P2187 – System too lean at idle, bank 1

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By Reinier (Contact Me)
Last Updated 2016-12-01
Automobile Repair Shop Owner
CodeFault LocationProbable Cause
P2187 System too lean at idle, bank 1
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Fuel pressure, injectors, intake leak

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Table of Contents

  1. What Does Code P2187 Mean?
  2. What are the common causes of code P2187?
  3. What are the symptoms of code P2187?
  4. How do you troubleshoot code P2187?
  5. Codes Related to P2187
  6. Get Help with P2187

What Does Code P2187 Mean?

SPECIAL NOTES: On engines with two banks of cylinders, “Bank 1” refers to the bank of cylinders that contains cylinder #1, while “Bank 2” refers to the bank of cylinders that does not contain cylinder #1. In practice, this means that on engines where the inlet tract is split, with one half feeding “Bank 1”, and the other half feeding “Bank 2”, a vacuum leak, or other issue on that half of the inlet tract can cause the same issues and problems as on “Bank 1”. However, if “Bank 2” is affected, the problem is indicated by code P2189 – “System Too Lean at Idle Bank 2”, but the diagnostic and repair procedures for this code are generally the same as for P2187 – “System Too Lean at Idle Bank 1”. END OF SPECIAL NOTES.

OBD II trouble code P2187 is a generic code that is defined by all carmakers as “System Too Lean at Idle Bank 1”, and is typically set when the PCM (Powertrain Control Module) detects a difference of about 25% between the desired air/fuel mixture when the engine is running at idle, and the actual, measured air/fuel mixture. Note that this code indicates a lean-running condition, which occurs when the air/fuel mixture on a gasoline engine contains more than 14.7 parts of air to 1 part of fuel. Note that diesel engines run with excess air at idle, and are therefore not affected by this code.

During normal operation when the engine is running at idle, (and assuming that the engine and all auxiliary systems are fully functional), the air/fuel mixture is controlled by the PCM by using data received from various sensors such as the #1 oxygen sensor(s), MAF/MAP Sensor, Crank -/Camshaft Position Sensor(s), and others, such as the fuel pressure sensor. Assuming that all relevant sensors are delivering valid data, the PCM uses the information thus obtained to make adjustments to the ignition timing, variable valve/cam timing (where fitted), the injector pulse width, and the position of the throttle plate on drive-by-wire systems in order to maintain a steady idling speed within the range specified for that application.

NOTE #1: Code P2187 indicates an issue with the air/fuel mixture when the engine is running at idle; issues with the air/fuel mixture when the engine is running above idling speed is generally indicated by other codes, some of which may be manufacturer specific.

NOTE #2: Added loads to the engine, such as the headlights, A/C, and others tend to decrease the idling speed, so when the PCM detects that the load on the engine has increased, it will make suitable adjustments via the components/systems listed above, as well as to the IAC (Idle Air Control) valve and sometimes the throttle position to increase the idling speed to counteract the effects of the additional loads. Be aware that this type of idle control problem is a separate issue altogether and it should therefore not be confused with the effects of code P2187, which is solely concerned with the effects of an incorrect air/fuel mixture.         

However, failures and/or malfunctions in one or more engine sensors or systems, such as leaks in the vacuum system can upset the air/fuel mixture, and while the PCM will attempt to correct the air/fuel mixture its ability to do so is limited. When the PCM detects that it cannot control the idling speed due to issues with the air/fuel mixture it cannot correct, or compensate for by making adjustments to various subsystems, it will set code P2187, and illuminate a warning light. Note that on some applications, the code will be set as soon as the fault occurs while on others, the code will be set if the fault occurs over two or more start-up cycles.

The image below shows typical damage in an inlet boot caused by hardening of the boot. This type of damage is a major cause of vacuum leaks that lead to code P2187 being set. Note that this is only one example of a typical vacuum leak; in some cases, some disassembly, and/or removal of various engine components may be required to gain access to all vacuum lines for the purpose of inspection or replacement.


What are the common causes of code P2187?

There is an extensive “laundry” list of possible causes of this code, some of which may apply to all applications, while others are specific to a particular application. Non-professional mechanics are therefore advised to read the relevant section (Fuel and Air Metering) in the manual for the application being worked on before attempting a diagnosis and repair of P2187, since some of the possible causes of this code as listed below may not appear to be relevant to the code, and may in fact not be relevant to this code on some applications. Consult the manual for the application for more detailed information on the possible causes/failures/malfunctions that are most likely to produce code P2187 on that particular application.

  • Unmetered air entering the engine through leaks in the engine vacuum -, and/or EVAP systems
  • Unmetered air entering the engine through leaks in the inlet manifold or other parts of the inlet tract, such as around sensor attachment points
  • Unmetered air entering the engine through leaks in the vacuum supply line for the brake system. Note that a leak in this line is likely to effect the operation/performance of the brakes
  • Defective upstream oxygen sensor
  • Damaged, burnt, shorted, disconnected, and/or corroded wiring. Note that this applies to the wiring of all relevant sensors, including the upstream oxygen sensor(s)
  • Open circuits in the wiring of any relevant sensor
  • Defective, or contaminated MAF (Mass Airflow) sensor
  • Defective MAP (Manifold Absolute Pressure) sensor
  • Low fuel pressure, which can be caused by a defective fuel pressure regulator, defective fuel pump, clogged fuel filter, or wiring issues that cause the pump not to develop full system pressure
  • Exhaust leaks that contaminate the upstream oxygen sensor’s reference air
  • Under boost condition(s) caused by leaks in the inlet ducting between the turbocharger and the inlet manifold, or a malfunction of the turbocharger boost control system
  • Loose, ill-fitting, or broken fuel and/or oil filler caps on some applications
  • Failed, or failing PCM. Note that this is a rare event and the fault must be sought elsewhere before any controller is replaced

What are the symptoms of code P2187?

In some cases, there may be no symptoms present other than a stored trouble code and an illuminated warning light. However, depending on the application and the exact nature of the problem, other symptoms could vary from slight and barely detectable, to severe enough to prevent the engine from idling at all. Typical symptoms could include the following-

  • Hard starting, especially when the engine is hot
  • Engine may stall frequently or unpredictably during idling, especially when the engine is cold
  • Idling may be rough, erratic, or fluctuate
  • Engine may misfire during idling
  • Engine may run hotter than normal during idling
  • Engine may stumble or hesitate upon acceleration
  • Loss of power is possible in some cases, depending on the application and the exact nature of the problem

NOTE: Be aware that some of the fault conditions that set code P2187 can cause engine and/or catalytic converter damage and even failure if they are not corrected within a reasonable time. Consult the manual for the application being worked on for detailed information on the possible long-term effects of not correcting all, or some code-setting conditions.

How do you troubleshoot code P2187?

SPECIAL NOTES: Code P2187 occurs most frequently on applications that are known, (or notorious) for their high oil consumption rates, such as some BMW, Mercedes-Benz, Audi, and many VW products. In many, if not most cases of code P2187 on these applications, the problem can often be resolved, (albeit temporarily) by a simple cleaning of the MAF (Mass Airflow) sensor element with an approved solvent, since the excess oil in the inlet tract(s) of these applications can cause false, invalid, or erratic signal voltages to be generated.

Note also that aftermarket MAF sensors and other components often cause this code on these applications; therefore, any diagnostic procedure for code P2187 on the above-mentioned applications should start with an inspection/cleaning/replacement of the MAF sensor. Finding liquid oil in the inlet tract(s) of these applications is common, and while the cure for this condition is often very expensive, the only way to reduce the incidence of code P2187 on these applications is to take all practicable measures to reduce the amount of oil that accumulates in the inlet tract.

Moreover, be aware that excess oil in the inlet tract often causes premature failure of #1 oxygen sensors (upstream of the catalytic converter), which set can code P2187 when the correct functioning of the oxygen sensor is inhibited sufficiently by carbon and/or oil fouling. END OF SPECIAL NOTES.

WARNING: Due to the large number of possible causes (and code setting parameters) of code P2187, the information presented here is intended for general informational purposes only. As such, this information should NOT be used in diagnostic procedures for code P2187 without making reference to the application’s repair manual. However, the “generic” troubleshooting tips/steps outlined in this guide should enable most DIY and non-professional mechanics to trace and repair most, if not all possible causes of code P2187 on most applications.    

NOTE #1: Diagnosing code P2187 requires that the engine be in perfect running condition, with no exhaust leaks, vacuum leaks, or other codes being present. If other codes are present along with P2187, these codes MUST be resolved before a diagnosis/repair of P2187 is attempted. Failure to repair exhaust and/or vacuum leaks, as well as all additional trouble codes will result in misdiagnoses, and the almost-certain unnecessary replacement of components.

NOTE #2:  Upstream oxygen sensors rarely cause code P2187. It is far more likely that either issues within the oxygen sensors’ control/heater circuit(s) or contamination of the sensors’ sensing element is at fault, as opposed to a failure of the sensor. Nonetheless, while invalid data from an upstream oxygen sensor can influence the air/fuel mixture, the cause of the invalid data will almost always be indicated by codes other than P2187, which specifically means that if oxygen sensor related codes are present along with P2187, these codes MUST be resolved before a diagnosis of P2187 is attempted.

Step 1

Assuming that the engine is known to be in good mechanical condition, record all fault codes present as well as all available freeze frame data. This information can be of use both should an intermittent fault be diagnosed later on, and when several other codes are present, to assist in pinpointing some causes of P2187.

NOTE #1: There is little point in continuing the diagnosis of this code if the engine is not in good mechanical condition. Problems such as worn piston rings, leaking turbocharger shaft seals, and blown/damaged cylinder head gaskets (among others) can either cause, or contribute to code P2187 being set, which means that this, and possibly many other codes will persist until these defects are corrected/repaired.

NOTE #2: Failures and malfunctions in the PCV (Positive Crankcase Ventilation) system of turbocharged 2.0L VW/Audi engines are common causes of code P2187 on these applications. Therefore, any diagnostic procedure for this code on these applications should start with an inspection, and replacement of defective components/parts of the PCV system. Consult the manual for detailed information on how to identify, and replace defective PCV system components.

Step 2

Perform a thorough visual inspection of the engine vacuum system, and look for split, cracked, dislodged, or hardened vacuum hoses and lines. Make repairs as required, clear all codes, and operate the vehicle for at least one drive cycle. Rescan the system to see if the code returns.

NOTE: If no damaged or leaking vacuum lines are found but the code persists, suspect leaking inlet manifold gaskets. Consult the manual on the correct procedure to replace inlet manifold gaskets. Rescan the system after the gasket replacement to see if the code returns.

Step 3

While performing the vacuum system inspection, also inspect the MAF (Mass Airflow) sensor for evidence of oil contamination. Clean the element with an approved solvent, refit the sensor, but make sure there are no vacuum leaks at any connection in the inlet tract.

Step 4

Consult the manual to determine the location of all sensors (and their wiring) that could conceivably be implicated in the setting of code P2187. Also, determine the location, color-coding, routing, and function of all associated wiring, and perform resistance, reference voltage, continuity, and ground connection tests on all wiring and connectors. 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 manufacturers’ specifications.

NOTE #1: Be sure to test all relevant sensors as well, since sensors form part of their control circuits. Consult the manual on the correct procedure to test all sensors, and replace any that do not comply with the manufacturer’s specifications.

NOTE #2: Merely testing the internal resistance of a relevant sensor is not enough. It is also necessary to check that sensor’s reference (input) voltage where applicable, as well as the signal voltage generated by that sensor. However, since test procedures are often manufacturer specific, it is imperative that all and any test procedures prescribed in the manual for that application be followed EXACTLY in order to obtain reliable and accurate results. Replace all sensors that do not comply with ALL the specified values stated in the manual.

NOTE #3: Pay particular attention to the dedicated heater control circuits of #1 (upstream) oxygen sensors, since these sensors have to be at very specific temperatures before they will return accurate signal voltages. Oxygen sensors, and particularly upstream oxygen sensors, can be seen as having the final “say” when it comes to short-term fuel trim adjustments, and as such, anything that influences their operation has a direct bearing on how well the PCM manages the air/fuel mixture. Note that it is necessary to remove the oxygen sensor(s) from the exhaust system to inspect it/them for signs of damage or contamination.

NOTE #4: There are many steps and testing procedures contained in Step 4, so clear all codes after each step and/or test, and rescan the system to see if the code returns. Since it is easy to be bogged down in small details, it sometimes happens that the code is resolved without realizing it, so rescan the system regularly to save time and effort.

Step 5

Once it is certain the engine vacuum system is fully functional and that there are no vacuum leaks present, and that all relevant electrical values fall within specified values, prepare to test the fuel pressure.

WARNING: This is a potentially dangerous step due to the risk of fire. It is therefore imperative that the testing procedure only be carried out with good quality test equipment and in strict accordance with the instructions provided in the manual.    

Be aware though that while low fuel pressure can contribute to code P2187, this condition will almost always be indicated by a code other than P2187. However, since there is some overlap between the symptoms of low fuel pressure and those of code P2187, it is important to eliminate (or confirm) low fuel pressure as a contributing factor in the presence of code P2187. Thus, compare the actual, measured fuel pressure with the minimum value stated in the manual, and make repairs as required or refer the vehicle for professional diagnosis and repair if the actual fuel pressure does not agree with the minimum value specified in the manual.

Step 6

If the code still persists, check the vacuum hoses and lines in the entire EVAP (Evaporative Emissions Control) system, and especially between the vapor purge valve and the inlet manifold. Leaks in this part of the system allow unmetered air to enter the inlet tract, which will definitely upset the air/fuel mixture, thereby setting code P2187. Make repairs to vacuum lines as required, but make sure all connections are made and tightened to industry standards to prevent a recurrence of the problem.

NOTE: While leaks in other parts of the EVAP system, such as through defective control solenoids/valves might also produce code P2187 on some applications, it is more likely that leaks elsewhere in the system will produce a rich-running condition at idle due to the uncontrolled delivery of fuel vapor to the engine. This condition will usually be indicated by a closely related code, P2188 – “System Too Rich at Idle Bank 1”

Step 7

The steps outlined up to this point are almost certain to resolve nine out of every ten instances of code P2187 on most applications, but if the fault persists beyond Step 6, it is likely that there is an intermittent fault present in the system. However, intermittent faults can be extremely challenging and time consuming to find and repair, and in some cases, it might be necessary to allow the fault to worsen considerably before an accurate diagnosis and definitive repair can be made.

TIP: If an intermittent fault is suspected, but the engine vacuum system is known to be good, the cause is more likely to be related to wiring issues than to anything else. One way to trace intermittent faults is to repeat all checks on all relevant wiring, but to perform the “wiggle” test on all connectors while either the scanner or the multimeter is connected to the system or wiring respectively. “Wiggling” a connector will often produce a change in a resistance, signal -, or reference voltage in response to the “wiggling”- if such changes occur, there is a poor contact in the connector that is often easily repairable simply by cleaning the pins or terminals in the connector with an approved cleaner.

  • P2188 – Relates to “System Too Rich at Idle Bank 1”
  • P2189 – Relates to “System Too Lean at Idle Bank 2”
  • P2190 – Relates to “System Too Rich at Idle Bank 2”

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