OBD-II trouble code
P0204: Injector Circuit/Open - Cylinder 4
Statistically the engine in front of you is a four-cylinder, which makes this the one member of the family where you do not need a numbering diagram — and the one where you have three known-good injectors sitting in the same row to measure the suspect one against.
Quick facts
- System
- Powertrain
- Category
- Fuel Injectors
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $50 – $1,500
- DIY difficulty
- Intermediate DIY
Browse every code in P0200–P0299, or start from the full code library.
What does P0204 mean?
P0204 means the module detected an electrical problem in the fuel injector control circuit for cylinder 4. Injectors are fed battery voltage on one side while the module switches the ground on the other for a precise number of milliseconds; the module watches that circuit as it switches and stores a code when the voltage or current it sees does not match what it commanded. "Circuit/Open" describes an electrical condition rather than naming a part, and the wiring is a more common cause than the injector itself.
What makes this particular member of the family worth reading on its own is population. Cylinder 4 is the highest-numbered cylinder that exists on essentially every engine capable of setting an injector code — four-cylinder engines make up the great majority of vehicles on the road, and every V6 and V8 has a cylinder 4 as well. In practice that means the engine in front of you is most likely an inline four, and that changes the job in three useful ways.
First, you do not need a numbering diagram. On an inline engine the cylinders are numbered in a row from the accessory-drive end, so cylinder 4 on a four-cylinder is the last one, next to the transmission. There is no bank to identify and no manufacturer convention to look up. That is not true for the higher-numbered codes in this block, where working on the wrong injector because of a numbering assumption is a standard and expensive mistake.
Second, all four injectors sit in one accessible row along the top of the cylinder head, with no intake plenum to lift and no rear bank hiding under the cowl. The same port injector that takes three hours to reach on a transverse V6 takes half an hour here. That is why the cost range for this code, while wide on paper, lands at the bottom of it most of the time.
Third — and this is the part that actually helps you find the fault — you have three known-good references built into the engine. Measuring the cylinder 4 injector's coil resistance is only half a test; measuring all four and comparing is a whole one, because three siblings of the same age at the same temperature are a better standard than a printed specification. On a four-cylinder with port injection you can go further and physically swap cylinder 4's injector with cylinder 1's. If the code follows the part, it is the part. If it stays where it was, it is the circuit. Very few codes allow a conclusion that definite for so little work.
One assumption about cylinder 4 is worth dismantling rather than repeating, because it sounds right and sends people to the wrong place. It is tempting to reason that cylinder 4 must sit at the end of the injector harness and therefore carry the most fault-prone connector. That does not hold up. Injector sub-harnesses are not routed to a common standard: the loom may enter from either end of the head or break out from a point in the middle, and the branch order does not have to follow cylinder order, so there is no general rule about which connector is last. Nor is cylinder 4 the corner nearest the bulkhead on a transverse engine — the cylinders run side to side across the bay, which puts all four at roughly the same distance from it. What is true, and useful, is that cylinder 4 is the end of the row closest to the transmission, and on a transverse installation that is the cramped end where mounts and brackets crowd the loom. So follow the harness on the engine in front of you and concentrate on where it is clamped, where it bends with engine movement, and where it runs near the exhaust side of the head. Those are where insulation hardens and conductors fatigue, whichever cylinder they happen to serve.
On consequences: an injector that is never pulsed leaves its cylinder without fuel, so this code very often arrives with a cylinder 4 misfire code. Together those two are one fault seen from two directions, and the pair points at the injector or its wiring rather than at the ignition side. A dead cylinder is worth attending to promptly, because unburnt fuel and air passing into the exhaust can overheat the catalytic converter, which is a far more expensive part than anything in this repair. The engine will usually run and drive, but treat it as a drive-it-to-the-shop situation, avoid heavy throttle, and stop if the check engine light is flashing — a flashing light means the misfire is severe enough to be causing damage now.
Common causes
- Corroded or backed-out terminal at the cylinder 4 injector connector
- Broken conductor in the injector loom where it is clamped or anchored near the transmission end of the block
- Chafed wiring where the injector harness passes a bracket or a mount at the crowded transmission end of a transverse engine
- Open coil winding inside the cylinder 4 injector
- Corroded splice in the shared power feed, which starves every injector downstream of it
- Blown injector fuse or open supply feed — quick to check, though a fuse normally feeds several injectors, so P0204 standing alone makes it unlikely
- Heat-hardened insulation on the loom section nearest the exhaust side of a transverse engine
- Failed cylinder 4 injector driver inside the module, which is uncommon
Symptoms
- Check engine light, flashing if cylinder 4 has gone completely dead
- Rough idle with a noticeable shake felt through the vehicle
- Loss of power and hesitation under acceleration
- Companion cylinder 4 misfire code stored alongside
- Smell of unburnt fuel from the exhaust
- Increased fuel consumption
- Hard starting, particularly when cold
- Fault appearing or clearing when the injector harness is disturbed
Diagnostic steps
- 1.Count the cylinders before anything else, because it decides whether you need service data at all. On an inline four — which is the most likely engine to set this code by a wide margin — cylinder 4 is the last one in the row, counted from the accessory-drive end of the block, and there is no bank question and no numbering convention to look up. On a V6 or V8, cylinder 4 may well be on the second bank, and then you do need the diagram before you touch anything.
- 2.Trace the injector loom on the engine in front of you rather than assuming where it ends. Branch order and feed point are not standardised — a sub-harness may enter from either end of the head or break out from a point in the middle, and the branch order does not have to follow cylinder order — so following it is the only reliable way to know which connector sits where. What is worth targeting whatever the order turns out to be is where the loom is clamped, where it bends with engine movement, and where it runs near the exhaust side of the head.
- 3.Wiggle-test the cylinder 4 connector and the last section of loom while watching live data or with the code monitor active. A fault that appears and disappears under hand pressure is a terminal that has lost tension or a conductor broken inside its insulation, and both are more common at anchor points, bends and terminated ends than in the middle of a free span.
- 4.Measure the cylinder 4 injector's coil resistance with the key off, then measure the other three and compare. On a four-cylinder engine you have three identical known-good references sitting in the same row at the same temperature, which is a better standard than a book figure — an injector reading open or well away from its siblings is condemned without needing a specification.
- 5.Confirm the module is actually pulsing the circuit. A noid light does this directly on a port-injected engine with low-voltage saturated injectors; on a direct-injection engine do not use one, because those injectors are driven at 60 volts and above from a boost capacitor — that is outside a noid light's range and a shock hazard at the connector, so use the scan tool's injector output test instead. Either way this separates a module or harness problem from an injector problem in one step: a pulse present at the connector with a dead injector means the injector, no pulse at the connector with good wiring means the drive side.
- 6.Check the shared power feed and the fuse before concluding anything about the module's driver. Injectors are fed from a common supply, so a feed problem usually takes more than one cylinder with it — but a corroded splice starves everything downstream of it, so when P0204 stands alone, trace the feed from whatever splice you can actually find rather than assuming cylinder 4 is the far end.
- 7.Swap the cylinder 4 injector with cylinder 1 if the resistance reading is borderline rather than clearly open. On an inline four with port injection this is a straightforward exchange, and if the code follows the injector to cylinder 1 you have proven the part; if it stays on cylinder 4 you have proven the circuit. Relieve the fuel rail pressure before unseating either injector, because the rail holds pressure long after shutdown, and keep this test to port injection — a direct-injection injector needs new seals and often a correction code after fitting, which makes it a replace item rather than a swap item.
- 8.Verify with a road test after clearing, and expect the companion misfire code to tell you whether fuelling was actually restored. A cylinder 4 misfire code should not return. If the injector-circuit code stays away but the misfire comes back, the electrical fault is fixed and there is a second, mechanical or ignition problem on that cylinder.
Repair cost
$50 – $1,500
This is the cheapest member of the injector-circuit family to fix in practice, because the engine is most likely an inline four with all injectors accessible along the top of the head. Diagnosis is $80 to $150. Repairing a connector terminal or a broken conductor at the end of the harness run is $50 to $200 and is the most common outcome. A port injector is $60 to $250 in parts plus half an hour to an hour of labour on a four-cylinder; a direct-injection unit is $150 to $400 plus one to two hours. If cylinder 4 turns out to be on the rear bank of a transverse V6, add two hours or more for plenum removal. A failed module injector driver is uncommon and runs $500 to $1,500 with programming.
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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.