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Fuel Trim & Fuel Control - Diagnosing Faults Part 1

Fuel Trim Background and Basics

In this series of articles, we will review gasoline engine fuel control, otherwise known as fuel trim.


Fuel trim is just that. The trimming or adding of fuel to the mixture depending on a feedback. Fuel trim began due to stricter tailpipe emissions and fuel economy. The DME’s job is to keep tail pipe emission below 1.5 ftp of that model year and vehicle class.

The DME makes adjustments from base fuel trim using a 3D map. The axis are usually RPM, Injector pulse width and load. Fuel trim is the correction for air seen by the feedback system in the engine. We will look at this in detail as work through this article series.

Fuel trim can be used to investigate possible fuel economy, rough idle and many other engine drivability issues. If your customer came in complaining of poor fuel economy and fuel trim was within specification, could there be an economy problem? Most likely not. Fuel trim would show the correction being made to correct for the economy problem.


Let’s begin by looking at generic fuel trim. Generic fuel trim should be similar to all vehicle manufacturers. These six codes are the standard generic fuel trim codes. They apply to all makes and models and when displayed, refer to the same type of fault.

Generic Fault Codes:


  • P0170 Fuel Trim Malfunction (Bank 1)
  • P0171 System too Lean (Bank 1)
  • P0172 System too Rich (Bank 1)
  • P0173 Fuel Trim Malfunction (Bank 2)
  • P0174 System too Lean (Bank 2)
  • P0175 System too Rich (Bank 2)

Short Term Fuel Trim (ST TRIM)


Short term fuel trim is the here and now of fuel trim. It is a direct reflection of what is happening in the exhaust stream, usually displaying the opposite range of the o2 sensor. For example, if the exhaust is lean, short term will be rich.

Short term fuel trim is not stored in the DME’s memory and resets to zero when the vehicle is restarted. Think of it as temporary of the moment corrections. Is short term used in open loop? NO Not until the oxygen sensor is active.





Narrow band o2 sensor. 20% correction of basic injector duration. After that a long term adjustment is needed.

Long Term Fuel Trim (LT TRIM)


Long term fuel trim is a learned air fuel mixture correction. This adjustment allows you to see how much of a correction was made to the mixture. It is stored in the DME when the key is turned off, when the engine is restarted, long term begins where it left off.

When using long term to diagnose a fuel trim fault. Keep in mind it may take longer for the DME to make an adjustment that it would with short term. You may force a change in the exhaust gas, yet long term will not change immediately.

Is long term used in open loop? YES It is used for starting.



Let’s take a look at how short and long term work in conjunction to correct for a vacuum leak. Normal pulse width is 3.0ms with a normal range of 10% correction. A vacuum leak occurs and short term goes to 20%. This may or may not be enough, in this case it is not. Long term has to make an adjustment. Long term goes to the first step +10%, still not enough, then to 20% and the air leak has been dealt with. More than likely a fault code is also set.


Mixture correction is a closed loop operation. What this means is it is determined by a feedback. Let’s call the oxygen sensor the end of the line. Really it all begins with a base calculation and fuel injector pulse, but once the engine is in closed loop, the oxygen sensor sends feedback to the DME, this feedback is used to make adjustments to the mixture. The adjustments are also monitored and changes are made to keep the vehicle tail pipe emissions in specification.

This graph shows the close relationship between oxygen sensor and short term fuel trim. Note the oxygen sensor pattern and how it matches the short term fuel trim graph.


Normal adjustments to the mixture will be made until it is out of range. This graph shows how fuel trim moves throughout the normal range, varying depending on rpm and load. Once it is out of range, a fault is set. This is why it is important to build a strategy to use with each diagnosis. A strategy that allows the same approach to each fuel trim fault.

To have a strategy, we have to first set a range for which we expect to see good or normal control occur in. When looking for a mixture problem, look at the whole picture. What is short term and long term doing together? Are they both rich all the time? Are they rich or lean at different engine load ranges? A good range is about + / - 10%, I usually see engines in the 8% range. When testing, you can add short and long together to see if there is a borderline issue or intermittent issue.

On V-style engines, it is important to look at both banks as individuals. Is one bank rich and one lean? Are they both doing the same thing? This can be helpful when diagnosing, as the problem may only affect one bank.


In the next part of this series, we will review the oxygen sensors role in fuel trim, narrow and wide band sensors.