Bad Spark Plugs Symptoms: Real Signs to Watch For
Key Takeaway
Spark plugs wear out. Here are the real symptoms of bad spark plugs and when to replace them.
Symptoms of Bad Spark Plugs
| Symptom | Why It Happens |
|---|---|
| Rough idle | Inconsistent combustion at low RPM |
| Misfires (P0300–P0308) | Worn electrode can't produce reliable spark |
| Poor fuel economy | Incomplete combustion wastes fuel |
| Slow acceleration | Delayed or weak spark response |
| Hard starting | Weak spark, especially in cold weather |
| Engine surge | Inconsistent combustion |
Replacement Intervals
| Plug Type | Lifespan |
|---|---|
| Copper | 20,000 – 30,000 miles |
| Platinum | 60,000 – 100,000 miles |
| Double Platinum | 80,000 – 100,000 miles |
| Iridium | 80,000 – 120,000 miles |
Visual Inspection
| Plug Condition | What It Means |
|---|---|
| Light tan/gray | Normal combustion |
| Black soot (dry) | Running rich |
| Wet black | Oil burning |
| White/blistered | Running too hot/lean |
| Worn electrode | End of life — replace |
ProHow Steer Helps
Steer tracks misfire codes and alerts you when patterns suggest worn spark plugs, helping you replace them before they cause catalytic converter damage.
Fix Any Engine Issue with Steer
Get instant plain-English explanations for any engine code on your iPhone. Steer stays connected 24/7 to monitor your car's health.
