What Does P1378 Mean?
The ignition circuit for cylinder 8 has a short circuit to ground. This prevents the ignition coil from building sufficient voltage to fire the spark plug, causing a misfire.
Common Causes
40%
Damaged wiring harness with bare wire contacting ground/chassis
30%
Internally shorted ignition coil to ground
15%
Water intrusion in connector causing short circuit path
10%
Mounting hardware or bracket rubbing through wire insulation
5%
Failed ECM driver circuit stuck low
Diagnostic Steps
1
Step 1: Disconnect cylinder 8 ignition coil and measure resistance from control wire to ground (should be infinite)
2
Step 2: Inspect coil connector for moisture, corrosion, or carbon tracking between terminals
3
Step 3: Test ignition coil resistance between primary terminals and from primary to ground
4
Step 4: Trace control wire from ECM to coil, looking for abraded insulation or pinch points
5
Step 5: With coil disconnected, command coil on with scan tool and verify ECM can switch circuit (voltage should drop to ground)
Estimated Repair Cost
$150 - $700
Parts + labor, varies by vehicle and location
The ignition circuit for cylinder 8 has a short circuit to ground. This prevents the ignition coil from building sufficient voltage to fire the spark plug, causing a misfire.
The most common cause of P1378 (Cyl. 8, ignition circuit short to ground) is: Damaged wiring harness with bare wire contacting ground/chassis
Typical repair costs for P1378 range from $150 to $700, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Cylinder 8 misfire will cause rough running, reduced power, and potential catalytic converter damage from unburned fuel. Drive with caution.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P1378 to identify the root cause.
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Quick Info
Category
Powertrain
System
Ignition System
Difficulty
Type
Manufacturer
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