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2007 Buick Terraza Traction Control, Reduced Engine Power

I recently saw the Service Traction Control light, Reduced Engine Power Message come on. I could only go about 5 miles an hour. I had it towed to the nearest Buick dealer. They replaced the throttle sensor, the gas pedal sensor and the knock sensor. Car ran fine for about 300 miles. It happened again. This time I was near another Buick dealer. So I turned on the emergency flashers and crept up to this dealer. By this time the engine was knocking horribly as if it had no oil (oil was ok).
The service manager suggested the engine needed to be replaced. As I was discussing and contemplating, after a few minutes, we tried starting the car, but it acted dead. Half an hour later we restarted the car with no knocking noise. Left it with this dealer over the weekend. He said he could not find anything wrong with it. We drove the car 300 miles home and it ran beautifully. Due to the intermittent nature of this problem and the fact that that 2 different Buick dealers could not fix it, I am afraid to drive it. Other than selling it, any suggestions? We really like this van; it has only 116,000 miles on it.

Did the check engine light come on? If so, what were the codes? You can get them read at many chain auto parts stores. Call ahead to make sure the store has a code reader if you go.

I forgot to mention - the Check engine light did come on both occasions. It is no longer on. I didn’t get the codes, but I assume they prompted the first dealer to replace those sensors. The second dealer cleared the light but did not reveal the codes.

Thank you for your input, but at this point the check engine light is not on and the van is drivable at the moment, but for how long?

Yow! That kind of flies in the face of my philosophy involving trying the “cheapest, easiest” things first when repairing vehicles.

Man, that Service Manager goes right for the throat (and the wallet)! What if you had agreed and it did the same thing on your way home?

GM publishes Technical Service Bulletins for use by their technicians to help them diagnose problems that are sometimes difficult to diagnose or resolve.

Please note that not every bulletin applies to each problem discussed in the bulletin. They can be a starting point for additional diagnosis. This may or may not help you.

Did they check for bulletins? Did they check for what is outlined in this one?

The knocking is puzzling.

Here’s another version with a more recent publishing date and probably revised…

Good advice above. You may have multiple issues going on there OP. You didn’t say what year your Buick is, but it sounds like it has electric throttle control; i.e. the accel pedal isn’t connected to the throttle body directly. instead a computer reads the pedal position and causes a motor in the throttle body to move the throttle valve to match the pedal movement. This has been done by a lot of manufacturers b/c it eliminates other gadgets that used to be needed for controlling the idle speed under various circumstances, like if the AC is on, or in D vs N, the engine is cold, alternator load on the engine, etc.

The problem is that function has to work perfectly, otherwise it creates a big safety issue for the driver. Unintended accelerations, stalling, unsteady engine rpm, etc. So there’s software built into the design that is constantly checking if the throttle body is exactly matching what it should be for the current pedal position and those other factors. If not, it turns on that dash light and puts the car in limp mode. The idea is you can still drive home that way, provided home is just a short distance, which it usually is.

So what’s the problem? You need a proper diagnosis. Based on what you say, the limp mode is likely from among problems with the pedal position sensor, the electric motor in the throttle body, or the throttle valve sensor. The engine knocking, that’s harder to say. It could be caused by the above problems perhaps. Pinging – if that’s what’s causing the knocking – can be caused by bad gas, spark plug problems, and valve & ignition timing problems. On older and less well maintained cars, internal carbon deposits in the engine can cause pinging, esp if it occurs only when the engine has been driven for some length of time, like 30 minutes or more. Knocking sounds can signal major engine damage too, like bearing problems. But if your engine has that problem, it should be easy for a shop to reproduce it, or even yourself, by driving (or even accelerating) up a really steep hill to put a big load on the engine. If you do that and there’s no knocking sound, internal major engine problems are unlikely.

Instead of a dealership, take it to an inde mechanic that specializes in American cars, Buicks, even better. Ask your friends, relatives etc who they use to service their own cars. Tell the shop the problems it has, and that as first priority you want them to advise you if the engine has major internal problems or not, based on what you heard, the knocking sound.