P0285 explained: Cylinder 8 only exists on V8 and larger engines. A low-voltage circuit fault often kills cylinder 8 entirely - misfires, rough running, and bad fuel mileage are the result. Inspect the injector connector before condemning expensive parts.
🎯 Top Causes & Probability
50%
#1 - Most Likely
Damaged Connector or Wiring
Injector connectors get baked by exhaust heat. Backed-out pins, melted plastic, and green corrosion are the most common findings.
🔨 Part
$15-$60
👨🔧 Labor
$60-$200
⚡ DIY
Easy
30%
#2 - Check First
Failed Cylinder 8 Injector
The injector coil has burned open or shorted to ground. On older engines this is common after 100k miles. On modern direct-injection engines, less common.
🔨 Part
$80-$450
👨🔧 Labor
$100-$350
⚡ DIY
Medium
15%
#3 - Less Common
PCM Injector Driver Fault
The PCM driver for cylinder 8 has failed. Confirmed only after injector and wiring test good. Replacing the PCM is expensive.
🔨 Part
$400-$1100
👨🔧 Labor
$120-$240
⚡ DIY
Hard
🚗 Most Affected Vehicles
🔧 Step-by-Step Diagnosis
- Locate cylinder 8 (refer to a firing-order diagram for your engine). On most GM V8s it is rear passenger side. On most Ford V8s, rear driver side.
- Inspect the injector connector. Pull it, look for green corrosion or melted plastic. Reseat firmly.
- Unplug the injector and measure resistance. Most port injectors read 11-18 ohms. Direct-injection runs lower (1-3 ohms). Out of spec = bad injector.
- Swap the injector with cylinder 7. Clear codes and drive. If the code follows the injector (now P0283), the injector is bad.
- Inspect the wiring harness for chafing - especially where it routes near the exhaust manifold.
- After repair, monitor short-term fuel trims to confirm cylinder 8 is firing normally.
❓ Frequently Asked Questions
Will my V8 run with P0285?
Yes but rough. Cylinder 8 may dropout entirely, causing a noticeable misfire and rough idle. MPG drops 5-15%.
Can I drive with P0285 short-term?
A few days is okay. Extended driving with a dead cylinder dumps raw fuel into the catalytic converter, which is much more expensive to replace.
Should I replace all 8 injectors?
Not unless several have failed. If only injector 8 is bad, replace that one. Keep the others as long as they test in spec.
How do I know which cylinder is 8?
Check a firing-order diagram for your specific engine. Most GM V8s number drivers-side front-to-back 1-3-5-7 and passenger-side 2-4-6-8. Fords reverse this.
Are aftermarket injectors okay?
Standard Motor Products, Delphi, and Bosch make quality replacements. Avoid no-name brands - injector tolerance is critical for emissions and MPG.