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Q: car won't start and keeps showing error code P0129.

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After replacing a faulty clutch pressure plate in my 2004 pt cruiser (base not turbo), I attempted to start the engine. The engine would not kick over and the check engine light kept flashing. I checked the codes and it was showing a bad cam shaft sensor and a faulty barometric sensor. I replaced both sensors and cleared both codes. After replacing both sensors the car still won't start and is continuing to show the code P0129. Any ideas as to what the problem could be?

My car has 105550 miles.
My car has a manual transmission.

With a reoccurring code such as this, after replacing the sensor, it is time to test the wiring harness and connector at the sensor. The orange wire should have 5V. This would be the 5 volt reference. The black and light blue wire is the ground and the dark green and red wire is the MAP sensor signal the is returned to the PCM through the sensor. This code indicates the voltage from the MAP sensor to the PCM is greater than 4.9 volts or less than 2.28 volts. When this occurs, there is usually a short to ground or a short to power. You will need to check the voltage on the dk grn/red wire to know which it is.

A problem like this will often affect all or more than one sensor with that uses the 5V reference. If you find the 5V reference is high or low, it is often necessary to disconnect each sensor that uses the 5V reference while monitoring the reference signal. When the signal changes back to 5v and not above 4.9 or below 2.28 volts, you have found the bad sensor.

If once you have checked all of this you have not found the source of the short to ground or power, it is time to suspect the PCM. You will want to check the 5v reference, grounds and power pins directly at the PCM by back probing at the PCM connector.

This can be quite an undertaking to diagnose such a problem. If you lucky, you will find a wire that is broken or something along those lines. If you should require further assistance with this, I recommend the following inspection; Car is not starting inspection.

Good luck!

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