Here's a statistic that should give contractors and technicians pause before they condemn a compressor at a jobsite: One-third of all compressors pulled from the field, returned to manufacturers, and torn down for failure analyses do not have observable defects. In other words, these compressors were most likely good, workable compressors when they were replaced.

Such a phenomenon has been taking place for years, and it even has a name. It's called the "no defect found" (NDF) compressor return.

Here's a typical situation. During a field service job, somehow the compressor was suspected to be problematic. After the replacement the system started to run, so the problem seemed to be confirmed and solved from the field service point of view.

But is that really the case? What was observed that led the repairperson to condemn the compressor?

Compressor manufacturers have received the following reasons for return that have accompanied compressors:

  • Moving parts locked.

  • Motor winding grounded.

  • Motor winding open.

  • Unacceptable noise.

  • Won't start.

  • Overheat/current too high.

  • Runs but won't pump.

  • Low capacity.

    When the compressor's moving parts lock or when the motor winding is grounded, it is sufficient to say that the compressor is dead. However, the rest of the reasons in the list are iffy. Let's take a look at potential problems related to these symptoms.

    Motor open: When you measure a compressor motor winding, and if the compressor is still hot shortly after a shutdown, make sure that the opened protector isn't causing the circuit to open.

    Noise: Noise can be a good indicator of mechanical failure in a compressor, but it is also very subjective. Many other problems can cause a loud-running compressor, such as improper installation, loose nut, bad or missing rubber grommet, etc. If the compressor lacks oil or is having some floodback while running, this could also cause it to make a peculiar noise. This is especially possible for scroll compressors.

    Won't start: It's true that a dead compressor will not start, but a compressor that won't start may not necessarily be dead. Many other things can prevent a compressor from starting, such as a bad capacitor, a malfunctioning delayed start from the controller, and, again, an opened protector. A system with low-pressure protection may prevent the compressor from starting if there is a slow refrigerant leak.

    Overheat/current too high: Worn or damaged mechanical parts can make a compressor run hot at too high of a current. Many other conditions in the system can cause these problems, too. Were the condenser and condenser fan working properly? Was there a refrigerant leak? Was the weather hot? Did the capacities of the outdoor and indoor units match?

    Runs but won't pump: A broken internal valve in the compressor could cause the runs-but-won't-pump-or-make-cooling failure. So could a block in the refrigerant line, such as a defective or improperly installed expansion device or a dirty filter-drier. You may not see the latter very often in a new air conditioning system, but you never know.

    Low capacity: Here we need to be more careful. Worn parts would make the compressor lose capacity, but returns are mostly new compressors within the first couple of years of warranty. Many system issues could cause low capacity, especially leaks or blocks in the refrigerant side.

    As you can see, among the reasons stated for a compressor return, only a couple of them actually justify a replacement.

    The compressor is the heart of the air conditioning system. Re-placing it is essentially a system overhaul. You need to reclaim the refrigerant, disconnect electrical connections, disconnect pipelines, unscrew foot nuts, and replace the filter-drier. Although you may not have a handle on the real problem, you could "accidentally" fix the initial trouble once you put a replacement compressor in, because you have touched and altered so many things at the same time.

    As was stated before, one out of every three condemned compressors is still good, and is being pulled out of a system in a time-consuming process as a result of a misdiagnosis of the problem.

    Figure 1. Cause-and-effect diagram of NDF returns.

    Cause And Effect

    There are four aspects involved in the NDF return process:

    1. Compressor engineering and manufacturing.

    2. System engineering and manufacturing.

    3. Sales, field services, and technical training.

    4. Warranty policy and ad-ministration.

    Problems from any one of the four will have some impact. But the significance of the impact and the cost for each action will differ. Figure 1 shows many of the causes that will lead to NDF returns.

    On the upstream of the chart are the compressors and the system manufacturers. Four aspects help reduce NDF returns:

    1. Eliminating defective components, such as thermal protectors with a wide range of action temperatures, unmatched pairs of moving parts, etc.

    2. Eliminating manufacturing defects as much as possible.

    3. Making an error-proof system from a design point of view (that could be expensive and time consuming).

    4. Generating good technical documents to guide field service.

    Among the four tasks, the last one is the most effective and practical measure for engineers and manufacturers to take. For each unit model produced, there should be a field service manual available to give step-to-step, detailed, and illustrative guidance for troubleshooting.

    On the downstream of the chart is warranty administration. The distributors deal with dealers and end users; the warranty money comes from the manufacturers' pockets. Warranty policies and procedures should be improved to discourage the casual replacement of compressors in the field, and to relay feedback from the field to manufacturers' engineering groups.

    The most critical link in the chart is the field service tech, whose required skills and knowledge are high. Unfortunately, the annual turnover ratio for field technicians is said to average 25 percent or higher.

    When competency is the issue, training is the way to address it. However, training programs re-quire money. Companies have been cutting their training budgets.

    Dealers are reluctant to sponsor technician training because on-the-job training seems more economical. Many techs don't want to spend money on additional training for the same reason.

    If there were a good diagnosis tool kit available that could be deployed to avoid the NDF returns, it would be a good solution. But who is going to develop such a package: the manufacturers' engineers or some outside entrepreneur?


  • The direct cause of NDF returns is misdiagnosis in the field.

  • To address this issue, all parties in the process need to be involved. The compressor and system manufacturers need to develop technical documents to guide field servicers. They also need to develop diagnostic tool kits for improved field service work.

  • Warranty policies and procedures should be changed to discourage techs from swapping out compressors in the field, and to make sure technical feedback is communicated from the field to the manufacturer.

  • The best way to improve the competency of the field service force is to enforce certification and qualification programs. This will raise the overall quality of field service. By the same token, the cost of service will be higher. That money is well spent for getting the job done properly.

    Shawn Lee is a freelance engineering consultant. For the past 11 years, he has worked exclusively in the HVACR industry, including seven years of reliability engineering experience with Carrier. He can be reached at

    Publication date: 04/04/2005