P1204 Fault Code: Manufacturer-Specific Injector Circuit Faults Explained
P1204 sits in the manufacturer-specific P1xxx range, which means its definition depends on who built the vehicle. In the diesel applications where UK workshops meet it — reported on Nissan and Renault dCi engines and on small Ford TDCi diesels — it corresponds to an injector circuit fault on cylinder 4: the manufacturer-specific sibling of the generic P0204. Because P1xxx definitions are not standardised, the first diagnostic act is always the same: confirm the code's meaning for this exact make and model with manufacturer-level diagnostics before spending a minute on parts.
Technical Background
Generic P0xxx codes mean the same thing on every OBD-II vehicle; P1xxx codes do not. A generic scan tool that displays P1204 with a text description may be showing you another manufacturer's definition entirely. Manufacturer-level diagnostics — or a database entry for the exact make, model and engine — is the only trustworthy source for a P1xxx meaning. The exact procedure and specification varies by manufacturer and engine.
In the injector-circuit interpretation, the diagnosis is the standard electrical sequence: the ECU cannot drive the cylinder 4 injector correctly, and the fault may be the injector's solenoid, the harness, the connector, or the ECU driver. On the small-capacity diesels where this code is reported, the injectors are compact solenoid common-rail units and the loom runs are short — which makes end-to-end electrical testing quick.
Vehicles Commonly Affected
- Reported on Nissan and Renault 1.5 dCi (K9K) applications as an injector circuit fault.
- Reported on small Ford TDCi diesels in community and trade sources with the same meaning.
- Definitions genuinely differ by manufacturer — always verify against manufacturer data for the specific vehicle.
Symptoms
- Misfire or rough running centred on one cylinder.
- Reduced power; possible stalling on small-capacity engines where one cylinder is a quarter of the output.
- Engine management light, often with a companion misfire or generic P0204 code.
Causes
- Failed injector solenoid on cylinder 4 — open circuit or out-of-specification values.
- Wiring harness damage or connector corrosion on the cylinder 4 circuit.
- ECU injector driver fault.
- A replacement injector fitted without the required coding/programming step.
- Misread code — a generic tool applying another manufacturer's P1204 definition.
Diagnosis
- 1Confirm the code's definition for this exact vehicle using manufacturer-level diagnostics — this is non-negotiable for P1xxx codes.
- 2Read all companion codes; a paired P0204 or cylinder 4 misfire code corroborates the injector-circuit interpretation.
- 3Inspect the cylinder 4 injector connector and loom for damage and corrosion.
- 4Test the injector circuit electrically per the manufacturer's procedure, from the injector and from the ECU connector. The exact procedure and specification varies by manufacturer and engine.
- 5Where permitted, swap the cylinder 4 injector with a neighbour — updating injector coding correctly — and see if the fault follows.
- 6Bench-test a condemned injector to confirm before replacement.
Common Mistakes
- Acting on a generic tool's text description of a P1xxx code without verifying the manufacturer's definition.
- Replacing the cylinder 4 injector when the fault was a connector a few centimetres upstream.
- Skipping the coding step on engines that require injector programming — a documented cause of repeat faults after repair.
- Ignoring the generic P0204 sibling code, which often stores alongside and confirms the interpretation.
When It's Not the Injectors
- Wiring and connector faults — statistically the injector's main rival on circuit codes generally.
- ECU driver failures.
- A code definition that, on this manufacturer, turns out not to be injector-related at all — verify first.
When Replacement Is Required
Replace when the manufacturer's electrical test condemns the injector at its own pins with the harness proven. On the dCi and TDCi families where this code is reported, injector coding or programming after replacement is required on most variants — confirm the procedure for the specific engine.
Repair
Compatible Engines
Compatible Injectors
Frequently Asked Questions
Is P1204 the same as P0204?
On the diesel applications where P1204 is reported as an injector circuit fault, it is effectively the manufacturer-specific sibling of the generic P0204. But P1xxx definitions vary by maker — verify the meaning for your exact vehicle before acting.
Why does my scan tool describe P1204 differently from the internet?
Because P1xxx codes are manufacturer-defined and generic tools often display another maker's text. Manufacturer-level diagnostics or a make-specific database is the only reliable definition source.
The injector was replaced and the fault returned — why?
The most common documented reasons are an uncoded replacement injector on engines that require programming, or the true fault being in the harness or connector all along. Both are checked before the second injector is ordered.