OBD-II trouble code
P0355: Ignition Coil 'E' Primary/Secondary Circuit
The fifth coil in the manufacturer's mapping. 'E' is the first letter in this family that cannot come from a four-cylinder engine, and on the engines that can produce it the coil harness has usually split into branches — which is where the fault often is.
Quick facts
- System
- Powertrain
- Category
- Ignition
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $45 – $500
- DIY difficulty
- Intermediate DIY
What does P0355 mean?
This is the letter where the family crosses a threshold. Coils 'A' through 'D' can all come from the four-cylinder engine that dominates the road. 'E' cannot. A module has to be driving at least five ignition outputs independently to name a fifth one, so the code itself narrows the vehicle down before you have looked at anything: an inline five, a V6, a straight six, or something larger. That inference is worth making explicitly, because it is the first thing to check when the code appears on a car you would not expect to carry five coils.
On the engines that genuinely produce it, the important structural change is not the extra cylinder. It is the harness. A four-cylinder ignition harness is usually one branch running the length of one head, with four connectors hanging off it. As soon as an engine has two cylinder heads, the coil wiring almost always splits: a common trunk leaves the module, then divides at a bulkhead connector or an in-loom splice into per-bank branches, each with its own routing, its own retaining clips and its own path across the engine. On a V6 the fifth coil is normally on the second bank, which means the signal for this code has travelled through that split before it ever reaches the coil.
That changes where the cheap tests are. On a four-cylinder, testing at the coil connector is testing nearly the whole circuit. On a bank-split harness it is not, because everything between the module and the branch point is still unexamined and is exactly the part that gets disturbed — the trunk usually runs under or behind the intake manifold, across the valley on a V engine, or along the bulkhead where other work happens. Backprobing at the branch connector rather than at the coil splits the circuit in half in one move and tells you which side of the split to spend time on. On a fault that has already survived one coil replacement, that is usually the test that ends the job.
The inline five deserves a separate note because it behaves unlike the rest. It is an odd-cylinder engine and it is deliberately unbalanced — the firing intervals are uneven by design, which is where its characteristic sound comes from. That matters diagnostically because an inline five idles with more inherent shake than a four or a six, so a mild misfire hides in the engine's own character for far longer before anyone notices it. On those engines the stored code will often be the first evidence, arriving before the driver reports anything at all, and freeze-frame data is more useful than the driver's description.
Finally, five or more coils gives you something a four-cylinder does not: a genuinely large set of identical, identically-aged parts. That makes the swap test more informative, but it also sets a limit on it. Swapping proves that a coil is dead. It does not prove the others are healthy, because they have all done the same miles under the same heat, and a coil that is merely marginal will pass a swap test on the day and fail six weeks later on a different cylinder.
Common causes
- Failed ignition coil on the fifth output, most often on the second bank of a V6
- Break, corrosion or high resistance at the splice or bulkhead connector where the coil harness divides into per-bank branches
- Chafed section of the coil harness trunk where it passes under the intake manifold or across the engine valley
- Connector left partially seated after intake or plenum work that required the branch to be unplugged
- Worn spark plug with an opened gap raising the voltage demand on the coil
- Oil in the plug well from a leaking valve cover gasket, degrading the coil boot
- Failed coil driver inside the engine control module
- Code reported on an engine with fewer than five coils, indicating a misread or manufacturer-specific definition
Symptoms
- Check engine light on, occasionally flashing under load
- Misfire that is easier to see in live data than to feel, particularly on an inline five
- Rough running under load with normal-seeming idle
- Hesitation and reduced power on acceleration
- Fault appearing shortly after intake manifold or plenum work
- Increased fuel consumption and an unburnt fuel smell from the exhaust
Diagnostic steps
- 1.Confirm the engine has at least five independently driven coils. If it does not, the code has been matched to the wrong definition list and no part should be ordered until the manufacturer's own meaning is established.
- 2.Read all stored codes and use any cylinder-specific misfire to identify the physical cylinder, rather than inferring it from the letter.
- 3.Find the coil harness branch point on this engine — a bulkhead connector, an in-loom splice, or a junction at the rear of the intake. On a multi-bank engine this is the single most informative place to test.
- 4.Backprobe the trigger and supply at that branch connector with the engine running. A clean signal there with a fault at the coil isolates the branch; a fault at both puts the problem upstream, toward the module.
- 5.Inspect the harness trunk where it passes under the intake manifold or across the valley, which is where heat and chafe concentrate and where recent work would have disturbed it.
- 6.Remove the coil and check the plug well for oil or moisture. A leaking valve cover seal will destroy a replacement coil as reliably as it destroyed the first.
- 7.Swap the suspect coil with one from another cylinder and confirm whether the fault follows it. Remember this proves a coil is dead, not that the remaining coils are healthy.
- 8.If the fault has already survived one coil replacement, treat that as evidence and go to the harness split rather than fitting a second coil.
Repair cost
$45 – $500
A single coil is typically $45 to $200 in parts. Labour is higher than on a four-cylinder if the fifth coil sits on the rear bank of a transverse V6, where reaching it can mean removing an upper intake plenum — that alone can add one to two hours. A harness branch repair rather than a coil is usually $90 to $300 depending on how much of the loom has to be opened up. If a valve cover gasket is leaking into the plug well, add $150 to $400.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with ignition coil replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.