Q: Service stable track light is on, service traction control lights on, rough idle. New throttle body.

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Had my throttle body go out on vacation. Had a mechanic replace it. Driving back home, the service stable track, and service traction control lights come on. The idle is rough now, engine is not firing on all cylinders when idling. Shifts very hard from first to second gear. Throughs code p0300.

My car has 208000 miles.
My car has an automatic transmission.

Having symptoms such as this after having something like a throttle body replacement definitely suggests something went wrong with the installation, but not necessarily so. Coincidences happen every day in automotive. If I were working on this vehicle, I would be wondering if the throttle body replacement was needed or if it was a misdiagnosis. Unfortunately, there are many questions that need to be answered here.

First, you need a scanner that can communicate with the stability and traction control systems. First thing I would want to know is what codes are present. There are more than a few modules on your car that will produce codes. The P0300 code is from the PCM, which is a random cylinder misfire. You need to see what codes the stability control systems are setting. This will set me down the correct path to diagnose your vehicle. While I have the scanner hooked up, I would be looking at all relevant data for the ABS, traction, and stability control systems. These systems use the same sensors to make decisions.

If I were to make a guess at what is going on in your case, based on the symptoms you describe, I would be looking for some pinched wiring that resulted from the throttle body installation. This is an easy thing to do as a mechanic. It has happened to me many times. The throttle body is used by the PCM (Powertrain Control Module) to make several calculations involved in all aspects of vehicle controls systems. Some of the wires connected to the throttle body, if pinched, will affect many other systems randomly. In addition to a pinched wire, I would be looking for a vacuum leak either from a hose that was left off, or from the improper mounting of the throttle body.

One thing I am wondering is how long was it before the symptoms you are experiencing started after the throttle body replacement? Usually, if a wire is pinched, it won’t take long. So lets assume the throttle body has been replaced correctly. The next thing I am wondering is, what is the brand of the part that has been installed on your vehicle? Many aftermarket parts don’t work well with factory computer systems. Small differences in the electronics can make a big difference to the PCM. There are many quality aftermarket products, but there are also a few I wouldn’t install if I could avoid it.

If all mentioned above is ruled out, then it’s time to begin diagnosing with the assumption that these are new problems. As stated before, retrieving the codes and accessing sensor data is paramount. There are many sensors involved with the stability control. The only way to pinpoint a specific failure is to study scan tool data. Most people don’t have access to a scan tool that is capable of diagnosing such systems, so if you inspect everything and don’t find a vacuum leak or a pinched wire or wiring harness damage, you are going to need to take this one to a technician. Often times it is something simple, but just as often these systems can be very difficult to diagnose. If you would like a proper diagnosis of your warning light, a technician from YourMechanic can come to your home or office to inspect your car and let you know exactly what is going on.

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