Fuel Trim Malfunction (Bank 1)


The PCM has detected a problem with its ability to control fuel/air mixture when necessary, or received a post catalyst fuel trim request which exceeds its limitations for a particular engine. Fuel trim malfunctions are rarely exhibited in domestic and Japanese vehicles but are more common in European vehicles. Bank 1 denotes the engine bank that contains the number one cylinder. Refer to the manufacturer’s service manual (or equivalent) for the location of the number one cylinder.

Code Set Parameters

Fuel trim is the measure taken by the PCM to increase or decrease the amount of fuel supplied to the engine in order to maintain the correct air/fuel ratio or mixture. This trouble code is set when the PCM detects a fuel trim level that exceeds the manufacturer’s specified percentage over a given period of time. Typically, deviations that exceed twenty-five percent of this long-term fuel trim desired value will cause a code to be stored and a malfunction indicator lamp to be illuminated. Short term fuel trim input data is less critical and less likely to contribute to a code being stored.


Symptoms for this code can vary greatly, due to its generality. Negative fuel trim indicates that the PCM is attempting to lean the air/fuel mixture and a positive fuel trim indicates that the PCM is attempting to richen it. Symptoms can include just a service engine soon lamp, poor acceleration, a hesitation or misfire upon acceleration or at start up, engine stall at start up, black smoke upon acceleration (or at start up), sub-par fuel economy, or there may be no symptoms at all. Some code setting conditions can lead to engine and/or catalytic converter damage if left unattended over a long period of time.

Common Causes

The most common cause of this code occurs primarily in European vehicles such as Mercedes-Benz, Volkswagen, BMW, and Audi. It can be traced to the mass air flow sensor. Either the mass air flow sensor connector becomes contaminated with engine oil, the sensor hot wire becomes fouled with oil or other debris, or the air inlet pipe is cracked or broken allowing unmetered air to enter the engine. Aftermarket mass air flow sensors have also proven to be problematic on this type of vehicle. Faulty oxygen sensors are also fairly common in European models, due to oil fouling in the exhaust system. Other possible causes include an excessively rich fuel condition caused by a faulty fuel pressure regulator, faulty manifold air pressure sensor, or a faulty PCM. This code may also be caused by a lean fuel condition caused from a faulty fuel pump, bad fuel pressure regulator, clogged fuel filter, or engine vacuum leak. Keep in mind that PCM failure is rare. Turbocharged engines present their own symptoms, including fuel contaminated engine oil leaks that are drawn into the induction system. If camshaft or crankshaft sensor codes, ignition misfire codes, or low fuel pressure codes are also present, diagnose and repair them before attempting to diagnose this code. Catalytic converter failure may also contribute to this code being stored.

Common Misdiagnosis

Technicians report that this code rarely occurs without the accompaniment of other fuel system codes. This code is rarely exhibited in vehicles that are not manufactured in Europe. Failure to diagnose other codes, before diagnosing this code, can lead to a misdiagnosis.


  • Several tools will be instrumental in successfully diagnosing this code
  • A suitable OBD-II scanner (or code reader), a digital volt/ohmmeter, and an infrared thermometer with a laser pointer will be most helpful in performing a successful diagnosis
  • A manufacturer’s service manual (or the equivalent) will also be necessary
  • Begin with a visual inspection of all wiring and connectors
  • Repair or replace damaged, disconnected, shorted, or corroded wiring, connectors, and components as necessary
  • Always retest the system after repairs are completed to ensure success. If all system wiring, connectors, and components (Including fuses) appear to be in normal working order, connect the scanner (or code reader) to the diagnostic connector and record all stored codes and freeze frame data
  • This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored
  • Continue by clearing the code and operating the vehicle to see if it returns
  • This will help to determine whether or not the malfunction is intermittent
  • After the codes are cleared, test drive the vehicle to see if the code returns
  • If the code fails to immediately return, you may have an intermittent condition
  • Intermittent conditions can prove to be quite a challenge to diagnose and in extreme cases may have to be allowed to worsen before a correct diagnosis can be made. If the code persists, then you should begin your diagnosis by determining whether the fuel trim level is negative or positive
  • Once this is determined, proceed with testing the engine and fuel system for proper operation (including testing fuel pressure, inspecting for vacuum leaks, etc.)
  • If these factors are found to be within the required specifications, then begin testing engine sensors, electrical wiring and connectors, as well as the PCM. Since mass air flow failure is by far the most common cause of this code, here is the process for diagnosing it
  • Using the scanner (with the data stream narrowed to include only the mass air flow) observe the sensor reading in grams per second
  • Compare your findings with the manufacturer’s specified reference value at varying RPMs
  • As the throttle is gradually opened grams per second (read by the PCM as voltage) should increase respectively
  • The voltage signal from the mass air flow sensor should increase smoothly as the RPM level increases and decrease as RPMs decrease
  • If actual data fails to coincide with the manufacturer’s specifications, replace the mass air flow sensor with an OEM quality part and retest. If the mass air flow sensor checks out, monitor oxygen sensor voltage using the scanner (again with the data stream narrowed to include only oxygen sensor data)
  • If a lean exhaust condition is indicated, check the fuel pump to make sure that it is making sufficient pressure and inspect the fuel system for leaks
  • Check the fuel pressure regulator for internal fuel leaks and proper operation
  • Repair or replace fuel delivery components as required and retest the system to ensure a successful repair has been performed
  • Check for vacuum leaks at vacuum hoses, throttle body gaskets, and intake gaskets
  • Also check the EGR valve and gaskets for potential vacuum leaks
  • Repair vacuum leaks and/or replace defective air intake parts as needed
  • Clear the codes and test drive the vehicle to ensure that the repair has been successfully completed
  • Test the positive crankcase ventilation system for proper operation and replace the PCV valve or hoses as required
  • Check turbo hoses for leaks and/or the presence of engine oil/fuel
  • The presence of oil in the exhaust system can also contribute to a fouled oxygen sensor and a resultant fuel trim code
  • Replace, repair, or reconnect turbocharger hoses as required, then clear the codes
  • Always test drive the vehicle to ensure that a successful repair has been completed