P2191 – System too lean at higher load, bank 1

By Benjamin Jerew (Contact Me)
Last Updated 2017-03-31
ASE Master Tech

Trouble CodeFault LocationProbable Cause
P2191 System too lean at higher load, bank 1 Fuel pressure, injectors, intake leak

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What Does Code P2191 Mean?

For both efficiency and performance, injecting the right amount of fuel is critical. Injecting too much or two little generally does not result in more power or better efficiency, but poor performance and excessive emissions. The modern engine control module (ECM) uses a number of sensors and programming to determine how much fuel to inject, depending on driver demand, engine operating conditions, and environmental conditions.

o2Sensor

The ECM monitors intake parameters, such as mass air flow (MAF) or manifold absolute pressure (MAP), intake air temperature (IAT), fuel pressure, engine coolant temperature (ECT), engine speed (RPM), and throttle position (TPS), to name a few, which it uses to determine injector pulse width (IPW). The, using feedback from knock (KNK), engine speed (RPM), and air fuel ratio (AFR) sensors, among others, the ECM fine-tunes IPW to ensure complete combustion. This “fine-tuning” is called fuel trim (FT). The result is the best performance and fuel economy per drop of fuel.

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Generally, the ECM strives to achieve the optimum AFR of 14.7:1 (stoichiometric ratio), but changing conditions always require a certain amount of FT, and 0% FT is ideal. For example, if the ECM detects higher oxygen levels, via the AFR or Lambda sensor, it interprets this as not enough fuel being injected to burn all the available oxygen, and will add more fuel. This is positive fuel trim. Conversely, if the ECM detects lower oxygen levels, it will interpret this as excessive fuel being injected, and it will reduce fuel injection. This is negative fuel trim. Usually, the ECM considers ±5% FT to be acceptable, but this threshold varies according to manufacturer.

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If FT goes over a certain threshold, again depending on manufacturer, the ECM considers this a fault and will set a diagnostic trouble code (DTC) and illuminate the malfunction indicator lamp (MIL). If the ECM is consistently injecting fuel at +25% FT, for example, the ECM determines that the engine is running lean (not enough fuel or too much air), and will set an FT code. At higher loads, such as on acceleration, towing, or highway cruising, if the system is running lean, the ECM will store DTC P2191, which is defined as “System Too Lean at Higher Load (Bank 1).”

What are the common causes of code P2191?

Depending on year, make, and model, DTC P2191 may have number of causes. Here are some of the most common.

  • Vacuum leaks are the most common problem we have encountered. Loose or broken vacuum hoses and intake leaks are common. Plastic intake gaskets on certain vehicles have been known to crack, leading to vacuum leaks.
  • MAF contamination – Certain vehicles seem to be more susceptible to contamination. Be sure your MAF is clean and unobstructed.

What are the symptoms of code P2191?

Aside from the MIL, you may not notice any drivability symptoms. Depending on the severity of the problem, you may experience poor engine performance, misfiring, and poor fuel economy, usually accompanied by other DTCs in ECM memory.

How do you troubleshoot code P2191?

NOTE: It does one good to note that the ECM uses the AFR sensors to monitor FT, which means that the ECM cannot run FT monitors unless the AFR sensor monitors have passed. Therefore, in 95% of cases, fuel-trim related faults cannot be repaired by replacing an AFR sensor or the ECM.

As mentioned, this fuel trim fault means that the ECM has determined that there is too much air or not enough fuel, so what we are looking for is unmetered air or insufficient fuel problems. If there are any other DTCs in the system, including those related to ignition, injection, air temp, throttle control, idle control, oxygen sensors, air flow, or air pressure (IG, FI, ECTS, IAC, HO2S, AFR, MAF, MAP) diagnose and repair these before attempting to correct a fuel trim lean condition. That the DTC refers to high engine load conditions may give you a better idea about when it happens, which may help in diagnosis of the problem.

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You’ll need a DMM (digital multimeter), fuel pressure gauge, and a mechanic’s stethoscope. A model-specific repair manual will come in useful when checking specific sensor readings and voltages, and a scan tool with live data capabilities can be especially helpful if you put up MAF/MAP, AFR, and FT data points.

  • Before troubleshooting, check with your dealership service center to see if any TSBs (technical service bulletins) apply to your vehicle. Some automakers have released ECM software updates to address this issue, which will require a factory scan tool and access to the update software. It may even be covered by your warranty.
  • General Check – Start by checking for obvious problems in the fuel and air metering system and repair as necessary.
    • Low fuel pressure, and therefore insufficient fuel, could be caused by a fuel leak or pinched fuel line. If you have a multi-speed fuel pump, check its fuses and relays. If the fuel pump doesn’t go to high speed on high load, it could lead to insufficient fuel pressure and a lean condition.
      • Unmetered air can find its way into the system if there are vacuum leaks. Check for intake gasket leaks, broken or disconnected vacuum lines. A faulty PCV (positive crankcase ventilation) valve, missing oil fill cap, or missing oil dipstick could also result in a vacuum leak. Intake gasket leaks can be very difficult to pinpoint, and may require disassembly to confirm. A mechanic’s stethoscope, with the probe removed (just an open hose) can be useful in pinpointing some vacuum leaks. Listen for a whistling or “sucking” sound. Another method used is to spray brake cleaner on suspected areas, listening for changes in engine operation (the engine will idle down if you temporarily plug the leak with fluid).
      • Exhaust leaks, particularly between the cylinder head and AFR (air fuel ratio) sensor, could allow air into the exhaust stream, skewing AFR readings. The AFR sensor would read the excess oxygen, which the ECM would interpret as a lean condition. The open mechanic’s stethoscope can be helpful here, as well, to pinpoint the location of an exhaust leak. Listen for a puffing sound.
      • As a matter of course, check for blown fuses, wire harness damage, and that electrical connectors are in good condition and properly seated. Repair as necessary.
  • MAF / MAP Check – Everything starts with the MAF or MAP sensors, which indicates how much air is getting into the engine. If the sensor is contaminated, bypassed, or otherwise compromised, an inaccurate measure of air will throw off every single reading down the line, including AFR and HO2S (heated oxygen sensor) readings.
    • MAP – Check that the vacuum line going to the MAP sensor is intact and unblocked. Carbon buildup can skew MAP readings, resulting in faulty FT adjustments. You can bench-test the MAP sensor by applying vacuum to the sensor and comparing voltage output from the sensor. Replace the sensor if MAP voltage doesn’t match repair manual specifications.
    • MAF – Remove the MAF and inspect visually with a flashlight. If you notice that the sensing wires or grid wires are “fuzzy,” it is not going to be able to measure air flow properly. You may be able to clean it with MAF sensor cleaner. Not all sensors respond to cleaning, however, and will require replacement.
  • Fuel Check – You can verify correct fuel pressure using a fuel pressure gauge and taking your vehicle for a test drive, attempting to recreate the high-load conditions of the DTC. Freeze-Frame data will be helpful in this situation. If fuel pressure is insufficient, not enough fuel will be injected. Check the rest of the fuel system if there is a fuel pressure problem, such as a leaking fuel line or defective fuel pressure regulator.

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Codes Related to P2191

  • P0171 System Too Lean (Bank 1)
  • P0174 System Too Lean (Bank 2)
  • P2177 System Too Lean Off Idle (Bank 1)
  • P2179 System Too Lean Off Idle (Bank 2)
  • P2187 System Too Lean at Idle (Bank 1)
  • P2189 System Too Lean at Idle (Bank 2)
  • P2193 System Too Lean at Higher Load (Bank 2)

All related codes are similar but depend on general operating conditions at the time the fault was noted, which may help in determining the cause. As usual, there are the similarities between Bank 1 and Bank 2, just different locations.

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