Emission Control Diagnostics

Emission control system problems can cause a host of issues for the vehicle owner. First of all, their car probably will have a drivability issue (but not always). Secondly, the check engine light, or MIL (Malfunction Indicator Light) illuminates. Also, emission levels increase. Finally, the annual license renewal might be in jeopardy. As a result, problems with the emission control system must be taken seriously.

Due to the fact that everything under the hood is computer-controlled, you can make the argument that almost any engine system can cause an emission control system problem. On one hand that is true, but in reality, we can apply some common sense to the situation and approach the problem logically.

In many cases, the problems encountered on a daily basis in your repair facility 
are somewhat basic in nature. When solving any problem, you always look for the most likely cause first. The same approach should be used when diagnosing emission control problems.

COMPONENT FAILURE

We’re going to approach emission control problem diagnosis from a component failure standpoint. Specifically, we are going to look at four different components that play important roles in controlling automotive emissions. These components are the catalytic converter, oxygen sensor, purge solenoid valve and the PCV valve.

Sure, if you have a fuel injector that goes bad, it will cause an increase in emissions. The same holds true for an oil-fouled spark plug. But when you look at what happens day in and day out, what do you see? Oxygen sensors fail. It’s the result of the harsh environment in which the sensor operates—and oxygen sensors fail more than any other sensor.

The plunger in the PCV valve can stick from the residue in the crankcase vapors that pass through it. And inside the purge solenoid is a diaphragm that is constantly moving back and forth. Guess what? A diaphragm, by the nature of its design and function, along with the environment in which this one operates, can fail. Let’s look at each of these components in some detail.

OXYGEN SENSORS AND THE CATALYTIC CONVERTER

The O2 sensor is basically a rich-lean switch. By monitoring the oxygen content in the exhaust stream, the air/fuel ratio is adjusted. When O2 sensors were first introduced, only one sensor was used and it was located before the catalytic converter (commonly referred to as the upstream O2 sensor, as shown in Figure 1).

Over time, a second O2 sensor was installed downstream of the converter. This second sensor is used to monitor the efficiency of the converter (Figure 2). It operates in the same manner as the upstream sensor, but the voltage signal (oxygen content) it generates is not used to adjust the air/fuel ratio; rather, this signal is compared to the upstream sensor to determine the efficiency of the catalytic converter.

The three-way catalytic converter, used on all late model Chrysler Group LLC vehicles, simultaneously converts HC, CO and NOx emissions into harmless gases (water vapor, carbon dioxide and elemental nitrogen). This conversion process is most efficient at an air/fuel ratio of 14.7:1.

As a vehicle accumulates mileage, the catalytic converter deteriorates. This results in a less efficient catalyst. To monitor this decline efficiency, the PCM compares the readings from the two oxygen sensors to calculate the oxygen storage capacity and converter efficiency. If the efficiency drops below mandated emission standards, the PCM stores a trouble code and illuminates the MIL.

When the trouble code is accessed, either P0420 or P0432 might be set. One of these trouble codes doesn’t necessarily mean that the converter is going bad. One of these trouble codes is set when the switch rate of one of the oxygen sensors approaches the switch rate of the other sensors. If both sensors are new, this indicates a problem with the converter.

On the other hand, if one of the oxygen sensors is aging, its switch rate will approach that of the other sensor. This will also set one of the two trouble codes, but it doesn’t indicate that the converter is bad. If P0420 is set, check the downstream oxygen sensor. If P0432 is set, check the upstream oxygen sensor. If either sensor is bad, replace it. Slow switch rate response times can also set P0133 and P0139 trouble codes. Check the appropriate diagnostic manual and perform the diagnostic test.

PURGE SOLENOID VALVE

The evaporative emissions system is a closed system that captures the gasoline vapors from the fuel tank and fuel system, preventing the release of these vapors to the atmosphere. In the pre-emission days, fuel tanks were not sealed. The fuel tank cap was vented, allowing mechanical pumps to draw gasoline from the tank to the carburetor. As a result, fuel vapors were vented to the atmosphere.

Today, the fuel tank has a sealed cap that prevents such venting. Vapors lines, connected to a purge canister, control the flow of air in and out of the fuel tank. Any vapors are drawn off the canister into the intake system to be burned with the air/fuel mixture. The purge solenoid (Figure 3) is activated by the PCM to draw these vapors from the canister. As mentioned, the diaphragm can go bad in this component.

If there is a problem in the evaporative emission system and the P0441 trouble code is set, check the diaphragm (this might be easier done with the valve removed from the vehicle). Connect a vacuum pump to the intake, or canister outlet, and apply about 15 psi of vacuum. The vacuum should hold. If not, the diaphragm is bad. Replace the purge solenoid valve. The vacuum pump can also be connected to the vacuum outlet. It doesn’t matter. If the diaphragm is bad, the vacuum will not be held.

PCV VALVE

The PCV valve meters the amount of crankcase vapors that are routed into the combustion chamber. It contains a spring loaded plunger whose position is based on intake manifold pressure. The maximum vapor flow is achieved when there is moderate intake manifold pressure.

Testing the PCV valve is relatively straightforward. At idle, remove the hose from the PCV valve. A hissing noise should be heard. A strong vacuum should be felt if your finger is placed over the valve inlet.

Re-install the hose on the valve. Remove the make-up air hose from the air plenum and hold a stiff piece of paper over the end of the hose. After about a minute, the paper should be drawn up against the hose with noticeable force. If not, replace the valve. If the paper is not drawn against the grommet on the hose, replace the PCV hose. Turn the engine off. Remove the PCV valve. The valve should rattle when it is shaken.

If the PCV system doesn’t operate as described above, replace the PCV valve. Retest the system to verify the repair.