P013A – O2 Sensor Slow Response – Rich to Lean

By Reinier (Contact Me)
Last Updated 2020-12-14
Automobile Repair Shop Owner

Trouble CodeFault LocationProbable Cause
P013A O2 Sensor Slow Response - Rich to Lean -

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What Does Code P013A Mean?

OBD II fault code P013A is a generic trouble code that is defined as “O2 Sensor Slow Response – Rich to Lean Bank 1 Sensor 2”, and is set when the PCM (Powertrain Control Module) detects a slow or delayed response from the #2 oxygen sensor. Note that “Bank 1” refers to the bank on cylinders on V-type engines that contains cylinder #1, and that “Sensor 2” refers to the oxygen sensor that is located downstream from the catalytic converter.

All modern gasoline engines use sensors that compare the concentration of oxygen in exhaust gas to the oxygen content of ambient air to determine the quality of the air/fuel mixture. Put differently, this means that the amount of oxygen in the exhaust gas is a direct indication of how close the air/fuel mixture at any given moment is to a stoichiometric value or ratio. For gasoline engines, this ratio is 14.7 parts of air to 1 part of fuel, and in this ratio, all the available air is used to burn all of the fuel.

In practice though, it is not easy to maintain a stoichiometric air/fuel ratio so most modern vehicles are fitted with 2 oxygen sensors for each bank of cylinders. One sensor is fitted before (upstream of) the catalytic converter and one is fitted after (downstream of) the catalytic converter, with each sensor having a different purpose.

As a practical matter, the purpose of the upstream oxygen sensor is to measure the actual level of oxygen in the exhaust stream, and it does this via a complex chemical process or reaction that generates an electrical current that changes in direct response to changing oxygen levels in the exhaust gas. The PCM interprets the changing signal voltages as changes in the oxygen level, and it uses this data to adapt the amount of fuel that is injected into the cylinders. This process is known as “closed-loop” operation, but to prevent large changes to the amount of fuel being injected, the PCM switches the oxygen sensor’s control voltage between reading rich and lean several times per second.

This has the effect of averaging out the changes in fuel delivery volumes over time, meaning that while the PCM has to make many adaptations per second, each change is small, which, makes it easier to maintain the air/fuel mixture at or close to a stoichiometric value.

However, to monitor the fuel delivery process, the PCM uses input data from the downstream oxygen sensor as well. In practice, the purpose of the downstream sensor is to monitor the efficiency of the catalytic converter, since much of the converter’s efficiency is a function of the composition of the air/fuel mixture. However, when this code is diagnosed, a technician will monitor the switching rate of the upstream sensor, which can be as rapid as 20 times or more per second.

In a fully functional system, the switching rate of the upstream sensor will establish a near-constant oxygen level in the exhaust stream, so the technician will confirm that the downstream sensor’s signal voltage remains stable at around 0.45V to about 0.50V. The PCM interprets this voltage as confirmation that a) the catalytic converter’s efficiency is within acceptable limits, and b), that the air/fuel mixture is at, or close to the stoichiometric ratio.

However, although the downstream oxygen sensors on most applications are not directly implicated or involved in fuel control, the PCM expects the downstream oxygen sensor to respond to changes in the air/fuel mixture within a pre-determined time. Thus, a PCM interprets a downstream oxygen sensors’ failure to respond within this time as a failure of the sensor, and when this happens, the PCM will recognize that it cannot control or manage the air/fuel mixture effectively, and it will illuminate a warning light and set code P013A as a result.

Where is the P013A sensor located?

Note that on some applications, it is sometimes very difficult to access the sensor. Therefore, the removal or disassembly of parts of the exhaust system may be required to access the sensor for the purposes of testing and/or replacement.

What are the common causes of code P013A?

The most common causes of code P013A are largely similar across all applications, and could include one or more of the following-

  • Failed, failing, or defective downstream oxygen sensor
  • Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors in the downstream sensor’s control and/or signal circuit(s)
  • Failed, damaged, or clogged catalytic converter
  • One or more serious exhaust leaks
  • Failed or failing PCM but note that since this a rare event, the fault must be sought elsewhere before any control module is replaced

What are the symptoms of code P013A?

Common symptoms of code P013A are largely similar across all applications, but note that the severity of one or more symptoms may vary depending on both the application and the exact nature of the problem-

  • Illuminated warning light
  • One or more additional codes may be set and stored along with P013A
  • Increased fuel consumption
  • Varying degrees of power loss may be present
  • Vehicle may fail a mandatory emissions test
  • One or more readiness monitors may either fail to initiate or fail to complete successfully

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