Soldering repair of wire and engine knock sensor failure

I had a leaking gas line that was repaired for about $35. Immediately the Check Engine light went on, in my 2001 Subaru Outback Wagon with 120,000 miles on it. (I bouught it new.) Returned to shop, they had cleaned up around the leak, and said a wire may be loose. Found loose connection to Engine Knock Sensor which was soldered. The repair got me to work just in time to be hit on the way home as I sat on the side of the road with all other drivers waiting for a salt truck as the impassible iced road had cars in the ravine. The only bold young lady driver who dared to continue on the road hit me as I sat well off the highway on the side of road with flashers on like everybody else. At least I kept the seat belt on. Now with the car back in one piece, the light revisited me, and the shop said the computer was shot. So, now I have colleagues who teach electronics at my college (I teach the math and science) and they say the wire may have been orignailly CLIPPED to the computer, and should not have been SOLDERED as the heat may have fried the computer. How do I know? I paid for the repair, and asked twice about the issue of why it went out after the work! These guys always take my car in at 7 AM when I need work so I can teach math all day and not miss a day’s work of confusing the college students with equations, but they have me stumped on this repair and its cause. I don’t want to make enimies but $500 friends are easy to come by. (They have my $500 for detailed testing, labor, and parts.) And OH, it’s a AAA guarantee service provider, with a 3 year 36,000 mile guarantee (that’s less than 1 1/2 years for me).

I have been dissatisfied with the dealership and did not go there for work due to the fact that the smell I could not stand for almost 5 years disappeared when they finally acknowledged the fact that yes, the head gasket is broken and was under 100,000 mile replacement due to report I read right after I bought the car, and they replaced it at 95,000 miles and the smell went away, right before the catalytic converter went out and they got the $1000 anyway. Was that due to the head gasket being defective for 95,000 miles???Could go on and on about misery at the dealership!

Sure would like advice on the engine knock sensor.

The computer should be somewhere under the dash and I believe the wire that was soldered would be under the hood. It would be unusual for the heat to run that length of wire and through the computer connection to cause any damage. Its always a good idea to use a heat sink when soldering on a circuit board but I don’t think this would have fried the computer. Computer guru’s don’t necessarily know anything about cars.

A hard jolt like that can cause a computer failure just depending on where it was hit and where the computer is so you may want to try and recover from the assailants insurance company. Or it is quite possible the computer was bad to start with and that the wire wasn’t the problem at all. A dealer would put a test computer in to make sure that was the problem.

The wire to the knock sensor has a shield (probably foil) around it which is grounded to the car body. After soldering the center wire, the shield should NOT be caught in the solder; nor, should the shield be discontinuous from car body ground to knock sensor body. Did the repairer succeed in this? A check with an ohm meter would reveal yes, or, no.
Besides heat being conducted into electronic circuits, another danger is from damaging voltages from the soldering iron (or, gun) being conducted into the electronics. Crimping is, actually, the preferred method of wire repair, because of this danger.
Is the engine computer toast? Is this shop qualified to determined that? What shop would be the best shop, is your new problem. Perhaps, some of your colleagues may know of a qualified shop.

The fact your colleagues teach electronics does not mean they’re automotive electrical experts. Auto is a whole different ball game.

It’s unclear to me at this point where the wire was soldered. The ECM (computer) is out of sight and out of mind. Repairing a fuel leak should have nothing to do with the ECM connector. Soldering the other end at the knock sensor should not affect it.

Can you provide any details as to what codes are present, not present, etc. and why the computer is claimed to be bad?
Was the car running or not running when brought in? If running, was it running poorly?
Computers seldom go bad but a wallop from another car could have fouled something else up.

One of my regular customers was a PHd in electronics and worked as a trouble shooter for a major corp but was not familiar with automobile DC cirtuits and was dumbfounded with an intermittant short-circuit on his wifes Honda.

It’s unlikely that the direct heat from soldering the wire harmed the car’s computer in any way. It is, however, possible that the soldering iron wasn’t grounded and electrically damaged the car’s computer.

It works like this: the soldering iron runs on house current, which is 120 volts AC. If the iron is not correctly grounded, that is, if it’s own internal ground is broken or otherwise missing, the hot tip used to repair the wire can carry the same 120 volts. That 120 volts is thus applied to the computer, which is still connected to the wire being repaired. The 120 volts can then damage the computer, which normally runs on only 12 volts.

Note that this is only one possibility. It would require an ungrounded iron, and the lack of ground MIGHT still not damage the computer. It’s still very possible that the collision caused your problem.

Anti-freeze from a leaking head gasket, which gets burned (vaporized) during engine combustion, is a poison to the oxygen sensors and the catalytic converter. The car makers know this. The catalytic converter should be included in the warranty coverage.

The car electronics need a path back to the AC supply in order to have any current pass between them. The car’s electronics are isolated and it sits on rubber tires with very little carbon black (enough to prevent static build up but not pass appreciable current at 120VAC) making a connection to earth ground. If you hard grounded the body and then applied the 120VAC fault to the wire, then it could be toast. When the circuit floats, no damage should occur.

After the impact with the brave young lady, what was the Code that the check engine light read out to be? It would be in the form of: PXXXX.


I doubt very much any damage was done to the ECU connected to the soldering of the wire. Knock sensors are somewhat fragile and it may be that it was damaged in the collision but that is hard to say without knowing what the code error is. Hellokit has some good advice in the last two previous posts, along with the others, it would help us to know what code(s) is stored in the ECU. You can have it checked for free a place like Autozone. Write down the codes for us to look at.

While it is possible the ECU is bad, I doubt it is. I would have it checked out by another shop since you have had trouble with the dealer service. Try to find one that specializies in electrical repairs. Another place that works on Subarus would be good also.

I also agree that the CAT was damaged due to the gasket leak and caused it to fail. Of course the dealer will most likely deny that. If you reported the smell early on and the dealer didn’t work on the trouble you may be able to talk to the Subaru area rep. and see if he can help get you a refund on the CAT repair.

Keep us posted on the progress.

Here is a link to a good Subaru site that can help you in the future if you want to join in on the fun.

(I own a’01 LL Bean OBW. I think these are great cars. Sorry yours got hit.)

I was, at first, inclined to agree with you in that the car would be floating above ground, but upon reflection I think the car almost certainly was grounded during the repair.

It was surely on a lift of some kind. Presumably this was a pneumatic or hydraulic lift, made of metal and making good contact to the metal of the car’s supporting structure. While I suspect that the lifting mechanism itself might not be in good contact with ground, these lifts always have a mechanical safety. That mechanical safety would provide an excellent ground path, completing the circuit from the car to ground.

I therefore submit that it is still possible that an ungrounded soldering iron could have damaged the computer.