Why is a high code easier to diagnose than a low code?

Because the list of possible faults is shorter. A low code can come from a harness shorted to ground, a shorted injector coil, or corrosion bridging two connector pins — three faults that look identical on a scan tool and have to be separated by measurement. A high code allows only two: the circuit is open, or something is feeding it voltage. A coil resistance check finds the first and a voltage check on the control wire with the ECM unplugged finds the second, so two measurements cover the whole space.

I replaced the injector and P0277 came back. What now?

That result narrows things usefully: it points at the harness or the connector rather than the component. Recheck continuity on the control wire from the injector connector back to the ECM with both ends disconnected, and check that same wire for unexpected voltage. Also verify the connector terminals still grip firmly — a spread female terminal can pass a static continuity test and lose contact under vibration and heat, which is exactly the condition a rear-bank connector ends up in.

Could a fuse be the cause instead of the injector?

Yes, and it takes two minutes to rule out. Injectors are fed switched battery voltage, usually grouped through a common fuse and relay, often one group per bank. If that feed is lost, the ECM's control-side measurement goes abnormal and can set high codes on several injectors at once. Check for battery voltage at the injector connector with the key on before condemning anything else — if it is missing, the fault is upstream and one repair fixes every code.

Is P0277 safe to drive with?

It is best avoided. Cylinder 6 is effectively dead, so the engine is down on power and pushing unburned fuel and air into the exhaust, which can overheat and destroy the catalytic converter on that bank. Drive gently and directly to a shop, and if the check engine light is flashing, pull over and have the vehicle towed.

OBD-II trouble code

P0277: Cylinder 6 Injector Circuit High

The cylinder 6 injector control circuit read higher than expected — an open circuit or a short to battery voltage. Because it narrows to only two possibilities, it is the faster half of the low/high pair to diagnose.

Medium severityPowertrainFuel InjectorsDo not drive

Quick facts

System: Powertrain
Category: Fuel Injectors
Severity: Medium severity
Drivable: No — stop driving until repaired
Repair cost range: $30 – $1,500
DIY difficulty: Intermediate DIY

What does P0277 mean?

P0277 is the mirror image of P0276 on the same circuit. The engine control module (ECM) fires the cylinder 6 injector by pulling its control wire to ground, and it watches that wire to confirm the voltage actually drops when a pulse is commanded. P0277 sets when the voltage stays high — the ECM tried to pull the circuit down and the circuit did not follow.

Only two conditions produce that. The first is an open circuit: a broken conductor, a disconnected or corroded connector, or an injector with a broken coil winding means the ECM's ground path connects to nothing, so the voltage never drops. The second is a short to battery voltage, where some other power source is feeding the control wire and holding it up against the module.

That two-item list is the reason a high code is usually quicker to resolve than its low counterpart. A low code has three plausible faults that all read alike on a scan tool — shorted harness, shorted injector coil, or a corroded connector bridging pins — and you have to separate them. A high code splits cleanly: measure the injector's coil resistance to test for an open winding, then check the control wire for unexpected voltage with the ECM disconnected. One of those two measurements almost always identifies the fault, and both can be done in the same session.

On cylinder 6 there is a practical wrinkle worth planning around. This cylinder usually sits at the rear of the engine — on the rear bank of a transverse V6, or well back on a bank of a V8 — where the harness runs across the hottest, least accessible part of the engine and past exhaust crossover pipes on some designs. Heat-hardened insulation that has cracked and let a conductor touch a powered terminal, and terminals that have corroded from cowl runoff, are both more common there than at the front of the engine. So before committing to the teardown needed to reach the injector, do the tests that can be performed from the ECM end.

With the circuit stuck high, cylinder 6 gets little or no fuel, so a misfire is essentially certain and P0306 commonly appears with it. Unburned fuel and air entering the exhaust can overheat and ruin the catalytic converter on that bank, so this is not a code to defer.

Common causes

Open circuit in the cylinder 6 injector control wire (broken conductor or corroded splice)
Open coil winding inside the cylinder 6 injector
Injector connector unplugged, unlatched, or with a spread or corroded terminal
Control wire shorted to battery voltage or another powered circuit through cracked, heat-hardened insulation
Loss of the switched power feed shared by that bank's injectors — a blown fuse or failed relay
Connector left unseated after intake manifold or valve cover service
Chafed harness contacting a powered stud or terminal at the rear of the engine
Rarely, a failed cylinder 6 injector driver inside the ECM/PCM

Symptoms

Check engine light on, sometimes flashing under load
Dead or very weak cylinder 6 with a rough, shaking idle
Misfire codes such as P0306 or P0300 stored alongside
Marked loss of power and hesitation
Raw fuel odor from the exhaust
Poor fuel economy
Hard starting or extended cranking

Diagnostic steps

1.Confirm P0277 and record companion codes. P0276 stored with it suggests an intermittent wiring fault rather than a clean open; high codes on other cylinders of the same bank point at a shared power feed.
2.Identify cylinder 6 for your specific engine before testing — bank and position vary between manufacturers.
3.Start at the ECM. With the ECM connector disconnected, check the cylinder 6 control wire for unexpected voltage. Voltage present there proves a short to power and tells you the repair is in the harness, not the injector.
4.In the same setup, check the control wire for continuity through to the injector connector. No continuity confirms an open in the harness.
5.Inspect and fully reseat the cylinder 6 injector connector once you have access. A connector left unlatched after prior service or a backed-out pin is a common and free fix.
6.Unplug the injector with the key off and measure its coil resistance. An infinite or very high reading confirms an open winding and condemns the injector.
7.With the key on and the injector unplugged, confirm battery voltage is present on the feed side of the connector. If it is missing, the fault is upstream — a fuse, a relay, or the feed wire.
8.Once wiring and injector both test correct, use a noid light or a scan tool injector test to verify the ECM is actually pulsing the circuit before considering the module.

Repair cost

$30 – $1,500

A blown fuse or failed injector relay is often under $100, and an unlatched connector costs nothing but time. Repairing a broken or shorted wire is typically $75-$250 when reachable and more when the intake manifold has to come off. A single fuel injector is $60-$400 in parts plus 0.5-3 hours of labor — cylinder 6 on a transverse V6 rear bank commonly falls at the top of that range and may include intake gaskets. Diesel injectors on an inline six run $400-$900 installed. A failed ECM injector driver is uncommon and runs $500-$1,500 including programming. Diagnosis alone is typically $75-$150.

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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.