|Trouble Code||Fault Location||Probable Cause|
|P2255||Heated oxygen sensor (H02S) 1, bank 2, negative current control - circuit low||Wiring short to ground, H025|
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What Does Code P2255 Mean?
OBD II fault code P2255 is a generic code that is defined as “Heated oxygen sensor (H02) #1, Bank 2, negative current control – circuit low”, and is set when the PCM (Powertrain Control Module) detects an abnormally low voltage in the control circuit of the heater element in the #1 (upstream of the catalytic converter) oxygen sensor on the bank of cylinders on V-type engines that does not contain cylinder #1.
NOTE: In the case of code P2255, the heater element is controlled through the negative (ground) circuit, which is supplied through the PCM. Note also that in the vast majority of instances of this code, the fault is located in the wiring between the oxygen sensor and the PCM.
The purpose of oxygen sensors is to provide the PCM with input data about the composition of the exhaust stream, and with particular reference to the amount of oxygen relative to ambient air. This data is used by the PCM to make suitable adjustments to the air/fuel mixture and other settings/parameters such as ignition timing, to ensure that the engine is always supplied with an air/fuel mixture that is as close to the ideal 14.7: 1 air/fuel mixture as possible.
However, since modern wide-band oxygen sensors work only when their sensing elements are at a specified temperature (typically about 6000C/1 1000F), all wide-band oxygen sensors are fitted with electronically controlled heating elements that can bring a sensor up to operating temperature in a matter of only a few seconds. Short warm-up times mean that the sensor enters closed loop operation sooner than is possible when only the heat of the exhaust stream is available as a heat source. Note that “closed loop” operation refers to a condition where the oxygen sensor generates a signal voltage that is used by the PCM to make adjustments to the air/fuel mixture in response to the input data from the oxygen sensor.
Until comparatively recently, the only way a PCM had of estimating the temperature of an oxygen sensor was to compare the known values of various operational parameters with values programmed into look-up tables. The obvious disadvantages of this system were that the PCM could not account for resistance variations between oxygen sensors, and that calibration of the system was complex, expensive, and inaccurate to an unacceptable degree.
To address these issues, car manufactures now use dedicated heater element control systems that are based on the resistance of the heater element, but with complex adaptive control mechanisms that can adjust the resistance of the heater element on any given oxygen sensor to eliminate sensor-to-sensor variations in resistance. Put in another way, this means that the PCM can “update” or adjust the resistance of an oxygen sensor’s heater element based on a deviation of the actual, measured resistance of the heater element from an expected, predefined (calibrated) value at engine start-up.
As a practical matter, current control to the heater element can be either through the negative or positive circuit, but in all cases, the PCM is able to control the current in the heater control circuit by adjusting the resistance of the heater element through closed loop operation. In this way, the PCM is able to minimize the deviation of the heater element’s actual temperature from the desired oxygen sensor temperature. The practical advantage of this system is that the PCM no longer has to calculate the temperature of an oxygen sensor based on variables it cannot control directly; instead, the PCM can now adjust the resistance of the heater element to keep all oxygen sensors at temperatures above the required minimum operating temperature, and it can do so reliably regardless of resistance variations in between oxygen sensors.
Where is the P2255 sensor located?
The image above shows the positions of both the #1 and #2 oxygen sensors relative to a catalytic converter. Note the direction of flow through the converter, as indicated by the red arrow; oxygen sensor #1 will always be located upstream of the converter, and sensor #2 will always be located downstream of the converter.
Note though that for the sake of clarity, only oxygen sensors are shown here. Most modern applications also use various other sensors in the exhaust system that may resemble oxygen sensors closely. Since some of these sensors may also be located close to the catalytic converter, non-professional mechanics are strongly urged to refer to the manual for the affected application for details on how to locate and identify exhaust sensors correctly to avoid confusion and misdiagnoses.
What are the common causes of code P2255?
Some common causes of code P2255 could include the following-
- Damaged, burnt, shorted, disconnected, or corroded wiring and/or electrical connectors
- Defective, contaminated, or oil-fouled oxygen sensor
- Exhaust leaks upstream of the catalytic converter
- Failed or failing PCM. Note that this is a rare event, and the fault must therefore be sought elsewhere before any control module is replaced