No one likes to pay for diagnostics, I get that. Me neither. If I spend my hard earned money I want to get something perceptible in exchange rather then someone telling me that I have to spend even more money now to get my broken thing fixed.
Unfortunately, quite often there is no way around that. Here is a simple example (in the real life simple examples are rather rare).
2009 Toyota Venza comes in with concern of “VCS system fault” message on the dash. Scan tool reads codes C0210 “Right Rear Wheel Speed Sensor Signal Fault” and C1332 “Open Circuit In Right Rear Wheel Speed Sensor Circuit”. So the computer tells me to replace right rear ABS sensor isn’t it?
Not really. Scan tools just reads fault codes stored in ABS control module. The module will set fault code C0210 for sensor signal if it detects any irregularities in the signal coming from right rear sensor when the vehicle is moving. Because when the vehicle is not moving there are no signals coming from ABS sensors. Code C1332 will be set if module detects open circuit anywhere in the right rear sensor circuit. This circuit consists of the sensor itself, wires connecting sensor to the control module and part of the control module circuitry as well. There are three connectors in this circuit (one on the sensor, one on the module and another one that connects two wiring harnesses that serve this circuit (among many others).
Most likely what’s happened on this vehicle is that open circuit somewhere in the right rear sensor circuit have been intermittent first and by that affected the signal coming from the sensor. Maybe the circuit have been broken for just a split second at first few times and then restored back to normal. Hence the code C0210 was set. Later on the problem became more persistent and therefore detectable when the vehicle was not even moving as an open circuit. So the code C1332 was also set.
Here I use wording “most likely” and “maybe” because actual line of events might have been different. I don’t want to go into too many details here but the way the control module monitors the circuits and detects shorts and opens in them opens a whole lot of different possibilities actually.
The main idea here is that if there is a code stored for the sensor fault is not necessarily lies in the sensor itself. An easy case is if I do visual check and see the broken sensor wire. Or some other visually detectable problem with it. On this vehicle there was no such a thing, visually the sensor and its wire looked perfectly fine.
What would be the next logical step? Next step would be to verify presence of the open circuit by some other method. For the electrical circuit its continuity can be checked or resistance can be measured with some instrument (DVM or circuit probe). I can start with measuring sensor resistance and comparing it with reference data (which is usually just a range of possible values and not always available). Alternatively I can compare resistance of a sensor in question with known good sensor. If the suspected faulty sensor resistance will differ significantly I can safely presume it is indeed faulty. Otherwise the problem is elsewhere or maybe even the problem is not present at the moment of testing (for example because there is 50 degrees Celsius difference in temperature in the shop and out there where this car came from).
Unfortunately on this Venza rear ABS sensor connectors are buried under the trunk trim panels so there is a whole lot of disassembly and reassembly involved just to get an access to them. Which would also add to the labor cost of replacing the sensor resulting in a total cost of $430. So if the problem is not in the sensor it would be quite expensive mistake, isn’t it?
On the other hand ABS module connector is easily accessible on this vehicle so at least I can compare the whole circuit resistances of the four ABS sensors. Good thing is that test confirmed that right rear sensor circuit is indeed currently open. So I can quite safely presume that at least fault is not in the ABS module itself (as it contains part of the right rear sensor circuitry, if you are still following me here). Unless there are two problems here present at the same time, which is very, very unlikely but not absolutely impossible!
It is still remains unknown if the open circuit is in the sensor itself or in the wiring between the sensor and ABS module. Ideally I would need to remove all the trunk trim panels, get an access to both rear sensor connectors, check sensor resistances and then swap sensors around (with jumper wires) and run self-diagnostics on the ABS module again. If the fault will follow the sensor to the other side showing only the code for left rear ABS sensor circuit open now then I can be 100% certain that problem is in the sensor itself and there are no other problems elsewhere. Estimated labor cost to do all this comes to $162 and this cost added to the time that I have already spent on connecting the scan tool, reading codes, checking circuit resistances at the ABS module connector and recording the results of these tests would make up the total cost of diagnostics. In this case it is just above $200 or half of the price of the new sensor. Don’t forget that after performing required tests I have to re-install all the removed trim pieces and other parts (rear seats, seat belts, etc.) unless customer is OK with leaving the vehicle with me until the replacement part arrives (which could take few days or even few weeks).
So here is the choice: order and replace $430 sensor hoping that it will fix the problem – or do the job properly, eliminate all the other possibilities first, spend another $200+ and only then proceed with ordering part or performing other required repairs. Like it or not but there is no way of knowing for sure what is wrong without doing this $200+ work first. And after you got it all done you might just confirm what you were guessing about first time you have seen the code on a scan tool screen!
Like I said in the beginning, this one is a very simple example. Often enough after spending significant amount of time on diagnostics I can only reduce the list of things that are possibly wrong but still have to take a guess on what is left in that list :-).