rule. In some applications, the downstream oxygen sensor has limited fuel control functionality.

However, during deceleration, the air/fuel mixture will typically tend towards a momentary lean-running condition because of the engine’s sudden decrease in demand for fuel, and the downstream sensor’s signal voltage should change to reflect the lean-running condition within a time limit set by the vehicle manufacturer.

Although this time limit varies between applications, it is strictly observed by the fuel control systems on all applications because a violation of this limit has serious negative effects on the total amount of emissions a vehicle produces. Therefore, when a fault, failure, or defect occurs that does not produce the expected lean condition on the one hand or prevents the PCM from detecting the expected lean condition, on the other hand, the PCM will recognize that it cannot control the air/fuel mixture effectively, and it will set code P2271 and illuminate warning light as a result.

Note, though, that on most applications, the fault must be detected on two consecutive trips before a code is set, or a warning light is illuminated.

Where is the P2271 sensor located?


This image shows the location of the downstream oxygen sensor on a MINI application. In this example, the red arrow indicates the actual oxygen sensor;  the green arrow indicates the direction of flow through the exhaust system, and the yellow arrow indicates the catalytic converter.

Note that while the oxygen sensor is relatively easily accessible in this example, this is not always the case in other applications. In some applications, it may be necessary to remove all or part of the exhaust system, and/or splash plates or other components such as fuel lines to gain easy access to the downstream oxygen sensor for testing or replacement.

What are the common causes of code P2271?

Common causes of code P2271 are largely similar across all applications, and could include one or more of the following-

  • Defective, malfunctioning, or contaminated downstream oxygen sensor
  • Use of some aftermarket oxygen sensors
  • Defective, malfunctioning, damaged, or clogged catalytic converter
  • Use of “rebuilt”, or substandard aftermarket catalytic converter
  • Exhaust system leaks both upstream and downstream of the downstream oxygen sensor
  • Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors
  • Excessive fuel pressure
  • One or more damaged or leaking fuel injectors
  • Excessive hydrocarbon loads caused by excessive mechanical wear in the engine
  • Oil level too high, or use of unsuitable engine oil
  • Engine coolant leaks that could contaminate the downstream oxygen sensors’ reference air
  • Oil leaks on the engine that could contaminate the downstream oxygen sensors’ reference air
  • Failed or failing PCM, but note that since this a rare event, the fault must be sought elsewhere before any control module is replaced or reprogrammed

What are the symptoms of code P2271?

Some symptoms of code P2271 are common to all applications, and could include one or more of the following, but note that this code rarely, if ever, causes noticeable drivability problems-

  • Stored trouble code an illuminated warning light
  • Depending on both the application and the exact nature of the problem, multiple additional codes associated with one or more (seemingly) unrelated systems and/or engine components may be present
  • Some readiness monitors may not initiate, or may not complete successfully
  • In rare cases, the engine may sometimes exhibit a slight roughness at some engine speeds
  • Affected vehicles may fail a mandatory emissions test

BAT Team Discussions for P2271

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    2000 ford taurus 3.0 dura tech 24 valve 90,000 miles. Yeah I’m in a road salt area. I had the codes ran at advanced and this is what came back: 1. P0136 O2 sensor ciruit bank 1 sensor 2 (the guy at advanced said this is the upstream sensor?) (P2270, P2271) 2. P1131 The code tester at advanced d...