OBD-II trouble code
P0203: Injector Circuit/Open - Cylinder 3
An electrical fault on the cylinder 3 injector circuit. Most of the wasted effort on this code comes from tests that answer a narrower question than people think they are asking — a flashing noid light and a good ohm reading can both coexist with an injector that never opens.
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 P0203 mean?
The module commanded the number three injector and its own circuit monitoring reported that the command did not produce the electrical result it expected. The label says open, and on most vehicles this code covers electrical faults broadly rather than pinpointing a direction.
Start with the one piece of good news specific to cylinder 3, because it removes a step that costs people real money elsewhere in this family. Two cylinder numbering conventions are in wide use on V engines: some manufacturers count down one bank and then the other, and some alternate from side to side. Those two conventions disagree about where several cylinders live. They do not disagree about cylinder 3. Counting straight down puts it third on the first bank; alternating puts it second on the first bank. Either way, on a V6 or a V8, cylinder 3 is on the same bank as cylinder 1. You still want the numbering diagram to know its exact position along that head, but you do not need it to know which side of the engine to open up, and that is not true of every cylinder in this code family.
The substance of this page is about test tools, because injector circuits are one of the few places where three common tests give three different answers and people treat them as interchangeable.
A resistance measurement asks whether the winding is continuous. It is a static question, taken with the circuit dead, and it will fail an injector whose coil has broken clean through. It will not fail a winding with an internal high-resistance fault that still measures inside a fairly generous specification window, and it says nothing at all about whether the module is commanding anything.
A noid light asks whether the driver is switching. Plug it into the injector connector, crank or run the engine, and it flashes if the module is pulling the circuit to ground. That is genuinely useful for proving the module is alive. What it does not prove is that the circuit can do work. A noid light draws very little current, and a circuit crippled by a corroded splice or a partially broken conductor can light it happily while being unable to pull enough current through a real injector coil to open the valve. A flashing noid light on a circuit that still will not run the cylinder is the classic false clearance on this code.
A current ramp asks the question that actually matters. Clamp a low-current probe around the injector lead with the engine running and watch the shape of the current rise. A healthy injector shows a characteristic rise with a small step in it where the pintle moves — the mechanical event is visible in the electrical trace. A circuit with added resistance shows a current that never reaches the proper peak. A coil that is failing shows an abnormal shape even when its resistance measures acceptably. This is the test that separates a circuit that conducts from a circuit that works, and it is worth borrowing or hiring a clamp for rather than guessing.
A fourth check is mechanical and free: a stethoscope or a long screwdriver against the injector body while the engine idles. A ticking injector is opening. A silent one is not, whatever the electrical tests said. That is not a precise instrument, but as a cross-check against an electrical result that looks fine it takes thirty seconds.
Use them in that order and each one narrows the field honestly. Run only the first two, and it is easy to sign off a circuit that is still starving cylinder 3 of fuel.
Common causes
- Broken or chafed conductor in the cylinder 3 injector control circuit
- Corroded splice or joint adding enough resistance to prevent working current while still passing a light load
- Backed-out or spread terminal in the injector connector
- Injector coil with an internal fault that measures within specification but no longer opens the valve reliably
- Injector coil that has failed fully open
- Poor ground or a degraded supply feed to that injector
- Blown injector fuse, affecting cylinder 3 alone or with others depending on the vehicle
- Failed module injector driver, which is uncommon and should only be concluded after the circuit is proved
Symptoms
- Check engine light, flashing if cylinder 3 is not firing at all
- Rough idle with a distinct shake at a standstill
- Reduced power and hesitation under acceleration
- A cylinder 3 misfire code alongside it in most cases
- In some cases no misfire code at all, if misfire monitoring was disabled by conditions when the fault occurred
- Fuel consumption up and sometimes a smell of unburnt fuel
- Hard starting on some engines
- Cylinder 3 injector silent to a stethoscope while the others tick evenly
Diagnostic steps
- 1.Note that on a V6 or V8, cylinder 3 sits on the same bank as cylinder 1 under both common numbering conventions, so you can identify the correct side before consulting a diagram for its exact position.
- 2.Inspect the cylinder 3 injector connector and lead for chafing, corrosion, backed-out pins and previous repairs.
- 3.Listen with a stethoscope at each injector while the engine idles. A silent cylinder 3 confirms the valve is not opening, whatever the electrical readings say.
- 4.Measure coil resistance and compare it against specification and against the other injectors, but treat a pass as one narrow question answered rather than as clearance.
- 5.Use a noid light to confirm the module is switching the circuit. Remember it proves the driver is working, not that the circuit can carry working current.
- 6.Where the noid light flashes but the cylinder still does not fire, use a low-current clamp to ramp the injector. A current peak that never reaches specification points at resistance in the circuit.
- 7.Read the shape of the current trace, not only its height. An abnormal shape with a normal resistance reading points at the injector rather than the wiring.
- 8.Voltage drop test the supply and ground for that injector under load rather than checking for their presence.
- 9.Consider the module driver only after the circuit has been shown to conduct working current and the injector has been shown to be sound.
Repair cost
$50 – $1,500
A wiring, splice or connector repair is $50 to $250. A single injector is $60 to $400 in parts with 0.5 to 3 hours of labour depending on access. The cost pattern specific to this code is testing rather than parts: shops that own a low-current clamp reach the answer in under an hour, and those relying on a noid light and an ohmmeter can spend two or three hours and still replace the wrong item. If you are paying for diagnosis, it is fair to ask whether the circuit will be current-ramped. A failed module injector driver is $500 to $1,500 with programming and is uncommon.
Estimate your repair
Run the numbers for your vehicle
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.