This 2010 Fiesta came in with a slight misfire while driving. The first thing that happened was a full diagnostic scan, and the result was the one that confuses people most: all green, no faults found. Not a pending code, not a stored code, nothing at all.
That result sends a lot of owners away believing the car is fine, or worse, being told there is nothing wrong with it. There plainly was something wrong with it. The scanner simply had not seen enough of it to log a code, which is a completely normal state of affairs and worth understanding.
The answer was found the old fashioned way, by taking the spark plugs out and looking at them. One had a hairline crack in the white ceramic insulator, which was letting the spark escape instead of jumping the gap. This guide covers that diagnosis: why a clean scan proves less than people think, how to get the plugs out without causing damage, and what you are actually looking for when you have them in your hand.
A clean scan is not a clean bill of health
The engine management system logs a misfire code once the misfire rate crosses a threshold over a set number of engine cycles. A genuine but intermittent misfire that stays under that threshold produces exactly what this Fiesta produced: a car that feels wrong to drive and a scanner that reports nothing. Live data misfire counters will often show it long before a code is stored.
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Step-by-Step Diagnosis
Scan it anyway, and read more than the code list
Always start with the scan even when you suspect it will come back clean, because a code would save you an hour. On this Fiesta it came back with nothing at all. Do not stop there: look at live data for misfire counters per cylinder if your scanner offers them, and at fuel trims. A counter creeping up on one cylinder tells you where to look even when no code has been stored.
Clean around the plugs before anything comes out
Grit sits in the spark plug wells and will drop straight into the cylinder the moment a plug is removed. Blow the wells out with compressed air, or brush them, before you loosen anything. This costs a minute and prevents an expensive mistake that will not show itself until later.
Pull the leads off correctly, one at a time
This engine uses HT leads rather than coil-on-plug. Pull each lead by the boot and never by the cable, because pulling the cable separates it from its terminal and creates the misfire you were trying to cure. Tuck each one aside as you go, and if there is any chance of confusion, mark them, since crossed leads on a running engine is a bad afternoon.
Remove the plugs and keep them in cylinder order
Undo each plug with a proper plug socket, then lift it out with a magnet or grabber rather than tipping it out. Lay them out on the bench in the order they came from the engine. This matters more than it sounds: a fault on one plug tells you which cylinder to investigate further, and that information is lost the moment they get mixed up.
Read the white ceramic insulator carefully
Look at the pale ceramic around the centre electrode. Discolouration and an orange or rusty ring is common with age. What you are hunting for is a crack, and cracks are subtle. On the plug from this Fiesta it looked at first glance like a line of dirt, and it was not, it was a genuine split in the insulator. A cracked insulator gives the spark an easier path to earth than the gap, so the cylinder gets a weak spark or none at all, intermittently, which is exactly why no code appeared.
Read the tip as well, because it tells you about the engine
The firing end is a free diagnostic report. A light greyish brown deposit is healthy. Black and sooty means running rich or a lot of short journeys. Oily and wet suggests oil getting into the chamber past valve stem seals or rings. Glazed or blistered points at running too hot. Note anything odd, because a plug can be both the fault and a symptom of another one.
Check the new plugs before they go anywhere near the engine
Compare the part numbers against the old ones and against the specification for your engine, and check the new plugs look identical to each other. Inspect each tip for transit damage, since a plug dropped in a warehouse can have its electrode bent over. Modern plugs come pre-gapped and in most cases need no adjustment, so leave the gap alone unless the specification says otherwise.
Start every plug by hand
Drop each plug into its well and start the thread with your fingers or a socket with no ratchet on it. Aluminium cylinder heads strip easily and a cross-threaded plug is a repair that costs vastly more than the plugs did. If it does not turn easily by hand, back it off and start again rather than forcing it.
Torque them properly and refit the leads
Nip each plug down and then tighten to the manufacturer's torque figure with a torque wrench. Overtightening damages the thread and the plug; undertightening lets combustion gases past and can blow a plug out. Push each lead back on until it clicks home on the plug, and make sure each one is on the cylinder it came from.
Start it, then confirm rather than assume
Start the engine and let it settle. It should idle smoothly. Then take it out and drive the conditions that produced the misfire, because an idle test proves very little on a fault that appeared under load. If you have live data, watch the misfire counters stay at zero rather than trusting how it feels.
Why No Code Was Stored
Engine management systems do not log a misfire the instant one occurs. The ECU counts misfire events per cylinder over a rolling number of engine revolutions and compares that against a threshold, and only when the rate crosses that threshold does it store a code and light the engine light. This is deliberate, because a single stumble going over a pothole or a tank of poor fuel should not fill the memory with faults.
A cracked insulator often produces a misfire that comes and goes depending on temperature, humidity and load, because whether the spark jumps the crack or the gap depends on which path is easier at that moment. That kind of intermittent fault can sit under the threshold for a long time while being perfectly obvious from the driver's seat.
So a clean scan on a car that plainly misfires does not mean the scanner is broken or the driver is imagining it. It means the fault has not yet been frequent enough to qualify. Physical inspection is the correct next step, not a second opinion from a different scanner.
When It Is Not the Plugs
If the plugs come out looking healthy, the next places to look are the leads themselves, which break down internally and leak to earth in exactly the same way a cracked insulator does, and the coil. On an engine with HT leads, a lead can test fine cold and fail once hot and damp.
Beyond ignition, an intermittent misfire with no codes can come from an air leak that upsets the mixture at certain throttle openings, a lazy injector, or low fuel pressure showing itself only under load. The advantage of starting with the plugs is that they are cheap, they are due anyway on most cars that misfire, and they hand you a written report on the state of each cylinder while you are there.
If the misfire survives new plugs and leads, that is the point to get a scanner on it during a proper road test and watch the live data rather than continuing to replace parts.
How Often Should Spark Plugs Be Changed?
It depends entirely on the type. Standard copper plugs are typically a 20,000 to 30,000 mile item, while platinum and iridium plugs commonly run 40,000 to 60,000 miles or more, and some manufacturers specify longer. Always follow the figure for your engine rather than a general rule.
The practical point is that plugs are one of the cheapest parts on the car and one of the most informative. If a petrol engine is misfiring, running roughly or drinking more fuel than it used to, and nobody can tell you when the plugs were last done, taking them out is rarely wasted work even if they turn out to be fine.
Common Mistakes
- Believing a clean scan means no fault. A misfire under the ECU's threshold stores nothing at all.
- Removing plugs without cleaning the wells. Grit falls straight into the cylinder.
- Pulling HT leads by the cable. Pull the boot, or you create the next misfire yourself.
- Mixing the old plugs up. Keep them in cylinder order or you lose the diagnosis.
- Only glancing at the ceramic. A crack can easily look like a smear of dirt.
- Starting a plug with a ratchet. Aluminium heads strip, and that repair dwarfs the cost of plugs.
- Guessing the torque. Too tight damages the thread, too loose can blow a plug out.
- Judging the repair at idle. Drive the conditions that caused it before calling it fixed.
