It's 2023.  All of the regurgitated talk that if your ABS lights are on it's absolutely the module and replacing it will make everything wonderful  is a thing of the past.  That doesn't apply anymore.  It was true when it started 20 years ago, not as much anymore but it still keeps getting repeated.  

You can pay somebody to scan the car, or you can guess. On the 96-2000 cars which would be the s70, v70, c70, s80 and 850, 85-90% of the time it's the module. That leaves you a 10-15% chance that it's fine and there's something else wrong.   We'll touch on that next. IF your speedometer is not working, that's typically a dead giveaway. Intermittent lights are usually a sign that the module is on it's way out and over the course of time, they'll just stay on.  

The only downside to paying for a scan is that the codes that come up are almost always wheel sensor codes which really don't fail very often. IF read correctly actual faults outside of the module can be figured out.   That being said your mechanic may want to replace the sensors when the reason those codes are coming up is the fault of the module.  Don't do that.    You'll also typically get a pump motor code which is along the same lines, the pump itself rarely fails on 96-2000 cars, but the pins inside the module that control it do.  Communication codes are almost a dead giveaway that there's a module issue.

Speed sensors themselves rarely ever fail.  Speed sensor codes always come up with a faulty module.  When troubleshooting with a multimeter and you actually find an open speed sensor circuit 99% of the time it is the wiring to the speed sensor that has failed, not the speed sensor itself.  This is starting to become common considering all of these cars are 20+ years old.  It ABSOLUTELY does not matter how well your car has been maintained, they are the only wires that move when the car moves.  After 20+ years they can and will break internally, a visual inspection will show nothing wrong.  

You likely googled the issue and everybody on every forum says "It's always the module".    5-10 years ago that was true, now that the cars have aged you can have a module issue and an issue elsewhere in the system and I see this come up about 25% of the time.  It comes with the territory when you consider the age of the car.

The normal sequence for a faulty module is intermittent ABS lights for a period of time, then they'll always be on, then in time the speedometer will drop out.  Most of this will tend to happen after the car is driven and the module gets hot.   If your ABS light suddenly came on one day and stayed on from that point on that typically is an external issue.  

It's important to remember that there are other parts of the ABS system, and like anything else problems do come up.  All of these cars have been on the road for over 20 years, based on that it is not uncommon at this point to have an issue in the system outside of the ABS module itself.  

AFTER INSTALLING.  All of our modules are road tested and cleared of codes before shipping.  The ABS/Traction lights should go off within 3 seconds of turning the key.  Always double check your large harness when installing, make sure it is 100% bottomed out and locked into place and then push down on it firmly for another 5 seconds.  It DOES NOT always like to seat properly the first time due to the way it is designed.  

1999 and later, the Check Engine Light and Flashing transmission light will not always go away on their own.  You have 1 option, have the codes cleared with a scanner....disconnecting the battery rarely clears the P0500, P1618 or P1633 codes or resets the transmission codes.  Realistically, any time your CEL is on, you should have it scanned to see why it is on, you can't guess.  Most common OBD readers will not read ABS or TCU codes.  You will have to clear the TCU and then the ECU in that order.
 

 1996-97 850

96-97 850's have a flimsy harness between the module and the ABS pump where the insulation rots off the wiring. Rubber/heavy electrical tape is needed to fix this.  If either of the bare wires touch each other or the aluminum ABS pump block, this will trigger the ABS lights.  Using a flathead screwdriver, pop the black cover off and then rewrap the wires individually with heavy electrical tape.  Due to the age of the cars, you may also have faulty wiring going to the wheel sensors.  

S80

99-2001 S80's.  Many of them have electrical issues, mainly involving the CEM or the wiring harness connecting the abs module to the ECU.  You can scan your codes and they will point to the module, but it may not be the problem especially if they are using the module with the 8619538 part number.  I've run into more than a few instances where the dealer/shop will scan and say to replace the module, but the module in it works perfectly fine after testing it on another vehicle.    IF you have a CEM issue you typically  won't be able to communicate with the ABS module, which will give you a false positive that the module is faulty, in reality the CEM won't allow communication. In this case a full scan of the car is preferable so you can see if the CEM has codes.   Pump motors also have a higher failure rate than the other 96-2000 cars.

2001 S60 and V70, 2001-2004 C70.

CEM issues are common along with faulty ABS pump motors.  In the case of a faulty CEM the ABS lights are the symptom of a problem and not the problem itself.   

-The Fuses.  Always check your fuses first.  99-2001 cars have two ABS fuses, one in the fusebox below the windshield and one for the pump motor in the box above the module.  Aside from checking if they are blown, check for corrosion/oxidation on the fuse legs itself.

-Blown Axle Boots. These will spew grease everywhere, and can foul up the wheel sensor and tone ring on the axle.

-Flashing service light.  The flashing service light (Not Check Engine light) is indicative of a instrument cluster problem on 1998 cars.  If your speedometer is not working and that light is flashing the cluster can be involved.

-P1618 Speed sensor code.  This is a TCU code.  It isn't always caused by the ABS module.  It can also be stubborn to clear with a common OBDII scanner.  Same thing for the P1633 and P0500 codes. 

-Loose Axle Bolts.  This seems to come up quite a bit, after a front axle replacement the bolt isn't torqued properly allowing the bolt to come loose.  This allows the axle to move in and out of the front hub...in turn the tone ring and wheel sensor don't line up properly. 

-Faulty ABS pump.  Very rare on 96-2000 cars, common on 99-01 S80's and 2001+ V70, C70 and S60.  Very easy to test using a multimeter.  You should get .5-5 ohms from it, testing at the small harness going to it.  Excessive resistance or an OL reading can indicate a faulty pump motor.  This rarely happens.  A pump motor code from the ABS module generally does not mean the pump motor is bad. 96-2000 pumps rarely fail, 2001 pumps will occasionally along with 99-01 S80's

-Alternator/Voltage issues.  This can be a tricky one to figure out, and you'll need a meter to do it.  Low voltage/high voltage output from the alternator or fluctuating voltage will cause the ABS lights to come on indicating various faults in the system.  The voltage issue tends to become more of an issue with the 8619538, 8619535 and some 8622093 modules. 99+ cars are more sensitive to voltage, 96-98 cars are more tolerant of it.

-Ignition switch.  Another one that will cause ABS lights.  If manipulating the ignition key back and forth with the car running makes various warning lights on the cluster come and go or affect the headlights (dimming), that is a contributing problem.  While driving move the key all the way forward without engaging the starter and all the way back without actually shutting the car off.  If everything goes back to normal, you need a new electrical portion of your ignition switch.

-Wheel speed sensors. If you think you have a sensor issue, it should test out between 900-1380 ohms. In reality they rarely go bad.  Always check the plug going into the wheel sensors.  They may not be plugged in all the way, they could be cracked or the wiring could be stressed. I've found that people that have recently had front end work done on their car routinely run into sensor problems.  Why?  People forget to unplug them, then when moving the steering knuckle out of the way they stress the connection.  They are located front and rear behind the brake rotor dust shield, and are held in place by one 10mm bolt.  I've also seen the wiring rub against the sway-bar end links, rubbing the insulation off down to bare wire.  Also, the wiring likes to fray where it enters the plug before the wheel sensor..almost always up front.  Sometimes the fray is internal to the sheath and won't be seen physically.  They're the only wires on the car that always move when the car does, so they can wear out...especially after 20+ years of service.

Realistically, it doesn't hurt to pull them out and clean them occasionally.  If you're trying to troubleshoot them, you should pull them to check for debris and damage regardless even if the resistance tests out okay.  

-About Aftermarket wheel speed sensors.  Hit or miss on whether they will work or not and I always say to not touch them.  The common scenario is the wheel speed sensors were replaced by a mechanic because wheel speed sensor codes came up, as they do with a faulty module.  Then a module is ordered but the problem isn't solved because the aftermarket sensors are faulty out of the box. Any used OEM sensor is better than any new aftermarket.
 

 To test the ABS wiring (from the sensors to the module) for an open circuit, put a multimeter on the following pins on the ABS module connector (not the module itself) . They should test out between 900-1380 ohms .  The wiring almost always breaks internally as it's a very thin wire inside a heavy sheath.  After years of moving they break and a visual inspection proves nothing.   A cheap multimeter is all of $10 and there's a zillion YouTube videos showing how to use one.  They're simple.  

This only works for 1996-1998 models, not the 1999 and later cars.  On 99-2001 cars you can test the sensor itself, not the continuity of the circuit unless you physically unplug that section of harness.

LF sensor: pins 11 & 4
RF sensor: pins 18 & 3
LR sensor: pins 10 & 2
RR sensor: pins 17 & 1

Use your spare ABS module to figure out the location of the pins on the connector. Here's a picture for reference on a 96-98, all the pins are numbered.