If the upstream Oxygen Sensor on your OBD II Ford Escort (or Mercury Tracer, Ford Focus) goes BAD, you'll see the Check Engine Light shining nice and bright, and when you scan for codes, you'll usually see P0131 or P0133.
Not only will you see the Check Engine Light lit up, but your gas mileage will also suffer. Now what sucks about O2 Sensor issues is that quite a few conditions can fool the PCM (Powertrain Control Module = Fuel Injection Computer) into thinking that the O2 Sensor has failed when it hasn't. Therefore, testing them becomes a good idea.
The test can be accomplish in a few easy steps and you'll be able to say, “YES, the O2 Sensor is BAD or NO, it's OK” before you spend the money and replace it.
To help you navigate this article, here are its main points:
TIP 1: You'll need to use a Scan Tool that has Live Data capability for the O2 Sensor Test I'm gonna' show you in this article.
In case you wondering, you don't need the Ford Factory Scan Tool or an expensive Professional Technician Level Scan Tool to follow the test procedures in this article (Don't have a Scan Tool? Need a Scan Tool? Check out my recommendation: Actron CP9580 Scan Tool).
TIP 2: You will not be removing the O2 Sensor to test it, since the O2 Sensor test is done with it in place and in action as the Engine runs.
Before jumping into the O2 Sensor test, it'll help you to understand how the Oxygen Sensor works. Now, I won't go into minute technical detail... after all, you don't want to reverse engineer it... I'll just explain the basics that'll help you to test it.
In a nutshell, the upstream Oxygen Sensor's job is to report back to the PCM if it's either injecting too much Fuel or not enough. When the PCM dumps to much Fuel... the resulting Air/Fuel Mixture is considered as Rich. If it does not inject enough... the resulting mixture is considered Lean.
Depending on what the O2 Sensor reports, the PCM will adjust the amount of Fuel it's injecting into the Engine.
Here are some more specifics:
If the PCM in your Ford vehicle injects too much Fuel, the Oxygen Sensor produces a Voltage above 0.500 Volts. This voltage can go as high as 0.900 to 1.0 Volt.
Too much Fuel means that the Air Fuel Mixture is Rich and once the PCM sees this report from the O2 Sensor, it starts to inject less.
As the PCM starts to inject less Fuel.. it may go too far and not inject enough. When this happens, the Oxygen Sensor reports the resulting changes by producing a Voltage that can go as low as 0.050 to 0.100 Volts.
Too little Fuel means that the Air/Fuel Mixture is too Lean and the PCM starts to inject more.
This process (of adjusting the amount of Fuel being injected) by the PCM, goes on the entire time the Engine is running (and if the O2 Sensor is working correctly).
All of these Oxygen Sensor Voltage changes can be easily observed with a Scan Tool in Live Date mode, and this is how I'm gonna' show you how to test them.
A correctly working O2 Sensor will produce a voltage that will switch between a Lean and Rich Condition several times every few seconds. So, if the Voltage output of the O2 Sensor stays fixed (when testing it)... the O2 Sensor has failed.
To find out what are some of the most common symptoms of a failed Oxygen Sensor, take a look at the section: Symptoms of a BAD Oxygen Sensor. OK, let's turn the page and let's get testing...
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