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 where cylinder 4 sits at the far end of the injector harness, the end where wiring faults concentrate for a physical reason.
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
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.
There is one physical fact about cylinder 4 specifically that is worth using rather than reading past. The injector harness is fed from the module and runs along the head, branching to each injector in turn and terminating at the highest-numbered cylinder. Cylinder 4 is therefore at the end of the run on a four-cylinder engine: the furthest point from the module, the last splice in the chain, and — on the transverse installations that dominate this engine class — the corner closest to the bulkhead. Harness faults concentrate at terminated ends and at anchor points rather than in the middle of free spans, so the connector and the last few inches of loom at cylinder 4 deserve more scrutiny than the equivalent hardware at cylinder 1. Start there rather than working along the engine from the front.
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, at the terminated end of the harness run
- Broken conductor in the last section of injector loom, where it is anchored near the transmission end of the block
- Chafed wiring where the injector harness passes a bracket or crosses toward the bulkhead
- Open coil winding inside the cylinder 4 injector
- Corroded splice in the shared power feed starving the far end of the injector run
- Blown injector fuse affecting cylinder 4 alone or a group
- 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.Go to the last connector in the injector harness run first, and give it more attention than the others. The injector loom is fed from the module end and terminates at the highest-numbered cylinder, so cylinder 4's connector is at the end of the run: the furthest from the module, the last splice in the chain, and on a transverse installation the closest to the bulkhead. That is not a coincidence in the fault statistics, it is a physical consequence of where the wire ends.
- 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 far more common at the terminated end of a harness 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 using a noid light or a scan tool's injector output test. 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 can starve the far end of the run specifically, which is exactly where cylinder 4 sits on a four-cylinder engine.
- 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. Very few codes in this family allow a swap test this cheaply.
- 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.
Estimate your repair
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Open the Repair Cost Estimator with fuel injector 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.