P0173 – Fuel trim (FT), bank 2 -malfunction

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By Reinier (Contact Me)
Last Updated 2016-07-08
Automobile Repair Shop Owner
CodeFault LocationProbable Cause
P0173 Fuel trim (FT), bank 2 -malfunction
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Intake leak, AIR system, fuel pressure/pump, injector(s), EVAP canister purge valve, HO2S

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Table of Contents

  1. What Does Code P0173 Mean?
  2. What are the common causes of code P0173 ?
  3. What are the symptoms of code P0173 ?
  4. How do you troubleshoot code P0173 ?
  5. Codes Related to P0173
  6. Get Help with P0173

What Does Code P0173 Mean?

OBD II fault code P0173 is defined as “Fuel Trim Malfunction (Bank 2)”, and is set either when the PCM (Powertrain Control Module) experiences difficulty in controlling/maintaining the ideal air/fuel mixture, or receives a fuel trim request from the post-catalytic oxygen sensor that exceeds the PCM’s ability to adjust the fuel trim to meet the requested level of adjustment over a specified period of time. Bank 2 refers to the bank of cylinders that does not contain cylinder #1.

Fuel trim is the action taken by the PCM to adjust, or maintain the correct air/fuel mixture during normal engine operation. As the engine speed/load changes, the post catalytic oxygen sensor (oxygen sensor #2) continuously requests that adjustments to the air/fuel mixture be made to maintain maximum engine efficiency. Fuel trim requests can be either negative or positive; negative trim is an adjustment to make the air/fuel mixture leaner, while positive fuel trim is an adjustment to enrich the air/fuel mixture.

Typically, code P0173 relates to a condition in which long term fuel trim requests cannot be met, meaning that when the requested trim value exceeds the PCM’s  ability to adjust the fuel trim to the desired level by more than about 25% a code will be stored, and the CHECK ENGINE light will be triggered. Differences smaller than about 25% between the requested trim value and what the PCM is able to achieve are known as short-term fuel trim requests, and are much less critical to efficient engine operation. As such, these requests are far less likely to cause a code to be stored.

It is worth noting that code P0173 rarely occurs on American or Japanese-made vehicles. Vehicles most plagued by this code are of European manufacture, and most notably BMW, Audi, Volkswagen, and Mercedes-Benz models that are notorious for their high oil consumption rates.  On these models the root cause of code P0173 can almost always be traced to the Mass Airflow Sensor.

The image below illustrates how the pulse width, which is the amount of time (in milliseconds) the injector stays open to inject fuel is adapted to compensate for lean -, or rich-running conditions. Using the grey vertical line as a median point, the injector pulse width is increased to the right (red), to inject more fuel, and decreased to the left (blue) to inject less fuel. The information on which these adaptations are made is derived from the post catalytic (#2) oxygen sensor.


What are the common causes of code P0173 ?

The most common cause of code P0173 on European cars involves the MAF (Mass Airflow Sensor) due to excess oil in the inlet tract. Other possible causes could include the following, but note that not all causes will be present on all applications all of the time-

  • Oil contamination of the MAF sensor element. In these cases, the underlying cause(s) of the oil contamination must be resolved before attempting to diagnose P0173.
  • Oil-fouled oxygen sensors.
  • The use of fuel and oil additives that contain silicone based compounds. The combustion of these compounds forms deposits on oxygen sensors that prevent effective operation of the sensors.
  • Defective MAP (Manifold Absolute Pressure) sensor.
  • Poor quality aftermarket MAF sensors.
  • Defective oxygen sensors.
  • Burnt, shorted, damaged, or corroded wiring and connectors.
  • Vacuum leaks that allow unmetered air to enter the engine.
  • Serious exhaust leaks that can contaminate the oxygen sensors’ reference air.
  • High fuel pressure caused by defective fuel pressure regulators.
  • Low fuel pressure caused by restrictions in the fuel system or defective fuel pressure regulators and pumps.
  • Unresolved Crankshaft and Camshaft Position sensor codes. Such codes must be resolved before diagnosing code P0173.
  • Clogged or damaged catalytic converter.
  • 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 P0173 ?

The symptoms of code P0173 on effected vehicles can vary greatly, but note that not all symptoms will be present in all cases. Also be aware that that the severity of one or more symptoms may differ between applications. Possible symptoms could include the following-

  • In some cases, there may be no symptoms other than a stored trouble code and an illuminated warning light.
  • Reduced fuel economy that can vary from slight and barely detectable, to dramatic.
  • Hard starting can occur on some applications.
  • Misfiring, rough idling, and poor acceleration are possible on affected vehicles i.e., Mercedes-Benz, Audi, BMW, and VW models.
  • Visible black smoke from the tail pipe is possible in extreme cases.
  • Hesitation upon acceleration.
  • In some cases, the engine may shut off immediately after start-up.

NOTE: Almost all of the above symptoms can lead to eventual engine and/or catalytic converter damage or even failure if the code is not resolved.

How do you troubleshoot code P0173 ?

NOTE #1: It is rare for code P0173 to present without other fuel system related codes being present as well. Due to the large number of possible causes of P0173, it is recommended that all other codes present be diagnosed and repaired before attempting to repair P0173 to avoid a misdiagnosis.

NOTE #2: Diagnosing code P0173 requires that the engine be in perfect running condition, and that no misfiring, fuel pressure, vacuum system, or air/fuel metering codes and issues are present.  Many of these codes are caused by issues that directly affect combustion, which could result in code P0173 being set.

NOTE #3: Apart from a suitable scanner or code reader, you will need a repair manual, a good quality digital multimeter, and a dedicated fuel pressure gauge to diagnose code P0173. It is also a good idea to retest the system after each step in the repair process to ensure a reliable repair.

Step 1

Record all codes present, as well as all available freeze frame data. This information can be of use if an intermittent fault is diagnosed later on.

NOTE: If the vehicle being worked on is of European manufacture, it is possible to save a lot time by starting the diagnostic procedure by inspecting, and cleaning the MAF (Mass Airflow Sensor) element with an approved solvent. High oil consumption can deposit a film of oil on the element, causing inaccurate readings, and in many cases, a simple cleaning of the element will resolve this code.

Step 2

Test the operation of the MAF sensor with the scanner. The signal voltage of the MAF sensor should rise smoothly to about 5 volts as the engine speed increases with a wide open throttle, and decrease smoothly to about 0.1 volt as the engine speed decreases. Note that the throttle should NEVER be opened fully with the engine running, since over revving could cause serious damage, if it does not actually destroy the engine.

If the code persists after cleaning the MAF sensor element, perform a thorough visual inspection of all associated wiring. Consult the manual to determine the location, color-coding, function, and routing of all wires in the circuit. Look for damaged, burnt, broken, shorted, or corroded wiring and connectors. Repair as required.

IF all electrical readings fall within the manufacturer’s specifications, replace the MAF sensor with an OEM part, and retest the system to verify that the MAF sensor and its control circuit work to specifications. Note that aftermarket MAF sensors is a leading cause of P0173 and many other related codes, so avoid using aftermarket parts as far as possible.

Step 3

Consult the manual on the input voltage, resistance, and reference voltage values for the post catalytic oxygen sensor. Disconnect the sensor circuit from the PCM to avoid damaging the controller, and perform resistance, reference voltage, continuity, and ground checks on the control circuit. Compare obtained values with those stated in the manual, and make suitable repairs if any discrepancies are found.

The #2 oxygen sensor itself forms part of the circuit, so be sure to check its resistance as well, but pay particular attention to the sensor’s heater control circuit during this step. The heater element in an oxygen sensor plays a critical role in the overall performance of the sensor, and a bad heater element or heater control circuit can cause wildly inaccurate readings, especially when the sensor is still cold. The heater circuit input is typically equal to battery voltage; be sure not to miss this measurement at the sensor connector.

Step 4

If all electrical values fall within the manufacturer’s specifications, including the resistance of the sensor itself, or after repairs had been made (including replacing the sensor if required) to ensure that they do, clear the code and operate the vehicle to see if the code returns.

If the scanner can monitor live data streams, narrow its monitoring function down so that it monitors only the #2 oxygen sensor. Note that for readings to be accurate, the PCM needs to have entered closed loop operation. At a steady engine speed, the displayed reading will be somewhere close to the mid-point between a rich- and lean running condition. Changes in engine speed/load will produce deviations to either side of this point as the PCM adjusts the fuel trim.

NOTE: Large throttle inputs will/must produce large changes in the displayed reading. However, the reading must immediately return to a lean(er) condition as the throttle opening is reduced. These changes are known as negative and positive fuel trim, and in a system where everything works as intended, these changes will occur constantly. Nonetheless, it is possible for these changes not to occur, even though the electrical side of the sensor and its control circuit is in perfect condition.

Step 5

If the code persists, but all electrical values fall within the manufacturer’s specifications, it is likely that fuel pressure is not up to specification. Even marginally out-of-spec fuel pressure can cause either a permanent rich-running condition that can defeat the PCM’s ability to control the fuel trim, or a permanent lean-running condition that has a similar effect on the PCM’s ability to control the fuel trim.

Although it is rare for fuel pressure problems not to be indicated by a warning light (or a stored trouble code for that matter), it does happen and especially on older vehicles. Consult the manual for the ideal fuel pressure range for the application being worked on, and follow the procedure outlined in the manual to test the actual fuel pressure in the system with a dedicated fuel pressure gauge.

There are many possible reasons why fuel pressure can be either too high or too low at the fuel rail, but resist the temptation to automatically assume that the fuel pump is at fault. Perform a thorough visual inspection of the entire fuel system, and check for damaged, kinked, pinched, or otherwise damaged fuel lines that could restrict the flow of fuel, since the volume of fuel that is delivered to the engine is as important as the pressure it is delivered at.

If damaged fuel lines are found, resist the temptation to repair the lines. To avoid future issues with leaks in the system, the better option is always to replace the entire line with an OEM replacement. Also be sure to check the condition of the fuel filter; replace the filter if there is any evidence of it being clogged or dirty.

Step 6

If the fuel lines all check out fine, consult the manual on the correct procedure to test the fuel pressure regulator, which could be located either on the fuel rail, or in the fuel tank as part of the pump canister. Replace the fuel pressure regulator if obtained readings do not fall within the manufacturer’s specifications.

Clear all codes, and retest the fuel pressure after repairs had been made to verify that the fuel pressure falls within the specified range. Operate the vehicle with a scanner connected until the PCM enters closed loop operation. Pay close attention to the operation of the post catalytic oxygen sensor, and how it registers changes in the fuel trim.

If the code does not immediately return after the completion of a few drive cycles, it would be safe to assume that the problem had been resolved. However, if the code does return, it is likely that the catalytic converter is at fault. It may be clogged, or damaged due to the long-term effect of code P0173.

NOTE: The conclusion that the catalytic converter is at fault must be weighed against the certainty that-

  • the MAF sensor and its control circuit both work as intended,
  • the #2 oxygen sensor is not defective, and that its control circuit is fully functional,
  • the fuel pressure is within range and that there are no leaking or defective fuel injectors,
  • there are no vacuum leaks present that could influence the air/fuel ratio,
  • there are no misfires present,
  • no serious exhaust leaks exist that could contaminate the oxygen sensor’s reference air,
  • and that there is no excess oil in the inlet tract, as is very frequently the case with forced-induction European engines.

Note that catalytic converter replacements can be hugely expensive, and that all other avenues must be explored to resolve the problem before a converter is replaced in error.

Step 7

If despite all attempts to resolve the problem the code persists, or returns sporadically, it is likely that an intermittent fault is present. Sporadic or intermittent causes of code P0173 can be extremely challenging to find and resolve due to the relatively large number of causes, and in some cases it may be necessary to allow the problem to worsen considerably before an accurate and definitive repair can be made.

P0170– Relates toFuel Trim Malfunction (Bank 1)”. Bank 1 refers to the bank of cylinders that contains cylinder #1.

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