Testing the PCV valve: Just shake it. Of course you have to remove it first, and you can hear the spring and valve rattling back and forth. If the engine is running, you should feel vacuum at the end of the valve when it is still attached to the vacuum line but removed from the engine. Or you can leave the valve in, but pinch off the hose and idle speed should drop at least 50 rpm. On some idling engines you can safely remove the oil filler cap and put paper on the cap and eventually a little vacuum will develop and hold the paper in place. Another test is to remove the valve so it draws in fresh air and watch the CO change. If the CO gets more than 1% leaner, you can have old engine oil that has trapped a lot of gas in it. Be sure you change the oil before the smog inspection or carburetor adjustment.

Testing the Fuel Evap system: Some shops are lucky enough to have a pressurizing tank of inert gas, like nitrogen, with the adapters to shoot into the special test port (it has that green cap on it). They can put some very small pressure in (inches of water) and see if it holds, or they can listen with a stethoscope for where it is leaking out. Or there is another tester that can pull a vacuum on parts of the system through that port and you can check where it is leaking by clamping off different hoses. With your factory scan tools with bi-directional capability you can turn some of these solenoids on and off while checking vacuum or listening to the crisp click of a unstuck solenoid. The simple folk at an independent shop can also activate some solenoids by using a test light to ground them and see if you now have the expected flow or vacuum. Don't forget the old fashioned vacuum pump can be used with a inches of water vacuum gauge to see if parts of a system are sealed. Just beware you are pulling gas fumes towards you, don't light your cigarette then. I have even heard a turkey baster with that collapsing bulb on the end can be used to check flow through some of these valves. Or you could use a hand pump for an air mattress. The hard part about all these tests is they built a car or truck around the parts you want to get to. When working on OBD II systems remember that your system will only run the monitor to check out these systems when the right enable criteria, or conditions, have been met. And it often takes two drive cycles to turn on the MIL. So you may think you have fixed the system, but it takes a while and very particular conditions before the system will test itself and verify you have fixed the system.

Air Injection Testing: One easy way to see if the air injection is working is to look at the exhaust readings. Higher O2 shows the pump is working. When the O2 reads 6-8%, it often shows the pump is working downstream. Pulse air systems may only show O2 in the 2-4% range. You can also put your hand on one of the big rubber hoses and accelerate the engine and feel the pulsing of a good air pump in that hose. This can tell you which direction the valve has switched the air into. But it won't tell you if a switching valve is not shutting one direction off totally. To do that, you do best to disconnect and block the hoses. But that takes time, or the tugging may break weak parts, so many will clamp off a hose with those special pliers, but be careful, sometimes those pliers let some air leak by. Burnt hoses are a dead giveaway that the check valve is leaking. Vacuum can be checked by teeing in a vacuum gauge, or a vacuum pump can tell you what leaks vacuum or doesn't switch when it gets vacuum. Please make sure all the vacuum lines are going to the right place. They sometimes aren't. Sometimes I use a photocopier to enlarge a vacuum diagram so it's easier to read.

EGR Testing: One simple test on EGR valves with a diaphragm is to just reach under the valve and pull up on the diaphragm. Use a rag or glove to protect your fingers from the heat. With the engine running, the open valve should flow and lower the idle speed and also make the idle rough, or maybe stall the engine entirely. You know the valve could flow EGR. If you rev the engine a little and the valve opens on it's own, now you know the control vacuum got to the valve and was able to open it. This is enough testing for many valves. If you need to, break out the vacuum pump or gauge and verify how far you have the vacuum going in the system and what happens to the EGR valve when you pump vacuum into it.
For the EGR vacuum transducers, you may need to verify they have an open port to the exhaust that isn't plugged up. Be careful, those red silicone vacuum lines can handle a lot of heat, but they are fragile. You can tear them with your fingernails. The backpressure systems may also need you to build up backpressure some way before they work. So rev up the engine and feel or watch for the valve opening. Or get careful help to rev up the engine in gear with the brakes on. Or put a socket in the exhaust big enough to give you some backpressure. Or bring the idle speed up to 2500 with the vacuum line disconnected, then connect it and see if the idle goes down because the EGR come on and is now flowing. One of these ways should work. Remember positive backpressure valves should be able to hold vacuum or actuate only when there is backpressure in the exhaust. And negative backpressure valves should only be able to hold vacuum when there is no backpressure or vacuum, that is when the engine is off.
When testing some of the computer actuated valves, you may need to use your bi-directional scan tool. If you activate the valve, and it doesn't move, verify with your meter that the signal change is happening at the valve. If the voltage is going low on the right wire, you know it's being grounded, change the valve. Or you can often ground the same wire that the computer grounds with a test light and get them to work. (On the linear valve I use a big brake lamp bulb to ground it, it needs a couple of amps to turn it on.)

Catalytic Converter Testing: Here are the do's and don'ts of how to determine if the CAT is working. It's really nice to be able to tell a customer this will fix the car or this will make you pass your smog test. Proper testing techniques will get you closer to this goal. And you will see that some tests don't work as well as we once thought. P.S. One more thing. When you find a bad CAT, you have to find out what killed it. They don't usually die on their own, something did them in.

Let's go from simple to more complex.

Rattle Test: I said we'd start with the simple. And just in case your brain fades like mine does sometimes, we'll include this. Remember how a meltdown can cause the internal pieces to come apart? Well if you hit the CAT with a rubber hammer and it rattles like a box of rocks, it's coming apart on the inside. This could be the cause of that lack of power when it plugs up. And you know it's not going to clean up the emissions much. Simple. But you can also use this to find rattles and buzzes from the heat shields too. If the noise sounds more like tin than rocks, it's a heat shield that's loose. Lots of times you don't even have to put the vehicle in the air to do this, just reach under with the rubber hammer and whack it. The shop does have to be kind of quiet though for you to hear this.
Vacuum Test: This can help decide if the CAT is plugged, but beware it's not foolproof. If you accelerate the engine while watching a vacuum gauge (remember someone else should be driving if you test this on the road) a plugged CAT will cause the vacuum to go to near zero faster than vacuum normally would. This is a judgment call, because most things that cause an engine to run badly will also help the vacuum to go down too fast. If you rev up the engine to 2500, then close the throttle quickly, the vacuum should jump up suddenly and it's not supposed to respond the same if the CAT is plugged. I don't use this test on it's own. Always get another way to double check the CAT is plugged. A plugged CAT also has a particular sound. As you open the throttle the engine goes "wwwaaaaaa" in a certain way as the air tries to get out the intake when the exhaust is plugged. Once you hear it, you'll recognize that sound when you hear it again.

Exhaust Back Pressure: In this test, you somehow tap into the exhaust with your standard vacuum/pressure gauge and read the pressure as you try to accelerate the engine and it doesn't run well. Usual specs are you should have under 3 PSI at 2500 fast idle. You have less at normal idle speed. And you have a lot more when driving under load. My normal Volvo has 4 PSI under full acceleration at 4000 rpm, but well under 1 PSI at idle. And turbo vehicles could easily have more. I heard 6 PSI once. And if your readings are too high, remember the plugged section could be any where in the exhaust. It could be in the muffler, from all the stuff that came apart last time and they replaced the CAT, but didn't replace the muffler. How to tap into the exhaust? Lots of ways. If there is an EGR transducer with a line to the exhaust, just tap into that. There are kits that allow you to drill into the exhaust and screw in an adapter to plug you vacuum hose to. (Make sure the plug seals well after, you don't want an exhaust leak to get CO into the passenger compartment. Some techs won't use these plugs unless they can weld the hole shut afterwards.) Use silicone high temp vacuum hose. Or you can remove the O2 sensor and screw in a special adapter that has threads the same size. Or if you like playing with rusty frozen metal that might break, you can remove an air injection check valve and tap in with a tapered plug adapter. As usual, there is "more than one way to skin a cat..." (It's just a saying, cat lovers. I am nice to our cat, even though her name is Useless.) The most foolproof back pressure test is to just loosen a few bolts in the exhaust pipe at the exhaust manifold so there is about 1/2 inch gap. Now you know the exhaust can get out. If you drive the vehicle and the power is now OK, you know some place the exhaust is plugged.

Temperature Test: This is where you measure the temperature of the pipes before and after the CAT. And you figure, "since the CAT helps the oxidation reaction to take place, that would create more heat, so it should be hotter at the back of the CAT than in the front." If you can't measure a certain amount of increase, like 100 or 200 degrees F, it means the CAT is not working. Sounds good, but it doesn't always work. Here's why:

1. The exhaust pipes can be different thickness', or they can be double walled. Then the heat doesn't transmit out with the same accuracy. And you don't know.
2. What if the engine is real clean, and there's not much exhaust to clean up. Then the temperature of a good CAT wouldn't get high.
3. Air injection (AIR) seems to send in cooling air that often lowers the temperatures. This is for the dual bad, three way CATS.
4. The oxidation reaction, which is like burning, does create more heat. But the reduction reaction absorbs heat, takes it away. So if there is a lot a reduction taking place, it can lower the heat coming out, even when the CAT is working well.
If you must do this temperature test, here's what I do. Disconnect and ground one spark plug wire for only a short time. And don't run the engine above idle. Now you will have lots of HC going through the CAT, and the oxidation reaction will make it hotter at the back. My Volvo increase by about 450 degrees F. Be careful, this creates a lot of heat. Also, this only tests the oxidation capability of the CAT, not the reduction capability. Some friends tested several CATs and found they would clean up HC and CO just fine, but the NOx cleanup ability was no longer working in them. Remember oxidation and reduction are two separate reactions.

By the numbers: When you look at the exhaust readings, you can 'kind of' tell whether the CAT is working or not, but it is only a guess. When the CO2 gets up to 15% or more, you know the CAT is working. And then the O2 and CO get down to below 0.1 %. At a Ford dealership where is used to work, I would often see CO2 about 15.2%, and O2 and CO would be down to 0.02%. And the HC would be around 10 ppm. This was on very new cars. But you want to know if the CAT is working when the emissions are too high to pass the smog test. Then you frequently see CO2 only 12% - 13%, the O2 stays in the 1% - 3% range, and the HC are frequently around 100 - 200 ppm. You can see the CAT just can't finish the job of cleaning up the emissions. There is an exact example of this in the 5-Gas Exhaust Analysis section.

Intrusive Test: This is the best way to really know. You tap into the exhaust before the CAT and then measure out the tailpipe, and you compare the two. Many European cars have a built in CO adjustment tap. You take off a blue silicone plug, or unscrew a metal plug (if it's not too rusty yet and you have the special adapter) and you have direct access to the exhaust before the CAT. For other ways to tap into the exhaust in front of the CAT, see the section on measuring back pressure above. You should read a noticeable drop between the before and after CAT numbers for HC, CO and NOx. The intrusive test is the only one that works to see if the CAT is lowering NOx. And if you do several like we have, you will probably see there are a lot of older cars out there where the CAT will clean up HC and CO, but doesn't work on NOx any more.

Cranking CO2 Test: This is my favorite. Maybe I shouldn't have left it for last, but after all the above, you can really appreciate this one. What you do is have the engine and CAT all warmed up, then disable the secondary ignition safely so the injectors will fire, but the engine won't run. Ground out the coil wire on a old distributor system. Or ground out between the coil terminals on a DIS system. Or just ground out all the spark plug wires at the plugs. On a COP system, you will have to disconnect all the coils. (And hope the injectors are still firing gas. They may not.) Next, just crank the engine. (For no more than 15 seconds.) The fuel system sends gas into the engine, and you watch how much CO2 comes out the tailpipe with your 4-gas analyzer. Since it can't burn in the engine, if lots of CO2 comes out, it must have been created in the CAT. The CAT is working. Good numbers are CO2 above 13%, and HC below 500 ppm. Watch the HC too, because you don't want much coming out. The CAT should be able to clean it up. This is a great test because it is simple, fast and very accurate. It takes away the guess work.