How Many Spark Plugs Need to Be Replaced at Once?
4 Answers
There is one spark plug per cylinder, so a 3-cylinder car requires replacing 3 spark plugs at once, a 4-cylinder car requires replacing 4, and a 6-cylinder car requires replacing 6. This is because the discharge gap and resistance values differ between old and new spark plugs. Failing to replace all of them may lead to issues such as engine shaking, difficulty accelerating, and increased fuel consumption. Steps for replacing spark plugs: Open the hood, lift the engine's plastic cover, and remove the high-voltage distributor wires. Mark each cylinder's position to avoid confusion. Use a spark plug socket to remove the spark plugs one by one. While removing them, check for external debris like leaves or dust and ensure everything is cleaned thoroughly. Place the new spark plugs into the spark plug holes, hand-tighten them a few turns, and then use the socket to tighten them further. Reinstall the high-voltage distributor wires in the correct firing order and secure the cover back in place.
When replacing spark plugs, it's necessary to change all of them at once. For example, if your car has a four-cylinder engine, you should replace all four spark plugs simultaneously. The main reason for this is to ensure consistent ignition performance across all cylinders. Otherwise, old spark plugs with lower ignition efficiency paired with new, more powerful ones can cause the engine to shake uncontrollably, like it's dancing. Last time, I tried to save effort by only replacing two, and not only did the acceleration become sluggish, but fuel consumption also increased by nearly one liter. The mechanic said that mixing old and new spark plugs can easily overload the ignition coils, and repairing that costs quite a bit. It's recommended to check the electrode gap every 50,000 to 60,000 kilometers and don't hesitate to replace them when needed.
Although you can replace just the faulty spark plug, I recommend replacing all four for the most reliable operation. Spark plugs operate in high-temperature and high-pressure conditions, and electrode wear varies over time. If you only replace one, the others could fail at any moment. I once experienced sudden cylinder misfire on the highway due to an old spark plug failing. After replacing all of them, the engine ran noticeably smoother, and throttle response improved significantly. The cost of materials isn't much—it's the repeated labor charges for disassembly and reassembly that add up. Nowadays, car maintenance apps often offer service packages; a set of four platinum spark plugs with installation costs around 400 yuan, which is far more economical than calling a tow truck after a breakdown.
Just check the maintenance manual and you'll understand. The manufacturer requires replacing all spark plugs in the same batch. Think about it - the ignition timing difference between cylinders must be controlled at the millisecond level. Old and new spark plugs have different ignition delays, and the ECU can't compensate fast enough. This leads to incomplete combustion, smelly exhaust, and easily triggered fault lights. Last time I replaced all of them at 30,000 km as per the manual. The mechanic showed me the old plugs with an endoscope - the center electrodes were completely worn out. After the full replacement, cold starts became instant - the engine fires up immediately when turning the key, whereas before it would struggle for two seconds.
