|Trouble Code||Fault Location||Probable Cause|
|P2198||Heated oxygen sensor (H02S) 1, bank 2 - signal stuck rich||H02S, fuel pressure, injectors, air intake restricted|
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What Does Code P2198 Mean?
SPECIAL NOTES: Non-professional mechanics should take note that in order to diagnose code P2198 (or any other oxygen sensor related code), it always helps to gain at least a working knowledge of what oxygen sensors do, how they do it, what the major difference between the various types of oxygen sensor are, and how oxygen sensors differ from air/fuel ratio sensors.
While a detailed discussion of these issues falls outside the scope of this guide, it is worth mentioning that some oxygen sensors generate a signal voltage on their own, while others rely on a 5-volt input voltage delivered by the PCM (Powertrain Control Module) in order to work. However, regardless of the type of oxygen sensor used in any particular application, all oxygen -, and air/fuel ratio sensors have the function of monitoring the concentration of oxygen in the exhaust stream in order for the PCM to control the air/fuel mixture, but it is important to know how these sensors do what they do.
In short, oxygen sensors that generate their own signal voltages work by changing the signal voltage in response to the constantly changing levels of oxygen in the exhaust gas, while sensors that rely on an input voltage work by changing their resistance in response to changes in the oxygen content of the exhaust gas. This is an important distinction when it comes to diagnosing this code, which makes it imperative to read the section on oxygen sensor-related problems and diagnostics in the application’s repair manual before attempting a diagnosis of this code.
Moreover, the two main types of oxygen sensor are also electrically opposite, meaning that a reading that indicates a rich-running condition on one type of sensor will indicate a lean-running condition on the other type. While the two types of oxygen sensor can be distinguished by the number of wires they have in their connectors, bear in mind that apart from being different from each other, both types of oxygen sensor are also fundamentally different from the air/fuel ratio sensors used on many Japanese applications. Therefore, it must be noted that the two types of oxygen sensor are NOT interchangeable, and NEITHER type of oxygen sensor is interchangeable with air/fuel ratio sensors.
In light of the above, it should be obvious that this guide cannot provide detailed diagnostic and repair information that will apply to all applications all of the time. Therefore, the information provided here is intended for general informational purposes only, and proper reference to the application’s repair manual MUST be made before any of the information provided here is used in ANY diagnostic and/or repair procedure.
Nonetheless, diagnosing code P2198 is not an overly complicated affair, and although an oscilloscope is the recommended tool with which to diagnose this code, most non-professional mechanics should be able to diagnose and repair this code on almost any application using the information provided here, even if an oscilloscope is not available. END OF SPECIAL NOTES.
OBD II code P2198 is a generic code that is defined by all carmakers as “O2 Sensor Signal Biased/Stuck Rich Bank 2 Sensor 1”, with “Bank 2” referring to the bank of cylinders that does not contain cylinder #1, and “Sensor 1” referring to the oxygen-, or air/fuel sensor (typically used on Japanese applications) located upstream of the catalytic converter. This code refers specifically to the fact that the oxygen or air/fuel sensor indicates a continuous rich-running condition, which is rarely caused by anything else than a failure of the sensor itself, but take note that in the vast majority of instances of this code, the code is merely the result of a sensor failure. Although not altogether impossible, actual rich-running conditions are relatively rare when this code is present, for reasons that will be explained below. Note also that the length of time during which the rich-running condition is indicated varies between applications, but be aware that if the condition is indicated by a flashing warning light, the condition is serious, and fatal catalytic converter damage may occur.
During normal operation, the #1 oxygen or air/fuel sensor(s) react(s) to changes in the oxygen content of the exhaust stream almost immediately (typically within 300 milliseconds or so), in response to chemical reactions between the sensing material in the sensor and the oxygen in the exhaust gas. Depending on the type of sensor, the reaction causes a change in either the signal voltage generated by the sensor, or in the electrical resistance across components in the sensing element, which change is used as the basis of a complex algorithmic calculation the PCM uses to maintain the air fuel mixture as close to the ideal 14.7 [parts of air]: 1 [part of fuel] ratio for gasoline engines as possible.
However, many factors such as the engine temperature, intake air temperature, humidity, barometric pressure (and others), have a direct bearing on the composition of the air/fuel mixture. To even out the effects of all of these variables, the fuel mixture is set to be rich by default, so as soon as the PCM enters closed loop operation, which happens when the #1 oxygen or air/fuel ratio sensor generates a signal voltage or resistance, the PCM starts switching the signal from rich to lean at the rate of about seven times per second.
Therefore, when the PCM sees a signal during the normal switching cycle that indicates the engine is running rich, it makes adjustments to the ignition timing, injector pulse width, and other systems to lean out the air/fuel mixture, until the sensor(s) register(s) a lean condition. When this happens, the PCM reverses the adjustments it had made to lean out the air/fuel mixture, thereby enriching the mixture again. By switching back and forth between enriching and leaning out the air/fuel mixture several times per second, the PCM is able to keep the air/fuel mixture at, or very close to the ideal 14.7: 1 ratio, which maximises engine performance, and reduces emissions while using the least amount of fuel.
In practice then, it follows that a fully functional upstream oxygen sensor or air/fuel ratio sensor can never generate a signal voltage or resistance that does not change. With an oscilloscope, or a professional grade scanner that can draw graphs, both the amplitude and frequency of switching cycle is displayed as a series of uniform waves that vary in amplitude (height) as the oxygen content in the exhaust gas changes. The typical range of the signal voltage (displayed as a waveform) varies from about 0.1 volt, to about 0.9 volts, with a value of about 0.45 volts representing a midpoint at which the air/fuel mixture is as close to the ideal 14.7: 1 ratio as the PCM can make it.
On sensors that change their resistance in response to changes in the oxygen content of the exhaust stream, the change in resistance is analogous to changes in the signal voltage generated by other types of sensors. The PCM interprets the changes in resistance in the same way, i.e., a change in resistance represents a change in the air/fuel mixture, and the PCM makes appropriate adjustments to various systems to even out the effects of variable conditions that influence the composition of the air/fuel mixture in the same way. However, the resistance values generated by sensors of this type vary between manufacturers and applications, so consult the manual for the application being worked on to determine the acceptable range of sensor resistances for that application.
Thus, from a diagnostic perspective, it is highly unlikely that the air/fuel mixture ratio will (or can) either remain constant for any appreciable amount of time, or that a fully functional upstream sensor will not react to the changes in the oxygen content of the exhaust stream for any appreciable amount of time after the PCM has entered closed loop operation. Therefore, when the PCM detects no change in the signal voltage or resistance of an upstream sensor after a period of time that is set by the manufacturer, it will set code P2198, and illuminate a warning light.
The image below shows the typical waveforms that represent the switching cycle of an upstream oxygen sensor. Note the 0.45-volt median line that represents the air/fuel mixture being at, or close to a 14.7: 1 ratio. Also note the waveform in the bottom right panel that indicates a continuous lean condition- if the condition were continuously rich, the waveform would be above the median line, but similar in appearance to the lean condition.
What are the common causes of code P2198?
While there exists a very long list of possible causes of over fuelling (rich-running conditions), most of these causes will generally not cause an over fuelling issue that is so severe that the upstream oxygen sensor is unable to react to changes in the oxygen content of the exhaust stream at all. Of course, there are exceptions to every rule, and in some instances of over fuelling, the problem could be so severe that the engine may not start or idle at all, or it may give off clouds of black smoke from the tail pipe.
However, the latter type of problem is almost impossible to be caused by a defective oxygen sensor, and even more so if no codes other than P2198 are present on the application. Therefore, non-professional mechanics should be aware that while code P2198 (a defective upstream oxygen sensor) might indicate what appears to be an over fuelling problem, the over fuelling is almost certain to be the result of the oxygen sensor failure and the PCM’s limited ability to compensate for the failed sensor, as opposed to an over fuelling problem that destroyed an oxygen sensor.
With the above in mind, it should also be remembered that oxygen sensors fail for relatively few reasons, some of which could include the following-
- Oil or carbon deposits that damage the sensing element
- Silicone contamination that also damages the sensing element
- Failure of the heater element or its control system. Note that this is unlikely to cause P2198, although it may contribute to the code being set
- Short circuits in the upstream oxygen sensors’ wiring, and especially if the signal wire is shorted to positive. Note however that this condition will almost certainly be indicated by a code other than P2198.
- Excessive fuel pressure. Note that while this may cause severe over fuelling, the excessive fuel pressure will definitively be indicated by a dedicated code.
- Clogged or dirty air filter element. Note that while this may also cause over fuelling, the oxygen sensor will still react to changes in the oxygen content of the exhaust stream.
- Defects in the ignition system that could cause poor spark plug performance, or the spark plugs not to work at all on one or more cylinders. Note that this too, is unlikely to cause an oxygen sensor to fail, and that misfires are always indicated by dedicated codes.
NOTE: Since almost all causes of over fueling will still allow the oxygen sensor to react to changes in the composition of exhaust gas, the prudent thing to do is to consult the manual for the application on the most likely causes of code P2198 on that application.
What are the symptoms of code P2198?
In some cases, there may be no symptoms present other than a stored trouble code, and an illuminated warning light, especially when there are no other codes present. However, when other codes are present, typical symptoms could include the following-
- Hard starting, especially when the engine is hot
- Rough, erratic, or fluctuating idling
- Engine may run rough from idle to about 2 500 RPM or so. Note that this symptom may not always be present, or it may vary in severity between applications
- Black smoke from the tail pipe may be present
- Engine may stall unpredictably at low RPM’s
- Engine may lose power at low speeds
- Engine may hesitate or stumble upon acceleration
- Fuel economy may worsen considerably
- Catalytic converter damage or even failure could occur over extended periods if the code is not resolved
- Oil dilution and possible engine damage/failure may occur over extended periods
NOTE: This list of possible symptoms is not exhaustive. Consult the manual for the application being worked on for detailed information on symptoms that are likely to present on that application.
How do you troubleshoot code P2198?
SPECIAL NOTES: Cheap, generic code readers are generally not able to display graphs, or even display the normal operation of oxygen sensors because of the rapid rate at which these sensors switch from lean-to-rich-to-lean values. If an oscilloscope or professional grade scanner is not available, the affected sensor can be tested by using a good quality digital multimeter, but be aware that most multimeters are also not able to display the switching cycle accurately. In many cases where a multimeter is used, the instrument will display a steady voltage, even though the sensor may be switching at the proper rate. The inability of most multimeters to display an accurate reading of oxygen sensor switching cycles is caused by the fact that the switching happens faster than the instruments’ ability to process the input before the next switch occurs.
Nonetheless, if the sensor is indeed defective and does not reflect the changes in the oxygen content of the exhaust stream, the display on the multimeter will not change when the engine speed, and therefore the oxygen content in the exhaust gas changes, which can confidently be taken as confirmation that the sensor is defective.
However, multimeter tests should ONLY be considered conclusive when it is certain that an actual rich-running condition does NOT exist. END OF SPECIAL NOTES.
NOTE: The key to diagnosing code P2198 correctly the first time is to be certain that codes that are present along with P2198 are the result of code P2198, and not the result of an actual rich-running condition that caused code P2198 to be set along with other codes. This might appear to be confusing at first, but it sounds more complicated than it actually is. The “generic” testing procedures outlined in the steps below should enable the average non-professional mechanic to distinguish between the two conditions.
Allow the engine to warm up so that all other oxygen sensors are able to enter closed loop operation. Record all fault codes, as well as all available freeze frame data. This information can be useful should an intermittent fault be diagnosed later on.
NOTE #1: Even through the post-catalytic oxygen sensor has a limited role when it comes to fuel control, data from this sensor can often be used as a reference against which to measure the performance of the pre-catalytic (upstream) converter sensor, hence the need to allow this sensor to enter close loop operation. The same is true for the other upstream sensor if the application is fitted with one upstream sensor per cylinder bank.
NOTE #2: If there are other codes present along with P2198, these codes MUST be resolved in the order in which they were stored, since in some cases, one or more additional codes may have contributed to code P2198 being set. Also, always clear all codes after each step in the diagnostic/repair procedure, and retest the system or operate the vehicle for at least one complete drive cycle to see if the code returns, or if the repair up to that point had been successful.
The first thing to do is to determine whether an actual rich-running condition exists or not. Bear in mind that even a marginally defective oxygen sensor will react to changes in the oxygen content of the exhaust gas, albeit in ways that will differ from how it should react. To this end, consult the manual to determine the color-coding, function, routing, and location of all associated wiring and connectors for future reference and steps in the diagnostic procedure.
The purpose of this step is to check whether the sensor reacts in any way or not. With the scanner connected, the display will likely reflect the point above about 0.45 volts at which the signal had gotten stuck. Sensors that work on resistance (and not a generated signal voltage) will also have a midpoint, so consult the manual to determine this value; note this value, and compare it to the displayed value. Keep in mind that the value can be stuck anywhere above the midpoint for that application.
NOTE: Keep in mind that if the sensor itself had failed, the scanner or multimeter will likely not display a value that exceeds the maximum output signal/resistance for oxygen sensors on that application. If the displayed value does exceed the maximum, it is almost certain that a short circuit in the sensor’s control circuit is present, a circumstance that will almost certainly be indicated by a code other than P2198.
Follow the directions in the manual on the correct procedure to connect test equipment, and once the display on the scanner, oscilloscope, or multimeter has stabilized, open the throttle suddenly (but momentarily) to effect a change in the oxygen content of the exhaust gas. If the displayed signal voltage or resistance does not change at all, the oxygen sensor is likely defective. Similarly, if the displayed value does not change at all when a vacuum hose or line on the inlet tract is unplugged (which also effects a change in the oxygen content of the exhaust gas), the affected oxygen sensor can confidently be said to be defective.
However, if the displayed value changes, no matter by how small a degree during both tests, it is possible that an actual rich-running condition is present, but bear in mind that if an actual rich-running condition is present, this will definitely be indicated by other codes, among which P2198 is likely to be present. The possible causes of rich-running conditions are many and varied, but if such a condition is suspected, resolving it will almost certainly resolve P2198 as well.
Once it is certain that the oxygen sensors’ signal voltage or resistance does not change at all in response to changes in the exhaust gas’ oxygen content and that no codes or conditions are present that could indicate a rich-running condition, perform a thorough visual inspection of all wiring and connectors associated with the affected sensor. Look for damaged, burnt, shorted, and/or disconnected wiring and connectors. Make repairs as required, and rescan the system to see if the code returns.
If no visible damage to wiring is found but the code persists, perform reference voltage, resistance, ground, and continuity checks on all associated wiring. Pay particular attention to the values in the sensors’ dedicated heater circuit, since defects in this wiring could conceivably cause erratic or inaccurate readings. However, deviations of more than a few percent from specified values will almost certainly be indicated by codes other than P2198.
Nonetheless, compare all obtained readings with the values stated in the manual, and if deviations from specified values are found, make repairs as required to ensure that all electrical values fall within the manufacturer’s specifications.
NOTE: Oxygen sensors need to be at around 6000F before they will start to work, hence the need for a heater element and a dedicated control circuit for each sensors’ heater element. Generally speaking though, an oxygen sensor cannot enter closed loop operation if the sensing element is not hot enough; should there be an issue with either the heater element or its control circuit, the PCM will set a code (or codes) other than P2198. Moreover, if the heater element does not work for whatever reason, the sensor will not produce a signal voltage, or a resistance other than its natural “at-rest” resistance.
If no electrical issues have revealed themselves up to this point, remove the sensor from the vehicle, and inspect it for the presence of carbon deposits, damage caused by contamination with silicone-based compounds, or deposits formed by aftermarket oil and fuel additives.
Note that while anti-freeze contains silicone-based compounds, these substances only affect oxygen sensors if there are internal coolant leaks in the engine, such as coolant seepage past the cylinder head gasket(s). This is a serious condition that must be corrected without delay, but if the sensor is damaged or contaminated by aftermarket additives, the use of these additives should be avoided.
The image below shows two typical forms of fouling and/or contamination that routinely cause oxygen sensors to fail. Most contamination/fouling problems can be avoided altogether simply by replacing all oxygen sensors in strict accordance with the vehicle manufacturers’ routine maintenance schedule, or more frequently on applications that are known for their high oil consumption rates, most notably BMW, Mercedes-Benz, Audi, and some VW products.
Note that contaminated or damaged oxygen sensors cannot be cleaned or repaired, meaning that replacement of the sensor with an OEM part is the only reliable remedy. Replace the sensor with its EXACT aftermarket equivalent if OEM parts are not available, but be aware that the code could persist, or return in short order even if the sensor is replaced with a high quality aftermarket part.
When replacing the sensor, make sure all wiring is connected properly, all wiring is routed away from hot exhaust components, and that all wiring is properly secured to prevent the wiring from rubbing and/or chafing against other components.
Clear the code, and operate the vehicle for at least one complete drive cycle to see if the code returns, which is very unlikely to happen if all other tests and checks turned out negative, or failed to reveal any condition(s) that could cause the air/fuel mixture to be seen as rich by the PCM.
If however, the code does return, it is likely that either the replacement sensor is defective as well (a distinct possibility with aftermarket parts), or that the monitoring circuits in the PCM are defective. It is not always easy for non-professional mechanics to determine if the latter condition is the case, so if this is suspected, the better option is to refer the vehicle for professional diagnosis and repair.
Codes Related to P2198
- P2196 – Relates to “O2 Sensor Signal Biased/Stuck Rich Bank 1 Sensor 1”
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