Bottom line recommendation
Use Iridum spark plugs gapped for the Scion tC. Most are pre-gapped, but it’s always a best practice to verify using your own gapping gauges. The most simple process is going to the local parts counter (O’Reillys or Autozone) and just pay retail from a known good source. There are tons of counterfeit spark plugs in circulation. Torque to 14ft lbs, Anti-seize not needed – source.
Top choices for Iridium spark plugs (non-turbo, non-supercharged)
- Denso 3297 SK20R11 Spark Plugs, Pack of 4
- NGK 4589 Laser Iridium Pack of 4
- NGK 3764 Iridium spark plugs Pack of 4
- Bosch 9603 Double Iridium spark plugs Pack of 4
Tools for spark plug service
Spark plug gauge / gap tools
Symptoms of Bad Spark Plugs
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Engine Misfires or Rough Idle
Worn or fouled spark plugs can’t ignite the air-fuel mixture consistently. This causes misfires, shaking at idle, and an engine that feels rough or uneven—especially when stopped. -
Hard Starting or No-Start Condition
Weak spark plugs make it harder for the engine to fire, particularly in cold weather. In severe cases, the engine may crank but fail to start at all. -
Poor Acceleration and Loss of Power
When spark plugs don’t fire properly under load, the engine may hesitate, stumble, or feel sluggish during acceleration. You might notice reduced power when merging or climbing hills. -
Decreased Fuel Economy
Incomplete combustion caused by bad spark plugs wastes fuel. If your MPG suddenly drops without other obvious issues, worn plugs may be the culprit. -
Check Engine Light (Misfire Codes)
Failing spark plugs often trigger misfire-related trouble codes (such as P0300–P030X). Even intermittent misfires can illuminate the check engine light. -
Engine Knocking or Surging
Uneven or delayed combustion can cause knocking, pinging, or surging while driving. This can feel like the car is briefly losing and regaining power.
Don’t over think it
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NGK IFR6A11 – NOW KNOWN AS THE IFR6T11 Spark Plug – 4589
Spark plug replacement interval
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Every 120,000 miles
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Toyota recommends inspection at 60,000 mi for signs of fouling or electrode wear, replace as needed
Additional details
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These plugs are designed to last up to 120k mi under normal driving conditions.
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Torque spec: 14 ft-lb (18 N·m) on aluminum heads anti-seize not recommended
Why no Anti Seize on threads?
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NGK states that modern spark plugs have a factory thread coating that acts as a release agent, and anti-seize isn’t needed because of that coating. Using anti-seize may lead to overtightening since the torque spec assumes dry threads.
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Denso’s published installation guide specifically warns not to apply thread lubricant, because it can cause the plug to be over-tightened which can lead to thread or plug damage.
