P0271: Cylinder 4 Injector Circuit High

Cylinder 4 exists on almost every engine ever fitted to a road vehicle — fours, fives, sixes, eights and everything larger. That makes this the injector circuit code with the widest population of any, and most of the vehicles that set it are four-cylinder engines, simply because there are more of them. It is worth starting from that, because a four-cylinder engine has a specific geography and a specific injection architecture that shape the diagnosis in ways that do not apply on a larger engine.

The electrical claim itself is narrow. The module fires an injector by pulling its control wire down to ground, and it watches that wire to confirm the voltage actually falls. When the voltage stays up, the current the module asked for never flowed, which leaves two possibilities: the path is broken somewhere — a severed wire, a disconnected or corroded connector, an open coil winding inside the injector — or something else is feeding the wire with power and holding it up. That is a short list compared with the tangle of causes behind a circuit-low code, and a continuity check across the injector plus a voltage check on the control wire with the module unplugged usually resolves it inside half an hour.

The first thing to establish on a four-cylinder is whether the injectors are fired sequentially or in pairs, and this is the question most likely to save a wasted afternoon. A good number of four-cylinder engines, particularly older ones and some current economy applications, use batched injection: cylinders 1 and 4 are companion cylinders in the firing order, they reach top dead centre together, and their injectors are driven from a single module output. On such an engine a control-circuit fault does not stay tidily on cylinder 4. It can take cylinder 1 down with it, and the wire you are testing may be shared rather than dedicated. Seeing a companion misfire on cylinder 1 alongside this code is a strong hint that you are looking at a paired driver rather than two independent failures.

Geography matters too. On an inline four, cylinder 4 sits at the end of the block, which puts its injector at the far end of the harness branch — the longest run, the most clamps, and on a transverse installation the corner nearest the bulkhead. That corner is the hottest part of the engine bay, gets the least airflow at idle, and is the least accessible for inspection. Harness insulation ages fastest where it is hottest, and connectors that live against the bulkhead see the worst heat cycling, so the end cylinder collects more than its share of open-circuit faults on high-mileage engines. It is also the connector most likely to have been left unlatched after previous work, because it is the one the technician could not see properly.

A four-cylinder gives you less to cross-check with than a larger engine, and it is worth knowing that in advance. There is one bank, one set of fuel trims and usually one catalytic converter, so there is no comparison to draw between two sides of the engine. What you do get instead is a very loud signal on the single bank: with cylinder 4 dead, a quarter of the engine's air is going through unburned, and the trims on the only bank there is will swing hard. That number confirms the cylinder is genuinely out rather than merely reported out, which is worth checking before pulling anything apart.

The consolation is cost. On an inline four the injectors sit on top of the head, in the open, reachable with hand tools. Reseating a connector is free, swapping the cylinder 4 injector with a neighbour to see whether the fault follows it is a genuinely cheap test, and even replacing the injector is a modest job. That is the opposite of the situation on a V engine, and it means the practical advice here is to test physically and early rather than reasoning from the module end.

What you should not do is leave it. With the circuit stuck high, cylinder 4 gets little or no fuel, a misfire is effectively guaranteed, and the air and unburned fuel going into the exhaust can overheat the catalytic converter — which on a four-cylinder is the only one the vehicle has.