There are a variety of ways to check suspected sensor problems. One is to simply plug a scan tool into the vehicle's computer
to see if there are any fault codes in memory. Or you can use software that converts your PC into a diagnostic scanner
(requires a special serial connector cable).
A Check Engine light or the presence of trouble codes (if there are any) is a good indication that something is wrong. But what?
A trouble code may indicate a bad sensor, or it might indicate a bad connector, wiring problem or even a fault in the computer itself. To find out what's wrong, you have to look at the sensor data and, if possible, follow the step-by-step checks in the vehicle manufacturer's diagnostic chart for that code to isolate the cause. And if you find no trouble codes? Then you have to assume the problem is intermittent, within the normal range of values for the sensors, or outside the computer system (fuel, ignition or mechanical).
If the vehicle allows you to read live data stream with your scan tool, you can check the various sensor inputs to see if they seem to be responding normally. But checking individual sensors with a scan tool or even a multimeter can eat up a lot of valuable time and may or may not reveal what's causing the problem.
One way to check sensors is to run an electronic "sweep" of the entire system using computerized test equipment that compares all the sensor inputs to known values. Anything that falls outside the normal range is red flagged for further diagnosis. A sensor sweep can identify sensor-related problems very quickly (in a couple of minutes or less). But once you've found a problem, you still have to go in and figure out where it is. Is it the sensor or the wiring? Using a multimeter (or a built-in multimeter function), you must still do the various voltage, resistance and/or frequency checks to isolate the fault.
To do individual sensor or circuit checks, you must have a DVOM or digital multimeter capable of reading amps, ohms and volts. A high impedance (10 megaohm) digital multimeter is a must when working on electronics to (1) protect the electronic circuits against possible damage, and (2) read small voltages and currents with a high degree of accuracy. How are you going to adjust a throttle position sensor to the proper idle voltage (which is specified to a hundredth of a volt) using an analog voltmeter? You aren't, at least not with the degree of accuracy that's specified by the vehicle manufacturers. Digital accuracy is a must.
The better models of multimeters also include a light or beeper to indicate continuity (handy for checking circuits), and some also have the capability to frequency, too (which may be needed to check certain types of MAP sensors). The high end models can also display information in analog fashion like a bar graph to make it easier to spot momentary glitches that are impossible to see when looking at numbers alone.
Also called a "pinout" box, this device allows you to probe the various circuits in the computer system. A breakout box plugs into the computer harness so you can check all the circuits from a single point without having to backprobe or break individual connectors (which always increases the risk of damaging something or introducing corrosion which can cause problems later on). You'll also need a sackful of adapters so you can use the box on a variety of applications. There are also "intelligent breakout boxes" (IBOBs) which are computerized and self-display the various circuit readings without having to use a separate multimeter to probe the individual pins. IBOBs also allow you to tap the data stream in vehicles that have to access to the data stream through the usual diagnostic connector (most imports and older Fords, for example).
Next (or first) on your list of "must haves" is a scan tool -- not just a basic one for pulling fault codes but one that has bidirectional capability for two-way communications. It should also be OBD2 compliant for 1996 and newer onboard diagnostics, too.
Few would disagree that a scan tool of some sort is absolutely essential to diagnose computer-related driveability and emission problems. A scan tool may also be necessary, however, if you're working on a vehicle equipped with antilock brakes (ABS). This may require additional software or an add-on cartridge to access the ABS diagnostics. The same goes for airbags and other body-related electronic systems such as climate control, steering and suspension.
PC software or some scan tools can also display sensor data as waveforms similar to an oscilloscope. Viewing data in a graphical format often makes it easier to see problems that are difficult to detect when data is shown as numbers only. Looking at an oxygen sensor trace on a scope, for example, can tell you at a glance whether it is good or bad. By comparison, a simple scan tool can display the O2 sensor's output voltage, crosscounts (the number of times it flips back and form from rich to lean), and long term fuel trim (which tells you if the system is running rich or lean). But it's often faster and easier to spot a bad O2 sensor by viewing its waveform rather than all the other data.
Displaying sensor outputs as waveforms on a scope can also help you spot momentary glitches that are difficult or impossible to see otherwise. This is especially true with variable resistance sensors like throttle position and vane airflow sensors.
Another piece of equipment that can help you identify and isolate sensor problems is an oscilloscope. In fact, it can give you a whole new way of looking at sensor problems. A scope displays sensor waveforms. There are both hand-held and PC-based scopes from which to choose. Displaying sensor outputs as waveforms makes diagnosis much easier by allowing you to "see" intermittent faults and other problems that are nearly impossible to detect by other means. Once you know what to look for, you can easily distinguish good waveforms from bad ones. You can also capture good waveforms to create a library for future reference.
For example, let's say an engine has a hesitation problem. It could be dirty injectors, a bad mass airflow sensor (MAF), manifold absolute pressure sensor (MAP) or the throttle position sensor (TPS). You hook up your scan tool but find no codes. What's more, the voltage readings all seem to be within normal ranges at idle and wide open throttle. But before you conclude it must be dirty injectors, you hook up your scope and display the MAF, MAP and TPS waveforms simultaneously. You watch the traces as you open the throttle wide -- and there it is! A sudden dip in the TPS output caused by a dead spot in the mid-throttle range.
Another tool you should have in your arsenal is a "sensor simulator." Such a tool can help you determine if a sensor is good or bad once you've pulled a code or isolated the problem to a particular circuit. By substituting a simulated sensor signal with the tool, you can determine if the system responds normally or not. If the system responds normally to the simulated signal and/or the problem goes away, it tells you the sensor is bad. You've isolated the fault and no further diagnosis or testing is necessary. And if there's no change? Then you know it's not the sensor but something else (like wiring continuity or a bad connector). A sensor simulator can produce variable voltage, variable resistance and variable frequency inputs to simulate virtually any sensor. One of the advantages of using this technique is that it's a lot easier than swapping "known good" sensors for ones that might be suspect, especially if you've gone through the entire diagnostic chart and still aren't sure if a sensor is bad or not, or if you've replaced a sensor and it failed to cure the problem.
A sensor simulator can also be used to alter the operation of the computer system when performing other diagnostic tests. By simulating a hot or cold input signal from the coolant sensor, you can change the operation of the computer from closed loop to open loop. This type of substitution may be helpful when performing a power balance test on some vehicles. Running in open loop prevents the computer from changing the idle speed when cylinders are deactivated. The tool can also be used to simulate a TPS signal to vary the idle fuel mixture (changes the duration of the injector pulses), to simulate a knock sensor signal to check timing retard, to simulate a baro sensor signal to switch the fuel mixture and timing between "high altitude" and "normal" settings, to simulate a MAP signal to check for changes in ignition timing and fuel enrichment, and to simulate a MAF signal to monitor changes in fuel enrichment.