C0638 Code:

OBD-II Diagnostic Chassis (C) Trouble Code

Possible Cause

To know real cause of C0638 based of if this code is present in your two-wheel drive vehicle, suspect a faulty PCM or programming error. In most four wheel drive applications the four-wheel drive switch is used to switch between two and four wheel drive operation, as well as between low and high ranges

Common Symptoms

  1. If the numbers are incorrectly entered, this type of code will be the result
  2. If either the reference voltage or ground circuits are open, use the digital volt/ohmmeter to check for continuity (disconnect all related control modules from the circuit before checking circuit resistance or controller damage may occur) and resistance in all circuits
  3. Some applications also utilize the inlet door to regulate air flow through the intake tube by adjusting the degree to which the door opens according to throttle position
  4. If the code fails to immediately return, you may have an intermittent condition

Similar Problems

Auxiliary Emission Controls
Crankshaft Position Sensor Circuit Performance
Kick-Down Switch Failed Short

C0638 Troubleshooting

If this is the case with the code, you will be hearing very loud noises, like explosions, from the rear of the vehicle upon acceleration. Firstly check the carburetor to ensure that the fuel and air combination are properly balanced. If these settings are correct, then thoroughly inspect the distributor cap for any cracks or hairline splits, as this can produce a backfire. Move on to the spark plugs and ensure that they are clean and undamaged and inspect the engine gaskets for any sign of damage, as they are responsible for ensuring the correct gas exchange within the engine. C0638 OBD2 reports a sensor fault, replacement of the sensor is unlikely to resolve the underlying problem. The fault is most likely to be caused by the systems that the sensor is monitoring, but might even be caused by the wiring to the sensor itself.

How to turn off C0638 check engine light ?

There are two ways to turn off the check engine light:

A repair technician can do so once repairs have been made.
The OBD II can automatically turn it off when it fails to detect the problem after several diagnostic cycles.

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