Code
P121B
LAND ROVER
P — Powertrain
Cylinder balance - injector stuck
Views:
UK: 3
EN: 13
RU: 4
AI status
Completed
Completed
100%
Causes
- Injector pintle or internal valve stuck (carbon, varnish, debris)
- Injector electrical fault (open, short, intermittent, poor connector)
- Faulty injector driver in the ECU
- Low or high fuel rail pressure or clogged fuel supply
- Mechanical engine issue causing apparent imbalance (low compression, valve problem)
- Contaminated fuel or incorrect fuel type
Symptoms
- Rough idle or vibration
- Single-cylinder misfire or sustained misfire detected
- Reduced engine power or hesitation under load
- Elevated fuel consumption and/or smoke from exhaust
- Engine warning light (MIL) illuminated; DTC P121B stored
What to check
- Read and record freeze frame and related codes (misfire codes P030x, injector circuit codes P020x)
- Monitor live data: cylinder contribution/balance, misfire counters, short/long-term fuel trims
- Check fuel rail pressure and regulator operation at idle and under load
- Visually inspect injector connectors, wiring harness, and grounds for damage or corrosion
- Measure injector resistance (Ω) and compare to spec
- Perform injector flow/bench test or pulse the injector with a known-good driver
Signal parameters
- Injector pulse width (ms) — commanded vs actual
- Injector duty cycle (%) at various loads
- Injector coil resistance (Ω) — static ohms value
- Injector current draw (mA) during operation
- Fuel rail pressure (bar / psi) and pressure stability
- Cylinder contribution or balance % from scan tool
Diagnostic algorithm
- Verify the code and note freeze-frame data; clear codes and attempt to reproduce the fault under the same conditions
- Scan for related codes (P020x injector circuit faults, P030x misfires) and monitor live data while engine is running
- Perform a visual inspection of fuel injector wiring, connectors, and grounds; repair any corrosion or damage and re-test
- Measure fuel rail pressure and flow to ensure fuel delivery is within specification; repair fuel supply issues if present
- Measure static resistance of the suspect injector(s) and compare to manufacturer spec; abnormal values indicate internal fault
- Use a scan tool to monitor injector pulse width and cylinder contribution while commanding individual injection events if possible
- Swap the suspect injector with a known-good injector from another cylinder and observe whether the imbalance/misfire follows the injector (isolate mechanical vs electrical)
- Use an oscilloscope or injector tester to check injector drive waveform and current; look for missing pulses, abnormal rise times, or truncated waveforms indicating driver or wiring faults
- If injector is mechanically stuck, remove and perform ultrasonic cleaning and flow testing or replace the injector if cleaning fails or damage is found
- If wiring and injector bench tests are good, consider ECU driver failure — confirm with manufacturer procedures before replacing ECU
- After repair or replacement, clear codes and perform a road or load test to confirm cylinder balance restored and codes do not return
Likely causes
- Injector mechanically stuck (carbon/varnish) preventing normal flow
- Injector wiring/connector fault causing loss of drive or intermittent signal
- Failed injector driver circuitry in the ECU
- Insufficient or excessive fuel rail pressure affecting flow
Fault status
Status
Cylinder balance fault — injector stuck (stuck open/closed or not responding). ECU has detected abnormal contribution from one cylinder and set P121B.
Repair difficulty: Medium
Diagnostic time: 1.5-3.5 hours
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