Repair Info - Emissions Repair Explained
What Is a Diagnosis?
A diagnosis or diagnostic can actually refer to two things - it is a conclusion that is drawn based on an evaluation of symptoms, and it is also the process of reaching that conclusion. The most important part of any emissions repair is often the most difficult - a correct diagnosis of the root cause of the failure.
A diagnosis is not a repair. Before a repair can be performed, the cause of the problem must be identified. Identifying the cause of the problem is the diagnosis. For example: a vehicle that has excess hydrocarbon emissions (the problem) is diagnosed and the technician concludes that the catalytic converter is defective (the cause of the problem). To correct the problem, the technician replaces the catalytic converter (the repair).
The information presented here is to help you to understand vehicle emissions diagnosis. You won't magically be transformed into a competent diagnostic technician, but you will be much better prepared to locate a competent diagnostic technician and repair shop, and be more aware of what you should expect from them.
The Diagnostic Process
Because the word diagnosis can refer to both the conclusion and the process used to reach that conclusion, to be clear we will refer the two as a diagnostic process and a diagnostic conclusion.
The diagnostic process is the process of evaluating symptoms and test information, performing tests on specific components, and re-evaluating until a diagnostic conclusion is reached. Unfortunately, there isn't a step-by-step procedure that will lead to the correct diagnostic conclusion for every possible scenario. Many factors come into play that affect what action a technician should take to arrive at a correct diagnostic conclusion in the most efficient way. Each situation must be evaluated on an individual basis considering all of the available information. This makes emissions diagnosis a difficult topic to explain.
That is not to say that it will always be a complex and difficult task to identify the defect that caused a vehicle to fail its AirCare inspection. In fact, in many cases the cause of the problem can be narrowed down to only a few possibilities without even looking at the vehicle. Just looking at the inspection results and considering other information and symptoms, the technician can often eliminate many of the possible causes of emissions failures. The possibilities that are not eliminated are the ones that will need to be checked further to arrive at a diagnostic conclusion.
Generally, the diagnostic process that the technician should follow will involve three stages:
- reviewing the inspection results;
- prioritizing the component tests that will lead the technician to the correct diagnostic conclusion in the most efficient manner possible; and
- performing the component tests that are necessary to pinpoint the defect.
In the case of OBD failures, the diagnostic trouble codes (DTC) may sometimes be misinterpreted as identifying a specific defective component. In fact, DTCs do not identify defective components, but rather, identify a condition where the vehicle manufacturer’s recommended diagnostic procedure must be followed to pinpoint the cause of the abnormal condition.
Re-setting an Illuminated Check Engine Light or Malfunction Indicator Lamp (MIL)
An illuminated MIL is your vehicle’s way of telling you it has found a problem related to the emissions control or engine management systems. Take it to a repair technician to have the problem addressed before it becomes more serious.
Temporarily disconnecting the car battery or re-setting your vehicle’s computer memory with a scan tool may temporarily turn the MIL off; however, this will also set the status of the On-Board Diagnostic readiness monitors to Not Ready. The status remains Not Ready until the vehicle’s computer checks the components to determine if they are operating properly. After the check is run, the monitors are set to Ready. Your vehicle will be rejected from receiving an OBD inspection if the OBD system has not performed enough readiness checks.
Review Inspection Results
The inspection results are the most meaningful indication of whether an emissions defect exists, but they are not always the most meaningful indicator of what is actually causing emissions to be higher than normal. That is the reason that a diagnosis is required to determine what is causing the problem.
However, the emissions measurements made at the inspection centre do tell a lot about a vehicle's emissions performance, and they provide a competent diagnostic technician with insight as to what areas will need to be checked in order to pinpoint the cause of the problem.
What Should The Readings Be?
The inspection report shows the average passing readings for similar vehicles. This is a good indicator of what is normal.
Another indicator of normal emission levels may be the vehicle's previous inspection results. However, one must be careful when comparing to previous inspection readings; you want to be sure that the vehicle did not also have a problem in the past even though it was able to pass the inspection. For example, if a vehicle has a degraded catalytic converter it may have been deteriorating over several years. In this case, even if it passed, last year's inspection results are not likely to be indicative of normal readings.
Identify Each Emission That Is Higher Than Normal
Even if only one emission exceeded the maximum allowable, the technician must carefully consider all emission levels and identify any abnormality. This is critical to understanding what is going on with the vehicle's emission control systems.
Narrow the Range of Possibilities
After evaluating the inspection results and comparing the readings to what they should be, the technician may have been able to narrow the focus of their diagnosis. At this stage they will usually still have a number of components or systems in mind that could be causing the high emissions. But what should the technician check first?
Before jumping in and testing every component they see when they lift the hood, the technician should establish priorities for performing the specific component tests that will lead them to pinpointing the defect the quickest way possible.
Perform Component Testing As Necessary
In every case, component testing is necessary in order to locate the cause of the problem. What components need to be tested and how they need to be tested depends on the specifics of the vehicle itself. Different vehicles use different emissions control systems and often have significantly different procedures for testing those components.
In some cases, the diagnostic technician will only need to test one component just to confirm what he or she suspected was the cause of the problem. In other cases, the technician will need to perform component testing on several components before arriving at a diagnostic conclusion.
The Diagnostic Conclusion
The goal of the diagnostic process is to reach a diagnostic conclusion. As a consumer, you should be aware that if a repair shop charges you for a diagnosis, you are buying a diagnostic conclusion. In other words, if the cause of the problem is not identified, you did not receive a complete diagnosis.
Once a diagnosis has been completed, the cause (or causes) of excess emissions on your vehicle will be known. At that point, the repair shop should also be able to provide an estimate of what it will cost to repair those defects. Armed with this information, you can then decide what you want to authorize the repair shop to do. You may decide to do one of the following:
- do not authorize completion of any repairs to your vehicle at this repair centre;
- authorize partial repairs that do not exceed the applicable cost limit for your vehicle;
- authorize all repairs that were determined to be necessary on your vehicle.
NOTE: if there is some repair (applicable to the excess emissions problem) that can be completed at a cost less than the repair cost limit, and you choose not to authorize the certified repair centre to complete repairs to your vehicle, it will not be eligible for a conditional pass if it fails the re-inspection.
