What does it mean?
P1538 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Differentiated Intake Manifold Potentiometer Voltage in Upper Diagnosis Range .
Symptoms
- Check engine light on with a smooth idle
- Loss of torque in one part of the rev range because the manifold stays in a single position
- Dull acceleration under load and a flat spot at the usual changeover point
- Slightly higher fuel consumption
- Intake note unchanged through the rev range
- Fault may be intermittent at first and then set permanently
Likely causes
- Open sensor ground, which pushes the signal towards reference voltage
- Signal wire shorted to the five volt reference or to battery voltage
- Worn potentiometer track with a break at one end of its travel
- Flap jammed by carbon at its upper stop, or a broken return spring
- Corroded or water-damaged connector at the potentiometer
- Vacuum actuator stuck in one position, holding the sensor at the end of its range
- Module input circuit failure as the last possibility
How to fix it
- Watch the potentiometer voltage in live data while the flap is actuated and while it is moved by hand
- Measure the five volt reference and carry out a voltage drop test on the sensor ground
- Check the signal wire for a short to reference or supply voltage with the sensor unplugged
- Inspect the flap shaft, bushings and linkage for carbon build-up and seizure
- Test the vacuum actuator and hoses with a hand pump and confirm it holds vacuum
- Replace the potentiometer or the actuator assembly if the signal does not sweep correctly
- Clear the code and confirm the feedback voltage tracks the commanded position through a road test