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Fuel Trim & Fuel Control - Diagnosing Faults Part 2

Fuel Trim Feedback System - A Closed Loop

In this, the second part of our series on Fuel Trim, we will discuss the feedback part of the system - The Closed Loop of fuel control.

Fuel trim is an adjustment from Lambda. Lambda 1 = 14.7-1



With early fuel control, single wire o2 sensors and OBD 1 vehicle, for example like General Motors. The would set mixture faults for Lean - Code 44, or Rich - Code 45. When fuel trim was viewed using a scan tool, it was displayed in a format with the range of 0 -255 and named RICH / LEAN Transition. This was one computer byte, and you were viewing more or less nontranslated compiuter data of the adjustment. Usually in the 149 range, in my area.






Why do we have pre and post catalyst oxygen sensors? To monitor catalyst efficiency. Is that all? 



The DME monitors catalyst oxygen storage to determine if a catalyst is failing or close to it. it does this by comparing pre and post catalyst switch times and amounts. If the DME sees a catalyst beginning to fail, it will adjust fuel control to provid a better feedgas for the catalyst in question.




When the catalyst requires a change in the feed gas the DME makes adjustments to rear fuel trim. Rear fuel trim has been around for some time and I would wager that most of you have dealt with these issues. Rear fuel trim has priority over front. Of the front o2 signal requires a change and the DME sees the catalyst is not operating efficiently it will over-rule that change and make another adjustment to change the feed gas to the catalyst.


This is a screen shot of all the available data PIDs of the same vehicle shown in the last slide. The factory scan tool displayed the rear fuel trim PID, the aftermarket tool using factory mode did not. Keep this in mind, not all data will be displayed. be sure to a factory equivalent software like Autologic.


Let's Review

Why regulate rear trim?


  1. Cat efficiency
  2. Proper feed gas to cat
  3. Keep degrading cat working efficiently
  4. FT control if front o2 fails
  5. Can over rule front o2 sensor




When dealing with cat efficeincy faults, Check exhaust system forward of downstream (post-cat) O2 sensor for leaks.


BMW & MINI New Generation engines all have rear fuel trim fault monitoring.


Now that we have covered generic fuel trim, let’s look at how BMW and other European makes control mixture. All of you are familiar with these terms. Lets take a look at each one and discuss what they do.


Additive Fuel Trim

"Add" means additive trim, which is addressing an imbalance at idle. When the ECU is using additive trim, it is telling the injectors to stay open a fixed amount longer or shorter. The malfunction (e.g. vacuum leak) becomes less significant as RPM increase. For additive adaptation values, the injection timing is changed by a fixed amount. This value is not dependent on the basic injection timing. 

This is the boring Bosch definition for Additive Trim. It does however tell us quite a bit about this mixture control strategy. Let’s look at a few things.

Additive Fuel Trim is mostly used at idle. It is stored in the DME for start up, that makes it a Long Term Fuel Trim. Additive fuel trim has the most influence at idle range loads, but is also used off idle for minor trimming adjustments.

The term additive is used because of the way fuel is added to the mixture. if the base fuel injection time is 3.0 millisecond and a change is needed, it is made by adding a fixed  number. For example 0.1ms or -0.1ms. Not a percentage based amount like generic fuel trim.

Multiplicative Fuel Trim

"Mult" means multiplicative trim, which is addressing an imbalance at all engine speeds. The malfunction (e.g. clogged injector) becomes more severe at increased RPM. For multiplicative adaptation values, there is a percentage change in injection timing. This change is dependent on the basic injection timing.

That is once again, Bosch's definition of multiplicative trim. Multiplicative is a percentage based adjustment made at part load engine load ranges. It is also a Long Term Fuel Trim like Additive, and used at start up as well.

If Additive and multiplicative are both long term fuel trim, where does the short term, or here and now happen. This can be seen in oxygen sensor feedback. You can use this to vie the short term correction in manufacturer specific fuel control when Additive and Multiplicative fuel trim are used.

One other way you may see Additive Trim displayed, instead of the millisecond correction mentioned above is in milligram per stroke. Treat this the same way. What has changed is the languae. Instead of displaying the fuel correction amount, the amount of excess air being corrected for is disolayed.

When using a code look up tool or searching for a definition, keep in mind that there may be multiple definitions for each code and they will vary depending on model.

For example, BMW N54 engines can have different fault codes that each pertain to a single Generic fault code.

N54 Engine  
Generic BMW


 10721–10 sec. mixture adaptation at travel limit in max. 60 sec.
10796–30 sec., lambda controller pegs in max. 60 sec



10720 - 10 sec. mixture adaptation at travel limit in max. 60 sec.

10795 - 30 sec., lambda controller pegs in max. 60 sec


11313 - Trim controller pegge



We know that when fuel trim gets out of range a fault is set. Is there information to tell us when a fault will set. Information that can help us determine is an engine is close to setting a fault.

At times, manufacturers will give us information as to when the fault codes set. This information can be found within the fault code description or Mode 6 data, for the spcific fault code.

When diagnosing, it is important to know good and bad values. BMW use to tell us these within their diagnostic tools. We have compiled a list of known good values from when this information was available. it can be viewed here: Known Good Set-Point BMW Values

For example, fuel trim faults set on New Generatiuon BMW engines at -23% to 23% of correction.

For Mercedes-benz it is a bit more difficult to find this information, as they leave the fault cirteria out of the Mode 6 data for most models. VW / Audi are pretty good at sharing the criteria for fault setting in the fault code description. See the fourth column from the left in the image for details on what it takes tro set a P0171.


when diagnosing or understanding a fuel trim fault, it is important to know why and how the fault is set. If the specific code can only be set at idle, it is important to know that for testing. Use manufacturer fault code descriptions, Mode 6 data and your own experience when going up against these faults.

In the next and fianl part of this series, we will discuss how to diagnose fuel trim and the feedback components that keep it working.