When you first plug an OBD2 scanner into your vehicle’s port, it will display live data that shows a number of different parameters. These live readings can help you identify any problems with your vehicle’s powertrain or driveability.
The first two live readouts you should look at are STFT (short term fuel trim) and LTFT (long term fuel trim). Normal STFT readings will shift between 0.0% and 0% when the engine is running and accelerating.
Pre-Cat Oxygen Sensor
The Pre-Cat Oxygen Sensor (or O2 sensor) is part of the global OBD II system and is designed to modify fuel delivery and maintain the efficiency of the catalytic converter. Typically this is done by constantly reading the oxygen levels in exhaust gas.
If an O2 sensor continually reads low (lean), the engine computer will enrichen the fuel mixture to compensate. This can result in lean misfires, hesitation and poor idle as well as increased fuel consumption and carbon monoxide emissions.
On some newer vehicles, a different type of oxygen sensor is used that allows the ECM to control emissions much more accurately. These are called “wide-band” sensors and they are capable of measuring the exact air/fuel ratio rather than just giving an output signal that is rich or lean.
These are often less prone to contamination and should be inspected or replaced every 60,000 miles. However, they also have multiple circuits that can go bad and cause electrical issues.
The Manifold Absolute Pressure (MAP) sensor is one of the most fundamental components in the engine management system. It provides instantaneous manifold pressure information to the Power Train Control module (PCM) and is used for fuel control, ignition and emissions.
The MAP sensor output signal is a variable electrical output voltage that is proportional to the intake manifold vacuum. As the throttle opens and vacuum drops, the return signal increases.
When the engine is not running, the MAP sensor voltage is typically around 4.5 V. As the engine fires up, this signal decreases to 0.5 V, and when the throttle is fully opened, it stabilizes at 1 V.
A good rule of thumb is to measure ancel obd scanner sensor readings with the key on and then with the engine off at different altitudes. In addition, check the sensor’s reference voltage and ground connections using a scan tool.
The Throttle Position Sensor (TPS) is an electronic device that signals the engine control module when the throttle is opened. This is necessary to maintain a proper air/fuel ratio.
A TPS can fail gradually or in large increments, which can result in a “check engine” light turning on and poor fuel economy. Depending on the vehicle, the TPS may also cause the engine to idle rough, surge, hesitate or stall out.
The TPS is usually a potentiometer-type device that changes its resistance as the throttle opens and closes. This change in resistance is sent back to the ECU, allowing it to accurately set the fuel mixture.
The ECM/PCM tweaks the air/fuel mixture via fuel trim to ensure a steady ratio of air to fuel in the combustion chamber. It does this by constantly monitoring the exhaust stream, generating signal voltages upstream of the catalytic converter that are proportional to the oxygen content of the exhaust.
Short term fuel trim (STFT) values are calculated several times per second as a result of changes in the oxygen sensor signal voltage. Long term fuel trim (LTFT) values are calculated by sensors further downstream in the exhaust system.
Positive STFT percentages indicate the ECM is adding fuel to enrich the exhaust while negative LTFT percentages indicate the ECM is subtracting fuel to lean out the exhaust.
For a normal OBD2 reading, the STFT and LTFT should deviate by no more than 10% to either side of 0% when the engine is running at a steady speed. Once they deviate more than 25% the problem is almost always related to a rich or lean fuel issue.