The P1394 GMC code is a generic manufacturer (OBD-II) code that indicates a specific problem with the circuit or performance of the Variable Valve Timing (VVT) System.
On General Motors group brands (GMC, Chevrolet, Cadillac, Buick), this code means that the powertrain control module (ECM/PCM) has detected a discontinuity or error in the camshaft position sensor (CMP) signal relative to the crankshaft position (CKP), related to the variable timing system.
The PCM constantly compares the signals from these two sensors. If the correlation between them falls outside the expected parameters for a specific duration, the P1394 code is stored and the check engine light illuminates.
2. Main Symptoms
The appearance of this code is often accompanied by the following symptoms:
- Check Engine Light (MIL) illuminated: This is the initial sign.
- Engine shaking or misfiring: Incorrect valve timing disrupts combustion.
- Unstable and irregular idle: The engine may jerk and even stall.
- Noticeable decrease in performance and power: Especially noticeable during acceleration.
- Increased fuel consumption: Poor combustion reduces efficiency.
- Starting difficulties: In the most severe cases.
3. Possible Causes (By Order of Probability)
Diagnosis requires a methodical approach. Causes range from simple to complex.
- Electrical Problems (Most Common):
- Failure of the camshaft position sensor (CMP). This is a very common cause.
- Damaged wiring: Loose, chafed, cut, or melted wires in the CMP sensor circuit.
- Faulty connector: CMP connector that is corroded, oxidized, poorly connected, or has bent pins.
- VVT System Problems:
- Failure of the VVT control solenoid: The solenoid valve that controls oil flow to the VVT actuator may be stuck or inoperative.
- Faulty VVT actuator: The physical mechanism that rotates the camshaft may be seized or clogged.
- Poor quality or low engine oil level: The VVT system depends on oil pressure to function. Dirty oil, incorrect viscosity, or an insufficient level is a frequent cause of malfunction.
- Mechanical Problems:
- Timing issue: The timing chain may have jumped a tooth (without causing catastrophic damage), often due to worn tensioners or guides.
- Insufficient oil pressure: Problem with the oil pump or a clogged circuit.
- PCM Failure: (Extremely rare)
- An internal problem with the engine control module is possible, but all other causes should always be ruled out first.
4. Diagnosis and Troubleshooting
WARNING: These procedures require knowledge of automotive mechanics. If you are not comfortable, consult a qualified mechanic.
Recommended tools: OBD2 scanner, digital multimeter, basic tools.
Step-by-step procedure:
- Scan and Visual Inspection:
- Read all fault codes with a scanner. Note any associated codes (e.g., P0340, P0341).
- Check the engine oil level and condition. Black, thick oil is often the cause. Perform an oil change if necessary.
- Visually inspect the CMP sensor wiring and connector. Look for obvious signs of damage.
- CMP Sensor Test (Multimeter):
- Resistance Test (Ohms): Disconnect the sensor connector. Measure the resistance between the sensor pins. Compare the value to that specified in the repair manual for your GMC engine. A value outside the specification indicates a faulty sensor.
- Signal Test (AC Volts): Reconnect the sensor. With the multimeter in AC Volts mode, backprobe the signal and ground wires. Have an assistant crank the engine. You should see a voltage fluctuation (usually 0.5V to 2V AC). No reading = dead sensor.
- VVT Solenoid Test:
- Locate the VVT solenoid (often on top of the engine).
- Disconnect its electrical connector. Check its resistance with a multimeter. An infinite (OL) or zero (0 Ohm) reading indicates a faulty solenoid.
- You can apply 12V directly to it (check the exact procedure). An audible “click” confirms it is working mechanically. Its absence indicates it is stuck or dead.
- Mechanical Check:
- If everything else is good, it may be necessary to check the alignment of the timing marks to ensure the chain has not jumped a tooth.
5. Common Repairs and Approximate Costs
- Oil change and filter replacement: Low cost (€50 – €100). Sometimes this is enough to solve the problem if the oil was very dirty.
- CMP Sensor Replacement:
- Part: €50 – €150
- Labor: 0.5h – 1h
- VVT Solenoid/Actuator Replacement:
- Part: €100 – €300
- Labor: 0.5h – 2h (depending on accessibility)
- Timing Chain Resynchronization/Replacement:
- Very high cost (€600 – €1500+), due to intensive labor.
Conclusion:
On a GMC, the P1394 code is serious and affects performance and fuel economy. The cause is often electrical (CMP sensor) or related to engine oil and the VVT solenoid.
Recommended approach:
- Check and change the engine oil if it is not in good condition.
- Inspect the wiring and connector of the CMP sensor.
- Test and replace the CMP sensor if necessary.
- Test and replace the VVT solenoid if the previous steps have not resolved the problem.
If the problem persists after these checks, a more thorough inspection of the timing system by a professional is highly recommended.