P2078 code definition
Intake Manifold Tuning (IMT) Valve Position Sensor/Switch Circuit High
What the P2078 code means
This code will set if the Engine Control Module (ECM), or Powertrain Control Module (PCM), detects a problem with the position of the IMT valve.
What causes the P2078 code?
This code is set when the ECM sees a higher than expected voltage from the IMT position sensor. This can occur if the circuit has a short to power or a bad ground.
What are the symptoms of the P2078 code?
- Check Engine Light
- Lack of power at higher engine RPM’s
- Erratic idle
How does a mechanic diagnose the P2078 code?
The first step would be to connect a scan tool to the vehicle's OBD II connector, check for codes and observe the scan tool data for the IMT valve. Most vehicles have the capability to command the IMT valve motor to move. This should be done while watching the scan tool data and observing the valve itself. Often times it is necessary to do some disassembly to physically check the valve and sensor.
Common mistakes when diagnosing the P2078 code
Some IMT valves are vacuum actuated and others are electrically actuated. Vacuum lines to the vacuum actuated valves frequently plug, collapse or break. Failing to thoroughly inspect the vacuum lines can lead to misdiagnosis of the IMT valve.
Some disassembly is often necessary to see if the valve flap has broken away from its shaft. If this happens, all data and the valve motor will work, but drivability issues will still exist. A mechanic could dismiss this code as erroneous and suspect other components as the problem.
How serious is the P2078 code?
This code will likely cause drivability problems that the owner will find annoying. Outside of drivability issues this code will not cause damage to the motor or be unsafe for the passengers.
What repairs can fix the P2078 code?
Replacing the IMT valve (the sensor that this code indicates is the problem is most often part of the IMT valve assembly and will be replaced as a single unit)
Repairing wiring to the control circuit
Repairing vacuum lines that may be plugged or leaking
Additional comments for consideration regarding the P2078 code
This system is designed to enable better fuel mileage and performance in modern vehicles. The IMT valve directs the intake air flow from a long path at low RPM’s (Revolutions Per Minute) to a shorter path at higher RPM’s. This affects the MAF (Mass Air Flow) sensor’s data. The MAF, or as it is sometimes called, the MASS air flow sensor, is one of the primary sensors used by the ECM to calculate air/fuel ratio.
Need help with a P2078 code?
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This article originally appeared on YourMechanic.com as P2078 OBD-II Trouble Code: Intake Manifold Tuning (IMT) Valve Position Sensor/Switch Circuit High and was authored by Robert Kulp.