OBD-II trouble code
P0270: Cylinder 4 Injector Circuit Low
The cylinder 4 injector control circuit is being pulled toward ground when it should be floating. Cylinder 4 is the one cylinder position that exists on essentially every modern engine, and on the four-cylinder majority it sits at the far end of both the engine and the injector harness — which shapes where this fault usually is.
Quick facts
- System
- Powertrain
- Category
- Fuel Injectors
- Severity
- Medium severity
- Drivable
- No — stop driving until repaired
- 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 P0270 mean?
Injectors are switched on their ground side. Battery voltage waits at one terminal, and the module fires the injector by pulling the other terminal to ground for a measured pulse. Between pulses that control wire should sit near battery voltage. This code stores when the module finds the cylinder 4 control line already low — grounded when it should be floating — or, on platforms that measure it, drawing current outside the expected window.
Two facts about cylinder 4 specifically are worth having before you start.
The first is population. Cylinder 4 is present on every engine from a four upward, which makes this the highest-volume code in the per-cylinder injector block and the one most likely to appear on an ordinary four-cylinder family car rather than on a V6 or V8. That matters because the majority of vehicles that set this code are inline fours, and on an inline four cylinder 4 is the rearmost cylinder — nearest the bulkhead. That corner of the engine bay is the wettest and the hottest place a connector can live. Water shed from the windscreen runs into the cowl and, when the cowl drains block with leaf litter, spills down the back of the block. The rear connector is also the one that gets flexed and dragged past on every job that involves reaching over the engine. Corrosion and chafe at that specific connector are not a general possibility here; they are the leading cause.
The second is harness geometry. The injector loom leaves the module and runs along the engine, dropping off at each injector in turn, so cylinder 4 on an inline four is the last stop — the longest conductor run of the four. Every chafe point, every bracket edge and every heat-soaked section that affects cylinders 1 through 3 is also on cylinder 4's route, plus the extra length beyond them. When a loom is damaged, cylinder 4 is statistically the most likely single circuit to be caught.
That geometry gives you a fast triage question. Many modules drive injectors in paired groups, and on a great many inline fours the pairing follows the firing order so that cylinders 1 and 4 share a driver group. If both cylinder 1 and cylinder 4 circuits are reporting faults, stop looking at the individual injectors and look at the shared driver and the shared feed. If only cylinder 4 is complaining, the fault is downstream of the split and belongs to that one branch. Check the pairing in the service information for the specific engine before leaning on this, because it is a design choice rather than a rule.
The practical consequence is a misfire on that cylinder, usually with a companion misfire code, and unburnt fuel reaching the catalytic converter. That is the reason not to defer this one: a chafed wire is cheap and a converter is not.
One last caution. Cylinder 4 means the manufacturer's cylinder 4. On an inline four that is reliably the rearmost cylinder, but on a V6 or V8 it may sit on the bank you would not guess. Confirm the numbering before you test anything, because the whole rear-of-the-engine argument above only holds once you know which cylinder you are actually standing in front of.
Common causes
- Control wire chafed through to ground along the long rear section of the injector loom
- Corrosion inside the cylinder 4 injector connector, fed by cowl water running down the back of the engine when the cowl drains are blocked
- Shorted coil winding inside the cylinder 4 injector
- Insulation heat-damaged where the rear of the loom passes close to the exhaust manifold or turbocharger
- Wire pinched during earlier work that required reaching over the back of the engine
- Spread, backed-out or water-filled terminal in the injector connector
- Loom damage shared with cylinder 1 where the two circuits are driven as a pair, which points at the group rather than the injector
- Failed injector driver inside the module, which is uncommon and should be the last conclusion
Symptoms
- Check engine light, sometimes flashing under load
- Rough idle with a shake felt strongly through the bulkhead on an inline four
- Cylinder 4 misfire code stored alongside this one
- Loss of power and hesitation when accelerating
- Smell of unburnt fuel from the exhaust
- Fuel consumption noticeably up
- Hard starting on some engines
- Fault appearing after heavy rain or after leaves have collected at the base of the windscreen
Diagnostic steps
- 1.Confirm which cylinder is cylinder 4 on this specific engine before testing. On an inline four it is the rearmost; on a V engine it may be on the opposite bank from the obvious guess.
- 2.Check whether the cylinder 1 injector circuit is also reporting a fault. Where the module drives cylinders 1 and 4 as a pair, both complaining points at the shared driver group rather than at either injector.
- 3.Inspect the cylinder 4 connector before anything else. On an inline four it sits at the bulkhead end, the wettest and hottest connector position in the engine bay.
- 4.Check the cowl drains at the base of the windscreen while you are there. Blocked drains put water down the back of the engine and are a genuine root cause of corrosion at this connector.
- 5.With the key off, unplug the injector and measure its coil resistance against specification and against the other injectors. A value well below the others confirms a shorted coil.
- 6.Disconnect both the injector and the module connector, then test the control wire for continuity to ground. Continuity there proves a harness short and no new injector will fix it.
- 7.Wiggle-test the loom along its full run to the rear of the engine, which on an inline four is the longest of the four injector branches and passes every chafe point the others do.
- 8.Inspect for heat damage where the rear of the loom passes the exhaust manifold or turbocharger.
- 9.With the wiring and injector proven good, use a noid light or the scan tool's injector test to confirm the module is pulsing the circuit correctly.
- 10.Only after all of that should the module's injector driver be suspected, and then confirm it against the manufacturer's pinout test before replacing anything.
Repair cost
$50 – $1,500
A chafed wire or corroded connector repair is $75 to $250 and is the most common outcome, though labour runs to the upper end where the connector is buried at the bulkhead. Clearing blocked cowl drains costs nothing and prevents the repeat. A single injector is $60 to $400 in parts plus half an hour to three hours of labour, with a rear-of-engine or under-manifold injector at the long end. A failed injector driver in the module is uncommon at $500 to $1,500 including programming. Diagnosis alone is $75 to $150.
Estimate your repair
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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.