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DTC codes / P03xx · Ignition system and misfire / P03C4

P03C4Cylinder 5 Pressure Variation Low

P03C4 means the engine computer has detected that cylinder 5 is producing less pressure variation during the combustion cycle than the other cylinders, based on crankshaft speed fluctuations. The powertrain control module (PCM) uses the crankshaft position sensor signal to calculate each cylinder's contribution to engine speed and flags this code when cylinder 5 consistently underperforms.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaIgnition system and misfire

Symptoms you may notice

  • Illumination of the check engine light, often with no other symptoms noticed by the driver
  • A noticeable rough idle or shaking that may be more pronounced when the engine is cold
  • Slight hesitation or stumble during acceleration, particularly under light throttle
  • Reduced fuel economy as the engine compensates for the weak cylinder
  • In some cases, a flashing check engine light if the misfire becomes severe enough to damage the catalytic converter

Common causes, most likely first

  1. Worn or fouled spark plug in cylinder 5
  2. Faulty ignition coil or spark plug wire for cylinder 5
  3. Clogged or leaking fuel injector for cylinder 5
  4. Low compression in cylinder 5 due to worn piston rings, valves, or head gasket
  5. Vacuum leak affecting the intake runner for cylinder 5
  6. Mechanical valvetrain issue such as a worn cam lobe or collapsed lifter on cylinder 5
  7. Carbon buildup on intake valves or piston top in cylinder 5

How to diagnose and fix it

  1. Inspect and replace the spark plug in cylinder 5, checking for wear, fouling, or incorrect gap
  2. Swap the ignition coil from cylinder 5 with a known-good cylinder and see if the code follows the coil
  3. Perform a fuel injector balance test or swap injectors to see if the misfire moves to another cylinder
  4. Run a compression test on cylinder 5 and compare with other cylinders to rule out mechanical issues
  5. Perform a cylinder leak-down test if compression is low to pinpoint valve or ring leakage
  6. Check for vacuum leaks at the intake manifold gasket and cylinder 5 intake runner
  7. Use a scan tool to monitor misfire counters and fuel trims to confirm the repair before clearing the code

Cylinder 5 pressure variation pitfalls

  • A P03C4 can be set by a perfectly running engine if the crankshaft position sensor signal is erratic due to a damaged reluctor wheel or sensor wiring, so always check sensor integrity before condemning engine internals.
  • Many technicians replace the ignition coil first, but a swapped coil that does not move the misfire points to fuel or compression rather than spark, saving unnecessary parts.
  • Carbon buildup on direct-injection engines can cause low pressure variation on a single cylinder without triggering a traditional misfire code, so a P03C4 may be the only clue.
  • A failing intake manifold gasket can leak vacuum only to cylinder 5, causing a lean condition that mimics a bad injector; a smoke test is often the quickest way to find it.
  • The code may appear after a recent repair if a connector to the cylinder 5 injector or coil is not fully seated, so always recheck your work before deeper diagnosis.

Can I keep driving with P03C4?

You can usually drive the vehicle with a P03C4 code, but avoid heavy acceleration or towing until it is repaired because a persistent misfire can overheat and damage the catalytic converter. If the check engine light starts flashing, pull over safely and arrange a tow to prevent costly damage.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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