OBD-II trouble code
U0306: Software Incompatibility With Fuel Injector Control Module
The injector control module's calibration does not match the rest of the vehicle. This module is not just a switch — it holds the per-injector correction values that make cylinders deliver equal fuel, so a mismatch here produces a fault that feels mechanical and is not.
Quick facts
- System
- Network
- Category
- Network Communication
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $100 – $700
- DIY difficulty
- Shop recommended
What does U0306 mean?
A dedicated fuel injector control module appears mainly on diesel engines and on some high-pressure direct-injection petrol engines, where the injectors cannot be driven straight from the engine controller. Piezo and solenoid injectors on these systems need short, precisely shaped pulses at voltages far above battery level, so a separate driver module generates them. This code stores when the network determines that this module's software or calibration is not part of the coordinated set the rest of the vehicle expects.
One thing about this module makes it different from every other member of the software-incompatibility family, and it is the thing that catches people out.
Injectors on these systems are not interchangeable in a data sense. Each one is measured at manufacture and given a correction value — variously called an injector classification, a trim code, or an IQA number — that describes how its actual delivery differs from nominal. That code is printed or laser-etched on the injector body and has to be stored in the driver module against the correct cylinder. The module then trims each injector's pulse to make the cylinders deliver the same quantity. Without those values, or with the previous vehicle's values, every injector is being driven to a generic assumption.
The result is a rough, uneven idle, a diesel knock that varies cylinder to cylinder, smoke, and sometimes cylinder-specific misfire or contribution codes. It looks exactly like worn or sticking injectors. Workshops have replaced perfectly good injector sets chasing it. So on this code specifically, the correct question after any module work is not only whether the software version matches, but whether the injector correction codes were re-entered for each cylinder in the right order. Reprogramming the module to the right calibration and skipping the correction codes leaves the vehicle running badly with no code to explain it.
The second difference is practical rather than diagnostic. This module switches high voltage — well above anything else in a typical engine bay — and its output circuits stay charged briefly after the ignition is switched off. Follow the manufacturer's stand-down time before disconnecting injector connectors or probing the module's outputs. This is not a circuit to back-probe casually.
The causes are the ordinary service-history ones: a module replaced without VIN-specific programming, a used unit fitted with another vehicle's data, an update applied to the engine controller but not to this module, a reflash interrupted or done with the wrong file. What is unusual is the frequency with which the mismatch arrives alongside injector work, because the two jobs are so often done together — and that is exactly the situation in which the correction codes get forgotten.
A vehicle with this code will usually run, but on a diesel it may run badly enough that the module falls back to a conservative strategy and limits power.
Common causes
- Injector control module replaced without VIN-specific programming
- Injector correction or classification codes not entered, or entered against the wrong cylinders, after module or injector work
- Used module fitted from another vehicle, carrying that vehicle's injector trim values
- Engine controller updated while the injector control module was left on its previous calibration
- Reflash interrupted by a voltage drop, leaving the module partly programmed
- Programming performed with a calibration for a different engine variant or emissions specification
- Injectors replaced and the module never told about the new correction values
- Aftermarket tuning that alters injection strategy without matching the driver module calibration
Symptoms
- Warning light with this code stored, often beside other network software codes
- Rough, uneven idle that feels like a mechanical injector fault
- Diesel knock that varies noticeably from cylinder to cylinder
- Visible smoke, particularly on start-up and under load
- Cylinder contribution or balance codes appearing alongside this one
- Reduced power or a conservative limited-performance strategy
- Hard starting when hot or cold
- Fault present since injector or module replacement
Diagnostic steps
- 1.Observe the manufacturer's stand-down time after switching off before touching injector connectors or module outputs. This module generates voltages well above battery level and its circuits remain charged briefly.
- 2.Establish the service history. This code follows module replacement, injector replacement, an update or a reflash in almost every case.
- 3.Read the module's software and calibration part numbers and compare them against the approved set for the VIN.
- 4.Check whether the injector correction codes are present in the module and whether they match the codes printed on the injectors actually fitted, cylinder by cylinder.
- 5.Treat missing or transposed correction codes as a finding in their own right. They produce rough running that looks mechanical, and no amount of injector replacement will fix it.
- 6.Note any cylinder contribution or balance codes stored alongside, since they tend to follow the cylinders whose correction values are wrong.
- 7.Confirm a stable programming supply before reflashing, because an interrupted flash is itself one of the causes on this list.
- 8.Reprogram the module to the matching approved calibration, then re-enter the injector correction codes for every cylinder in the correct order.
- 9.Road test and review live injector correction or balance values afterward rather than only confirming the code has cleared.
- 10.If rough running persists with correct software and correct correction codes entered, only then investigate the injectors mechanically.
Repair cost
$100 – $700
This is a programming repair. Reprogramming the injector control module and entering the injector correction codes is typically $150 to $400, and $400 to $700 where the calibration is dealer-only or several modules must be brought into line together. Entering correction codes alone, where the software is already correct, is often at the low end and takes under an hour. The expensive mistake on this code is the one it invites: a set of diesel injectors replaced to chase rough running that was caused by missing correction values runs $1,500 to $4,000 and does not fix it.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with pcm replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
Leave this one to a qualified shop. It typically involves emissions-critical components, refrigerant handling, or other work that requires manufacturer-grade tooling, training, or certification. DIY attempts often produce a more expensive problem than the original code.