Magazine for the auto service professional
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38 | September/October 2014 incorrect data to the PCM and cluster (being that the PCM's VSS PID would act up when the cluster did. Before selling a module, and because interpreting serial data is close to impossi- ble, a soft reset was done to the vehicle (see Figure 6). After the soft reset, the vehicle did not act up. The driver was informed if the vehicle were to act up again, that a TCM replacement would be recommended. How- ever, it has been over three months and the vehicle continues to drive problem-free. 2005 Toyota Matrix P0171B A frst-time customer rolled by the shop with a check engine light. It was a Toyota four-cylinder with a P0171 DTC. This is a very common issue on these vehicles. Here are the common problems: a TSB for bad PCMs (and a recall to replace them), bad intake manifold gaskets, and bad MAF sensors. Of course, major vacuum leaks and other causes are possible, but with the Toyota 1.8L engines these are extremely common (see Figure 7). Because these Toyota lean codes are so common, they can be diagnosed very quickly. First, the telltale sign of a bad intake manifold gasket is to check freeze frame. It is wise to look for highly posi- tive LTFT (long term fuel trim) and the ECT at ambient temperature, generally during cold weather conditions. If the freeze frame shows this, but the vehicle presently has good STFT and LTFT, even though the DTC is on, the condition is caused by the intake manifold gasket. Generally, the moment the engine gets warm, the gasket expands and seals up everything, and fuel trim returns to normal. A smoke test will not catch these. Now, if freeze frame does not lead one to a faulty intake manifold gasket, it's a good idea to check LTFT to see if it is highly pos- itive in datastream. If it is, graph STFT and go WOT (wide open throttle). If STFT goes from highly positive to about zero, there is a massive vacuum leak. If there is not, the likely culprit is a bad MAF sensor. Now, before replacing the MAF, there's one more quick test to perform that only takes about 20 seconds. Go to the calculated load PID and when graphing it go WOT. If the PID does not go higher than 90%, as long as you are not at extreme altitudes such as in Colorado (in that case the technician will need to consult a volumetric effciency calculator), you've just diagnosed a bad MAF sensor. Suffce it to say, the frst two tests passed on this Matrix. Now, before telling the Connections Figure 9: Unplugging the O 2 sensor must have spiked the PCM, because all of the sud- den the PID on datastream came to life! Figure 8: Even when testing the vehicle at WOT, the sensor's readings did not budge. Figure 7: This 2005 Toyota Matrix 1.8L came into the shop with a P0171B DTC.