|Trouble Code||Fault Location||Probable Cause|
|P2251||Heated oxygen sensor (H02S) 1, bank 1, negative current control - open circuit / - no signal||Wiring, H025 / Wiring, H025, ECM|
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What Does Code P2251 Mean?
OBD II fault code P2251 is a generic trouble code that is defined as “Heated oxygen sensor (H02S) 1, bank 1, negative current control – open circuit / no signal”, or, sometimes as “O2 Sensor Negative Current Control Circuit/Open Bank 1 Sensor 1”, and is set when the PCM (Powertrain Control Module) detects an open circuit or other circuit failures in the control circuit of the upstream oxygen sensor on Bank 1.
Note that “upstream” refers to the oxygen sensor that is located before the catalytic converter and that Bank 1 refers to the bank of cylinders on V-type engines that contains cylinder #1.
While most vehicles today have two oxygen sensors for each bank of cylinders, one upstream of the catalytic converter, and one downstream of the converter, the two oxygen sensors have very different purposes. In all cases, the upstream sensor monitors the oxygen content of the exhaust stream as a means to control the air/fuel ratio, while the downstream sensor’s purpose is to monitor the efficiency of the catalytic converter.
In practice, the upstream sensor is supplied with a reference voltage, which changes in direct response to the fluctuating oxygen levels in the exhaust stream. An increase in the oxygen level (referenced to ambient air) means that the air/fuel ratio contains too much air relative to fuel, while a decrease in the oxygen level means the air/fuel ratio contains too much fuel relative to air.
To correct the balance between air and fuel, the PCM uses the changing signal voltages from the upstream sensor to adjust the fuel injector’s pulse widths, aka duty cycles, to add fuel to the air/fuel mixture or to subtract fuel from the air/fuel mixture to maintain the air/fuel mixture at a stoichiometric level, depending on the observed imbalances.
For gasoline, this ratio is 14.7 parts of air to 1 part of fuel, but to avoid large adjustments to the injectors’ pulse widths, the PCM switches the input data from the oxygen sensor between reading rich, to reading lean several times per second to average out the fluctuating oxygen level readings. This allows the PCM to make several small adjustments to the air/fuel mixture per second, as opposed to making a few larger adjustments over a longer period because this method maintains the air/fuel mixture at a stoichiometric ratio more accurately and consistently than a few larger adjustments could.
Note though that while all upstream oxygen sensors do the above on all applications, the switching mechanism differs between applications. In some cases, the control circuit is controlled with a negative current, which means that while the PCM supplies a constant positive current to the sensor, it also supplies/controls the ground connection that completes the circuit.
Therefore, when a failure, defect, or malfunction occurs in the PCM, or in associated wiring that prevents the PCM from grounding the control circuit, the upstream oxygen sensor becomes inoperable and PCM will recognize that it cannot control the air/fuel mixture effectively. When this happens, the PCM will set fault code P2251 and illuminate a warning light that might flash, since this condition has the potential to affect exhaust emissions negatively.
Where is the P2251 sensor located?
This image shows the location of the upstream oxygen sensor relative to the catalytic converter on a typical transversely mounted four-cylinder application. In some cases though, and especially on V-type engines, the upstream oxygen sensor may not be located directly on, or as near to the engine as shown here, but in all cases, the upstream sensor will be located on the engine side of the catalytic converter.
What are the common causes of code P2251?
WARNING: Note that negative current control sensors are typically of the four- and five-wire variety, since three-wire sensors are typically directly grounded to the vehicle via their casings. Therefore, before attempting to diagnose possible causes of code P2251, be sure to consult reliable service information for the affected application to determine the routing and color-coding of the upstream oxygen sensor’s wiring to ensure correct identification of the ground wire. Testing the wrong wire could result in fatal damage to the PCM.
The most common causes of code P2251 are largely similar across all applications, and could include one or more of the following-
- Burnt, damaged, shorted, disconnected, or corroded wiring and/or connectors
- Defective or malfunctioning upstream oxygen sensors
- Use of unsuitable, incorrect, or substandard aftermarket upstream oxygen sensor
- Failed or failing PCM, but note that, unlike most other codes, PCM failure is a relatively common cause of this trouble code
What are the symptoms of code P2251?
Common symptoms of code P2251 could include one or more of the following, but note that the severity of one or more symptoms listed here could vary between applications-
- Stored trouble code and illuminated warning light but note that in some cases, one or more additional codes may also be present
- In some cases, there may be no discernible symptoms (apart from an illuminated warning light) present
- Fuel consumption may increase as a result of the PCM reverting to a default fuelling strategy
- One or more readiness monitors may not initiate, or may not run to completion, which could affect a mandatory emissions test
- Note that this code will typically not cause drivability issues; if such issues and/or symptoms are present, they are almost certainly the result of secondary damage, such as a clogged or damaged catalytic converter
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