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How To Diagnose High Negative Long Term Fuel Trim

Fuel trims, along with the engine control unit, work together to keep the fuel and air in a stoichiometric balance. This means that the fuel and air are mixed to a preset ratio. The percentages of air and fuel minimize emissions. They also maximize power and fuel use. Trimming the fuel and air mixture helps with diagnosing if the vehicle is running rich. 

What Should Long Term And Short Term Fuel Trim Be

There are two types of fuel trims: long-term and short-term. With long-term trims, the ECU looks at the oxygen-to-fuel ratio over a longer time. The sensors farther away from the catalytic convertor read the ratios. But, those sensors do not make changes to the way fuel is sent to the engine. 

The sensors downstream from the catalytic converter check the part’s efficiency. The ECU checks that both the upstream and downstream sensors are registering the same voltage. This can only be done over long-term monitoring. If there is a difference, something is amiss with the exhaust system, and possibly a negative long-term fuel trim. 

The downstream sensor should stay relatively similar to the upstream sensor. If it does not, then the catalytic converter has a problem. If all is functioning properly, the long-term fuel trim is balanced and adjusted correctly. 

Short-term fuel trim happens with the oxygen in the exhaust changes. This trim is monitored by the upstream exhaust sensor. When the exhaust passes the sensor, the ECU quickly adjusts the balance of the air and fuel. The ECU changes the mixture quickly – many times per second that the engine is running. 

The sensors that read the exhaust require a voltage of 0.2 to 0.9 volts. If those volts change, the ECU adds fuel or subtracts fuel with the injector pulse width. That width is the fuel trim. The proper stoichiometric voltage for the air and petrol is 0.45. If the number is high, that means the engine is running rich. 

Why Car Running Rich, Negative Long Term Fuel Trim

If the engine is running properly, fuel trims should be close to zero. However, some run high, like five percent. A scan tool can read the fuel trim. A negative long-term fuel trim means that the ECU has to decrease the injector pulse because the car is running rich. The vehicle needs less petrol in the blend. 

How To Diagnose Negative Long Term Fuel Trim

A scanning tool can diagnose negative long-term fuel trim. The scanner will give a number. If the number is negative, then the vehicle is running rich, with too much fuel in the mixture. 

How To Diagnose High Negative Long Term Fuel Trim

The sensor will not diagnose if the negative long-term fuel trim is high. But, you can watch the sensor share the negative trim numbers. If the negative trim goes past -10, then it is excessively high. The negative number shows you how much fuel the injectors are not sending into the mix. So, there is too much fuel in the blend. 

Possible Causes Of Rich Fuel Trim Mixture

If the vehicle is running rich, there are several underlying reasons, including:

  • High fuel pressure
  • Leaking petrol injectors
  • Malfunctioning MAF sensors
  • Leaks in the exhaust system
  • Loss of compression in at least one cylinder
  • Occasional misfires 
  • Defective O2 sensors

How To Reset Long Term Fuel Trim

The steps to reset the long-term fuel trim vary based on makes and models. Some reset by disconnecting the battery or removing a fuse. If you are removing a fuse, follow these instructions:

  • Idle car so it reaches average running temperature
  • Turn off the car
  • Remove the ECU fuse
  • Turn on the ignition, but don’t start the vehicle
  • Let the dashboard light up
  • Turn off ignition
  • Insert fuse
  • Turn on the ignition and watch lights
  • Push the gas pedal down a dozen times
  • Start engine


Q: What’s the difference between running lean and rich?

When the vehicle is running rich, the injectors are not feeding fuel into the mixture. If the vehicle is running lean, it has to add fuel. In both cases, the stoichiometric blend is off, so the ECU needs to alter air and petrol percentages. 

Q: Does running rich damage my engine?

Running rich can cause problems in the engine. When too much fuel enters the engine, it can leave carbon on the pistons and valves. This slows the performance of the engine and dirties the exhaust. The engine will continue to run, but it will be noticeably slower as parts get sooty from excessive fuel. 

A rich-running engine can also have problems with clogged catalytic converters. Excessive fuel can also damage the O2 sensor. 

Q: What causes long term fuel trim negative at idle?

If an engine is idling and running rich, there could be a problem with the intake manifold. When airflow is slowed, fuel trims are affected. The intake manifold vacuum could be running high if the throttle plate is closed while idling. An open throttle plate should fix the problem. However, if the vacuum is leaking, the fuel trim goes negative. 


Watching the fuel trim keeps your engine running longer. When the fuel trim is negative, the engine can have damage from running rich. Pay attention to your exhaust, fuel consumption, and performance to determine if you need to test your fuel trim.

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