P2099 – Post catalytic converter fuel trim (FT), bank 2 -too rich

Reinier

By Reinier (Contact Me)
Last Updated 2017-01-22
Automobile Repair Shop Owner

Trouble CodeFault LocationProbable Cause
P2099 Post catalytic converter fuel trim (FT), bank 2 -too rich Catalytic converter

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What Does Code P2099 Mean?

OBD II fault code P2099 is defined by most manufacturers as P2099 as “Post catalytic converter fuel trim (FT), bank 2 – too rich”. Note that although GM defines this code as “Post Catalyst Fuel Trim System High Limit”, both definitions have the same basic meaning. In both cases, the PCM (Powertrain Control Module) sets code P2099 when it detects an excessive amount of un-combusted hydrocarbon molecules in the exhaust stream downstream of the catalytic converter on the one hand, and that it (the PCM) is unable to correct the air/fuel mixture, on the other. “Bank 2” refers to the bank of cylinders that does not contain cylinder #1 on engines with two cylinder heads.

Non-professional mechanics should note that “fuel trim” refers to the adjustments a PCM on any given application makes on a continual basis to the air/fuel mixture to maintain maximum engine performance while using the least amount of fuel. In general terms, there are two types of fuel trim adjustments, the first being “short term” fuel trims, which is monitored and controlled by the #1 oxygen sensor (upstream of the catalytic converter), and the second type being “long term” fuel trims which is monitored and controlled by the #2 oxygen sensor, located downstream of the catalytic converter. This statement is admittedly a gross oversimplification, but since long-term fuel trim/control is a highly technical subject, this guide can only provide a brief description of this process.

In simple terms, #1 oxygen sensors react to changes in the oxygen content of the exhaust stream that occur as a function of changes in the throttle opening that in turn, produces changes in the air/fuel mixture. Thus, assuming that the engine is in perfect running condition and that there are no other codes or defects present, the PCM uses input data from the #1 oxygen sensor to maintain the air/fuel mixture at a point close to stoichiometric, which in gasoline engines, is where all the fuel is combusted using all of the available air. Note that since diesel engines almost always run with excess air, this generally does not apply to diesel engines.

Nevertheless, to understand code P2099 (and why it sets), it is necessary to understand that the correct functioning of the upstream oxygen sensor is directly related to what happens after the exhaust stream enters the catalytic converter. It is also necessary to understand that catalytic converters are NOT the same; each application uses a converter that is designed for that specific application, and the emissions control/monitoring system on that application is therefore specifically calibrated in a manner that is calculated to ensure that the catalytic converter always performs within the design parameters for that particular application.

Thus, to ensure that the catalytic converter is able to clean up the exhaust stream effectively on any given application over the entire planned lifespan of the converter, manufacturers use a second oxygen sensor downstream of the converter, but this is where matters can get complicated. For instance, on many applications, the downstream oxygen sensor has the sole purpose of monitoring the operation of the catalytic converter, which it does by comparing the composition of the exhaust gas leaving the converter with readings obtained by the upstream oxygen sensor. On these systems, the downstream oxygen sensor plays almost no part in fuel control, and therefore, the difference between the readings obtained from both sensors represents the efficiency (or otherwise) of the catalytic converter.

On other applications, such as General Motors, the downstream oxygen sensor plays an active and critical role in fuel control, in addition to monitoring the operation of the catalytic converter. On these applications, the signal voltages of both the upstream and downstream oxygen sensors are used to calculate a fuel delivery strategy to suit a particular set of operating conditions, which means that the efficiency of the catalytic converter is determined in a different way than on systems where the downstream oxygen sensor does not contribute to effective fuel control.

However, regardless of design specifics, code P20197 sets when the PCM detects a rich-running condition, but only when the rich-running condition is defined by input data from the downstream oxygen sensor. In practice, this means that when a rich-running condition occurs, and the deviation from desired values exceeds a specified upper limit for a period of time set by the manufacturer, the PCM will set code P2099 and illuminate a warning light.

NOTE: During normal operation, the PCM will correct the rich-running condition without codes being set, but it must be understood that generally speaking and regardless of the application, a PCM’s ability to correct large deviations from desired air/fuel mixture values is limited.

The image below shows the typical arrangement of the upstream and downstream oxygen sensors (circled in red) relative to each other on either side of the catalytic converter. The red arrow indicates the direction of the exhaust gas flow; the sensor near the middle of the image is the #2 or downstream sensor, being located furthest from the engine, which is at the left of the frame.

Pre Post Cat Sensor Location

What are the common causes of code P2099?

Assuming that there are no additional codes present, the possible causes of code P2099 could include the following-

  • Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors
  • Defective oxygen -, or air/fuel ratio sensor
  • Defective catalytic converter
  • Major exhaust leaks upstream of the catalytic converter

NOTE: If there are other codes present along with P2099, possible causes could include almost anything that relates to those codes, with specific reference to “anything” that can cause a rich-running condition. Consult the manual for the application for detailed information on how additional codes can either cause P20197, or contribute to setting the code.

What are the symptoms of code P2099?

Apart from a stored trouble code and an illuminated warning light, common symptoms of P2099 could include the following depending on the application, the nature of additional codes if any are present, and the root cause(s) of additional codes-

  • Increased fuel consumption
  • Varying degrees of power loss, depending on the application and the nature of the problem
  • Rough, erratic, or fluctuating idling
  • Idling speed may exceed the recommended speed
  • Engine may not idle at all
  • Hard starting condition may be present when the engine is hot
  • The odor of fuel may be present
  • In extreme cases, the application may emit black smoke from the tail pipe
  • Engine may stumble, or hesitate upon acceleration
  • Vehicle may not pass an emissions test

WARNING: On some applications, there may be no discernible symptoms other than a stored trouble code and an illuminated warning light. However, the absence of other symptoms does NOT mean that this code could safely be ignored- in fact, not resolving this code as soon as it appears can cause the catalytic converter to melt, which if it happens, can set the vehicle on fire.

How do you troubleshoot code P2099?

SPECIAL NOTES: In terms of practicalities, it should be noted that diagnosing P2099 could be made a lot easier simply by paying close attention to which codes are present along with P2099. For instance, if this code appears alone, the problem is likely to involve only the catalytic converter itself, or the downstream oxygen sensor and/or its control circuit.

If there are several other codes present along with P2099 on the other hand, and especially if some additional codes were stored before P2099, it is almost certain that one or more of these codes either caused P2099, or contributed to its setting. For instance, if there are codes present that indicate misfires or issues with the MAP (Manifold Absolute Pressure) sensor, MAF (Mass Airflow) sensor, upstream oxygen sensor, or excessive fuel pressure, among others,  these codes must be investigated and resolved first, since all or any issues of this nature can produce rich-running conditions.       

For this reason, this guide cannot provide detailed diagnostic and repair information for code P2099 that will always be valid for all applications under all conditions, and the information provided here should therefore NOT be used in any diagnostic procedure for this code without making proper reference to the manual for the application being worked on. It is therefore particularly important to refer to the manual for the application both to determine the exact definitions of additional codes, and to determine the relationship between P2099 and ANY other code(s) that may be present, even if all or any additional codes are still in the “pending” stage.   

Although the generic diagnostic/repair information provided here should enable most non-professional mechanics to diagnose and repair code P2099 on most applications, it should be noted that failure to make proper reference to the manual for the application being worked on will almost certainly result in confusion, misdiagnoses, and the possible unnecessary replacement of a catalytic converter, which on some applications, could cost several thousand dollars if the converter is no longer covered by a warranty. END OF SPECIAL NOTES.  

Step 1

Record all fault codes present, as well as all available freeze frame data. This information can be of use should an intermittent fault be diagnosed later on. Be sure to refer to the SPECIAL NOTES above with regard to other codes that may be present along with P2099.

NOTE: Due to the high oil consumption rates of BMW, Audi, VW, and Mercedes-Benz applications, code P2099 is much more likely to be present on these applications than on AMD (American Domestic Market) or Japanese applications. Therefore, if there are no additional codes present on the aforementioned applications, inspect the oxygen sensors for oil fouling as the first step in the diagnostic repair procedure for code P2099. If replacing the oxygen sensors does not resolve the code, perform the procedures recommended by the manufacturer to test the operation of the catalytic converter.

Note however that rebuilt or non-OEM catalytic converters are prone to premature failure, so always replace these components with OEM parts only.

Step 2

If other codes are present along with P2099 on AMD or Japanese-made applications, resolve these codes in the order in which they were stored, before attempting a diagnosis of P2099. As stated elsewhere, the possible causes of rich-running conditions are many and varied, and in many cases, both cause and remedy may be manufacturer specific.

Consult the manual for the application to determine the most likely causes of all and any additional codes, and follow the directions provided in the manual to diagnose and repair all additional codes. Clear all codes after all repairs (including all relearning procedures where applicable) are complete, and operate the vehicle for at least one complete drive cycle before rescanning the system to see if any code(s) return.

NOTE: Pay particular attention to the condition of the exhaust system upstream of the catalytic converter since exhaust leaks can (and do) set this code because a serious exhaust leak contaminates the reference air used by both oxygen and air/fuel ratio sensors.

Step 3

If there are no other codes present, consult the manual to locate the affected oxygen sensor, as well as the location, function, routing, and color-coding of all associated wiring. Perform a thorough visual inspection of all wiring, and look for damaged, burnt, shorted, disconnected, or corrode wiring and/or connectors. Make repairs as required, clear the code, and rescan the system to see if the code returns,

Step 4

If the code persists but no visible damage to wiring or the sensor is found, perform resistance, continuity, and ground connectivity checks on all associated wiring. Compare all obtained readings with the values stated in the manual, and make repairs, or replace wiring as required to ensure that all electrical values fall within the ranges specified by the manufacturer. Note that the oxygen sensor itself forms part of the circuit, and as such, its internal resistance must be tested as well. Replace the sensor if its measured resistance does not agree with the value specified by the manufacturer.

NOTE #1: Pay particular attention to the affected sensor’s dedicated heater control circuit during this step. Oxygen sensors, and for that matter, the air/fuel ratio sensors used on Japanese applications, need to be at specific temperatures before they will work. This means that if the heater element does not work properly, the sensor might not work at all, or deliver inaccurate, erratic, fluctuating, or delayed signals to the PCM.

NOTE #2: Disconnect the sensor from the PCM before performing resistance, ground, and/or continuity tests to prevent damaging the controller.

Step 5

If the code persists but all electrical values agree with specified values, remove the sensor from the application and inspect it for signs of oil fouling, or the presence of other harmful deposits. Bear in mind that there is no reliable way to clean fouled oxygen or air/fuel ratio sensors without running the risk of damaging the sensor, so replace the sensor if fouling or contamination of any kind is present on the sensor. Clear all codes after the replacement, and rescan the system to see if the code returns.

NOTE: Bear in mind that both oxygen- , and air/fuel ratio sensors have limited life spans, which rarely exceed 100 000 miles. As these sensors age, they become less effective/accurate, and it is therefore entirely possible that merely replacing the affected sensor will resolve code P2099, and especially in cases where the sensors’ wiring is in good condition with no defects present. In light of this, it is important to consider both the sensors’ age, and the average oil consumption of the application before a catalytic converter is condemned out of hand.

Step 6

If the code persists beyond Step 5, there are two possibilities to consider. The first is that the catalytic itself is defective, or that there is some kind of problem with the PCM. The latter possibility is rare, and will almost always be indicated by a code other than P2099, which leaves the catalytic converter as the most likely cause of this code.

One useful diagnostic tip to bear in mind is that if no other codes are present, and there are no exhaust leaks, a downstream oxygen sensor that is known to be good will generate a steady signal voltage close to the mid-point between a rich-, and lean running condition. If the downstream sensor is good, but the scanner indicates a reading that fluctuates at close to the same rate as the reading from an upstream oxygen sensor would, or remains significantly above the midway point, the catalytic converter is almost certainly defective.

Although the same general principle holds true for air/fuel sensors, this type of sensor is best diagnosed with an oscilloscope.

Step 7

WARNING: The only reliable and accurate way to test the operation and efficiency of any catalytic converter is to follow the instructions provided by the manufacturer. One favourite, but severely misguided “back-yard” method is to induce a misfire by disabling one or more spark plugs to see if the converters’ temperature rises- DO NOT attempt this “test”, because doing so could melt the catalytic converters’ core in a matter of mere seconds.

Moreover, this “test” has set many vehicles on fire when their catalytic converters melted, so ONLY follow the directions provided to test the converter, or refer the vehicle to a competent repair shop for professional diagnosis and repair.

Codes Related to P2099

P2098 – Relates to “Post Catalyst Fuel Trim System Too Rich Bank 2”

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