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Cadillac Repair/1999 cad sts pcm communication loss


QUESTION: this car sat for several years with a dead battery.  now, with a fresh battery, everything lites up, but when the key is turned -- nothing.  dtc shows p0603, u1016, u1255, and b1652.  service engine lite is also on.  What should I do next ?

ANSWER: Hello,
The first thought that comes to mind since the engine doesn't crack is that the key memory is lost in the VATS (vehicle anti theft system). This is not a transponder but a resistance based system. That is a resistor pellet in your ignition key.

I do not know if by resetting the system if this will work, but give it a shot. This will reset all of these issues in theory. Try it and get back to me.
Put the key in ignition and turn to on. Leave it like that for 10 minutes. Turn off. Turn on for 10 minutes. Turn off. Turn on for 10 minutes and turn off again. See what you have and get back to me. See if the engine starts.


---------- FOLLOW-UP ----------

QUESTION: Thank you !  I did as you instructed ,  and still the same results.
Also, there is a buzzing noise coming from the left side of the dash, near the a/c vent.  There is a small vent, 1" x 1" approx. , between the a/c vent and the steering column.  Don't know what this is, but the buzzing seems to come from that area.  This car has the Northstar system.  

After attempting to start the car just now, after following your instructions, I checked for DTC's , and there were none.


The buzzing may just be from the key buzzer and chimes creating a noise you would not normally hear if the engine started. I remember these things making such noises when the engine did not start.

Here is my concern with the battery being dead for so long:

The VATS (Vehicle anti theft system) is a resistor based ignition system. There are 14 different resistor values. The computer stores in its memory as to which resistor value is assigned to the vehicle. If over an extended period of time, that memory is lost, you may have big problems. This is a common issue with transponder systems in which the battery is discharged. In the case of a GM transponder system, there are ways tat the owner can reset a transponder key for the vehicle.

In the case of VATS, it isn't that simple. One way to do it would be go to a junk yard, get the vehicle's vats module and steering column with ignition key for that vehicle. Swap the computer and VATS module and then take the steering column apart and remove the ignition and harness and swap that. In theory, if the VATS was not bypassed and if this is why the engine is not starting, the engine will run.

You could also obtain a NEW VATS module from the dealer (pricey at over $100), install that. Try starting the vehicle. The new VATS module will burn in the very first ignition resistor it sees, which would be from your key.

The biggest problem here is that we do not know if the engine is not cranking because of the VATS or something else.

The first thing I would suggest is to take a test light to the solenoid terminal at the starter and have someone put the ignition in the start position. If you have power going to the starter, the starter is junk. The problem you may run into is that you don't have power running to the starter, and the most common issue then goes back to VATS. Commonly bypassed because the wires broke inside the steering column. They are very thin like the old land line phone wires. The wire would break inside the plastic insulation and any movement would put the broken wire back together.

You can take a test light with the key in the start position and see if there is 12 volts running to one of the contacts that you need to look for at the ignition lock. the other side will have less than 12 volts due to the resistor in the key.

The problem is that if the memory is missing in the VATS module, the system can't even be bypassed.

What you can do is go to a junk yard and find a car from 1992 up, and it doesn't need to be a Cadillac. It can be a Buick, Pontiac, Chevrolet, Olds any model equipped with VATS (all were after 1990) but te 14 resistor system was used in the 1992 and before that was 15. Make sure the plug going in is the same as yours. Then take an ohm meter and measure the resistance on the key. If you do this, you do not need the steering column for the ignition lock and key. Write down the resistor value. All you are doing is measuring the resistance value in the key that matches up with that VATS module.

Remove the VATS module (Usually behind the glove box. Replace the VATS module in your Cadillac. Go to Radio shack and get a resistor which value is within 10% of your key reading.

Go under the dash an go to the junction block at the firewall. You will find two thin white colored wires commonly in an orange sheath. Make sure you are not looking at an orange wire! Once you find the correct wires, give yourself abut 6 " from the fire wall and cut the two thin wires running up and down the steering column. Connect the resistor bypassing the steering column altogether.

If it is a VATS problem with the unknown as to if the memory is any good from the car sitting at a junk yard with a discharged battery and you might run into the same issue you currently have.

I hate to see where someone has to change parts on a guess, but if this junk was not in the vehicle, we wouldn't have to guess.

Make sure that if this car was not subject of being a rodent's home under the hood where wiring wasn't chewed up.

There is also the possibility that the VATS wires are broken in the steering column. Titling the steering column in various positions moves these wires and sometimes they go back together to start the engine.

I have given you just about every scenario I can think of as to why the engine will not crank. You may want to have someone crank the engine and see if resistance power is getting to the VATS module. That way, you know for sure that the memory for the VATS chip is gone in the module.
Good luck and let me know what you find.  

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Rob Painter


Alarm system questions cannot be answered on this forum. These systems are not what I can answer. Without being physically at the vehicle and not knowing what kind of electrical service has been done on the vehicle, there is no possible way to give an accurate answer over the internet. My expertise is in Ignition/key based anti-theft systems. These issues include GM VATS (resistor chip in key blade) PASSLOCK (MRD)-ignition lock rotation based, no special ignition key and the PKIII Transponder (computer chip in key) systems. These systems are not alarm based and are integral with the starting of the engine. This is why I cannot diagnose alarm problems without physically looking at the vehicle: Alarm systems are a completely different annimal than ignition key/lock based anti-theft system. Many alarm questions come from vehicles 10 years old, and since older, many hands that had been involved over the years.I am an expert in all GM factory (ignition/key based)systems. Alarm system questions pose to many situations beyond my knowledge as to what has been done to the vehicle over the years. Some guy may have actually wired the stereo into the alarm system. Who knows? Over my past 30 years in vehicle wiring repair, I have seen unbelievable wiring disaters done by guys that consider themselves "mechanics." I have seen stereos and alarms intalled using surgical tape. I have seen modules burn up, un-fused circuits, wiring jambed between the doors and even lamp cord used for a starter kill. To answer alarm questions over the internet without examining the vehicle is like asking; What does it take to remove a dent?


Education/Credentials-ASE certified. 11 years with a GM dealer and 17 years with a repair facility dealing with only the repair of theft recovered vehicles.

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