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How to use an OBDII scan tool — Live Data

How to use an OBDII scan tool — Live Data

How to understand live data from your OBD II scan tool

Using the live data feature on your scan tool is analogous to a doctor reading an EKG or seeing lab results. It tells you what’s going on behind the scenes and helps you spot data that’s outside normal limits.

Start with the most important data — Fuel Trims

What are Fuel Trims?

Every vehicle is programmed at the factory to calculate air/fuel mixtures so the engine complies with EPA emissions standards and you get the best performance and best gas mileage.

If you were to run the engine at a steady throttle (RPM), with a steady load and on a flat surface, at the same engine temperature, and with the same outside air temperature the ECM would never need to adjust the air/fuel mixture. But that never happens in the real world. In other words, those variables are constantly changing, so the ECM must make constant adjustments. In addition to the changes in the variables listed above, the factory software is based on the performance of a new engine.

However, as engines and sensors wear, the computer must make adjustments to the factory programming to keep the emissions within spec and provide the performance you expect. Those adjustments are referred to as FUEL TRIMS.

Fuel trims are shown as percentages and can be either positive (+) when adding more fuel or negative (-) when the computer subtracts fuel.  So a reading of 0.0% means that the ECM didn’t have to make any adjustments to the factory programmed air/fuel mixtures. However, this is rarely the case in the real world.

How does the ECM know when to make changes to fuel trim?

The ECM uses inputs from these sensors to calculate air/fuel mixtures:

• Engine coolant temperature tells the ECM engine temperature
• Ambient air temperature
• Manifold absolute pressure allows the ECM to calculate barometric pressure. It reads manifold pressure before engine start (barometric pressure) and manifold vacuum after engine start. Barometric pressure minus manifold vacuum = manifold absolute pressure.
• Mass airflow sensor tells the ECM the mass of the air entering the engine.
• Throttle position sensor tells the ECM whether your foot is on the pedal and by how much.

Based on all those inputs, the ECM calculates an air/fuel mixture it thinks will be correct.

Then, it reads the oxygen sensor data to see how well it did with its calculations. If the exhaust shows high or low oxygen levels, the ECM adjusts the air/fuel mixture. Those adjustments are referred to as fuel trims.

Short-term fuel trim versus long-term fuel trim

As you accelerate and decelerate, the ECM is constantly making adjustments to short term fuel trim (STFT) based on feedback from the oxygen sensors. If the ECM determines that it must add or subtract fuel constantly, it moves those adjustments into longer term memory referred to as long term fuel trim (LTFT).

Normal and abnormal STFT and LTFT

• Normal STFT data will shift between -10.0% and 10.0%
• Normal LTFT data will shift between -5.0% and 5.0%.
Readings outside of these ranges are an indication of engine or fuel system problems.

High fuel trim readings

A fuel trim of +25% for example means the ECM is adding a LOT of fuel. That can be caused by many things like:
• A vacuum leak that’s allowing unmetered air into the engine, resulting in a lean air/fuel mixture. To bring the mixture back into spec, the ECM is adding fuel up to it’s maximum of 25%. If the ECM still can’t get the right air/fuel mixture after adding 25% more fuel, it will set a check engine light.
• Low fuel pressure that’s not injecting enough fuel into the cylinder.
• A clogged fuel injector(s) that are restricting the flow of fuel into the cylinder.
• A dirty of faulty MAF sensor can cause the ECM to run a lean mixture which then requires a positive fuel trim

Low Fuel Trim Readings

A negative fuel trim means the computer is subtracting fuel. A fuel trim of -25% means the ECM is subtracting a lot of fuel from the mixture. A negative fuel trim can be caused by:

• Leaking fuel injectors that are sticking open when they should be fully shut off.
• A clogged air filter
• A dirty or faulty MAF sensor


Live data will also show your engine’s RPM. Unless you’re touching the throttle, this will be the engine’s idle RPM, which is what we want to check. For cars, idle speed is usually between 600 and 1000rpm, while for trucks its around 600, and motorbikes are higher at between 1200 and 1500rpm.

If your RPM is falling outside these ranges, then it’s probably down to an issue with a faulty or carboned idle control valve or electronic throttle body, or a vacuum leak. These are probably two good places to start looking if your OBD2 scanner reports strange idle RPM values.

Oxygen Sensors

Oxygen Sensors show the level of oxygen in the exhaust stream before and after the catalytic converter. All cars made after 1996 are required to have two oxygen sensors. By monitoring the oxygen in the exhaust, the computer can determine how to adjust the air/fuel mixture.

An OBD2 scanner will display the oxygen levels in terms of voltage. If the air-fuel mixture is at the perfect mix, called the stoichiometric ratio, then you should get a reading of 0.45V. Of course, nothing is ever perfect, so you want to be looking for a fluctuation around 0.5V. Since the computer is constantly changing the air/fuel mixture based on engine RPM, it’s normal to see rapid fluctuations in the pre-cat oxygen sensor readings, from lean to rich.

However, if the pre-cat sensor reading is fairly steady at close to 1.0 volt, that indicates that the fuel mixture is too rich, while anything going towards 0.0V would suggest that the fuel mixture is too lean. The first sensor should be switching more often between rich and lean readings.

Since the post cat sensor is reading “cleaned up” exhaust, the reading should remain steady at around 0.5V.

Intake Air Temperature

The intake air temperature data lets the user know the temperature of the air that the engine is pulling in. The engine system needs this data itself so that it can estimate the air density and then calculate the correct fuel-air mixture. Cold air is denser than warm air, so requires more fuel to maintain the correct fuel-air ratio. The opposite is true for warm air.

The sensor should display a temperature very similar to the air temperature in the workshop. A value that is quite different could suggest a few problems. The most likely though is that the sensor itself is not functioning correctly. Symptoms of this problem could be a drop in acceleration due to an inefficient air/fuel ratio, or poor fuel economy. A damaged IAT sensor will produce trouble codes. These trouble codes will be processed by the code reader aspect of the OBD2 scanner, so you’ll know when the sensor is broken.

Ignition Timing

The live data for Ignition Timing represents the amount of delay the vehicle’s computer is adding before the ignition commences. If the engine detects a rich fuel mixture, then the ignition will happen sooner, to prevent flooding. If the fuel mixture is lean, then the engine will delay the ignition slightly to allow more fuel into the combustion chamber. This is all based on the Ignition sensor inputs.

If the CPU is adding on a lot of time, then the fuel is too lean and this will be reflected in the Fuel Trim recordings. The opposite goes if the fuel is too rich. Ignition Time is a good early warning about a problem with the air/fuel mixing process, but it won’t tell you any more about the specific issue.

Manifold Absolute Pressure (MAP Sensor)

The MAP sensor data readings are usually specific to fuel injected engines. MAP sensors measure the absolute air pressure (barometric pressure minus intake vacuum) in the intake manifold, which helps the engine CPU decide on the correct air/fuel ratio. The values it displays are pressures, measured in Inches of Mercury (in. Hg). 1 bar of pressure is equivalent to 29.4 in. Hg.

Like the Ignition sensors, faulty MAP sensors will lead to an incorrect fuel/air balance, resulting in excessive fuel consumption, lack of power, and failed emissions tests. Again, this is useful data to indicate possible problems with your car.

Posted on by Rick Muscoplat

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