Broad question about sensor replacement in cars in general

Question

As a shade-tree mechanic, I am rather frightened of doing repair work that involves disconnecting or replacing sensors in general.

One of my fears goes like this: I hear stories of people performing repairs on cars. During the repair, they disconnected a sensor of some kind, and after finishing the job, reconnected the sensor only to find that the car won't start or is undriveable until some fancy computer that they don't have talks to the car's OBD (or other control computer) and resets the sensor. One such situation like this is replacing the throttle position sensor.

My thinking is that every car out there has some way to reset the OBD (or other control module) to factor conditions. Sure, the car's computer may have to re-learn a few things, but a factory reset should get the car back to startable and driveable, provided that every other aspect of the repair went correctly.

Is my thinking above correct?

I realize that my question is based on paranoia and lack of experience, and probably oversimplifies and mis-states many aspects of late model cars. Also, I am trying to avoid giving any examples that are specific to a make or model of car because I'm trying to generalize this question to all cars.

Answer 1

In many (or most?) cases at least, simply disconnecting the battery for awhile will clear sensor conditions. If it's a case where it's not true I'd expect to find it documented in the manual. Personally I've never actually run across anything like that outside of audio/anti-theft systems.

Answer 2

Unfortunately, your broad question may get nothing but broad hand-wavy answers.

That said, speaking at a very high level, modern car electronics generally fall into three camps:

• Situations where electronics are serialized (i.e. a specific "unit X" has a serial number or address that's different than the "unit X" in another vehicle of the same make/model.) These electronics are linked to each other in the sense that they keep track of each other's IDs. If you try to replace one of these components, you'll typically encounter a situation where it no longer functions, until reset by a special procedure or a connection to a "fancy computer" that only the dealer has. Typically, this approach is only implemented for more complex situations, and is often done as a theft or hack-preventative measure, as much as for any other reason. As an example, many high-end motorcycles have an ECU that's programmed to match other components on the bike, as a way of stopping a thief from stealing the vehicle by swapping the ECU to one which they've hacked to run without a key - the bike won't start until the new ECU has been matched to the vehicle. Many modern vehicle touchscreen entertainment units or other high-value electronics components are serialized to the vehicle, again to prevent theft. From a certain perspective, this is also true for modern keyless entry keys, which contain computer chips that must be linked to the vehicle. Obviously, you don't want to give people the ability to "replace" a key on their own, otherwise theft would be rampant.

• Situations where electronics "learn" each other, to account for manufacturing or installation variations. This does sometimes apply to individual sensors, i.e. the TPS sensor you mentioned. In the "old days" when such sensors first came into use, it was sometimes the case that there would be an arduous calibration procedure required any time they were replaced - for something like a TPS sensor, you might have to "clock" it to set it's relationship to the throttle. These days, manufacturers generally develop systems that use secondary information to self-calibrate. Sometimes, this calibration needs to be manually triggered, but in other cases, the vehicle will do it automatically over time. In some implementations, replacing one of these sensors will cause warnings to be displayed, and whether or not you will need a special computer to calibrate the unit and clear the warnings is somewhat vehicle-dependent.

• Finally, in this day and age, there are still some electronics and sensors that are completely dumb and the vehicle "doesn't know" when you've replaced it.

To apply this all to your scenario - where you're scared that replacing a random component will cause a no-start condition - the only way to really answer this is to research the specific component for the specific vehicle you're trying to replace (which, these days, with the proliferation of online documentation and youtube videos, is pretty easy to do - much less having to look at a paper repair manual). That said, generally the "it won't work any more" issues from the first bullet are very special cases, and fairly obvious as they're related to anti-theft measures. So, your concern about doing shade-tree mechanics and unintentionally making your car inoperable because you replaced a run of the mill sensor are probably unfounded.

Answer 3

I can't comment on everything but I can provide information about some specific types.

I would say the following: for anything that is a screw in type sensor with a simple plug, (e.g. Temperature Sensor) you are fairly safe to replace without worry. There is likely no "smarts" inside the sensor and it is not communicating with the ECU itself so the ECU can't tell the difference.

For sensors that appear more complex (an ABS Speed Sensor) these very well may require calibration or tools to talk to the Engine Computer, especially if they are safety related (ABS System etc)

The following sensors should never require any self-learning or changes to the Engine Computers as they are essentially dumb sensors that don't have any self-calibration/adjustment possible.

1. Oxygen Sensors (Providing it is an OEM sensor and replaced with an OEM sensor)
2. Temperature, Pressure, Speed, Gear Position, etc (Fixed Value type sensors)
3. Other sensors with no adjustment possible.

Things that can be adjusted, such as Distributor, Camshaft/Crankshaft Position Sensors, Throttle Position Sensors, Drive by Wire Throttle Bodies, etc will typically require some kind of calibration.

This really depends on the nature of the sensor.

For example:

A coolant temperature sensor is in reality just a big resistor that changes with temperature.

At 100 degrees it may have 500 Ohms resistance, and at 50 degrees it may have 250 Ohms.

The engine computer just looks at the wire the sensor is attached to and sees 500 Ohms and knows it means 100 degrees, because it cannot be adjusted, and just gives a fixed value the engine computer doesn't care about its calibration.

A throttle position sensor for example can be adjusted, typically they may have more than 45 degrees of rotation available when installing. In theory if you install in the exact same location as the previous one it should be fine, however the ECU (engine computer) should be calibrated for the new values just in case. Some ECU's will automatically learn this by making some smart guesses, others may require a unit to be plugged in to tell it to re-calibrate, and some may just have a diagnostics jumper that can be put in to calibrate it.