Code
P01B1
Generic
P — Powertrain
Alternative Fuel Tank A Level Sensor Circuit Low
AI status
Completed
Completed
100%
Causes
- Open or shorted wiring in the level sensor harness (short to ground)
- Faulty alternative fuel tank level sensor (failed sender or float)
- Corroded or poorly connected sensor connector
- Missing or low reference voltage from PCM (typically 5 V reference)
- Bad PCM input/ground or internal PCM fault
- Contamination (fuel, water) or mechanical binding of the float/sender
Symptoms
- Wrong or missing fuel level indication from the alternative fuel tank
- Illumination of the MIL/check engine light
- Possible drivability unaffected unless system relies on tank level data
- Inconsistent or jumping gauge reading
- Other fuel‑system related warnings if tank level data used by other systems
What to check
- Read and record freeze frame and live data with a scan tool (fuel level, related PIDs).
- Visually inspect sensor connector, wiring harness, and tank area for corrosion, damage or chafing.
- Backprobe sensor connector with key ON (engine OFF) and verify reference voltage and ground presence.
- Measure signal voltage at the sensor connector while moving the float (if accessible).
- Check connector pins for corrosion, bent pins, or poor contact and wiggle harness for intermittent faults.
- Verify continuity from sensor ground to chassis ground and from sensor signal to PCM input.
Signal parameters
- Typical reference voltage: ~5 V supply from PCM (vehicle-specific).
- Expected sensor signal: proportional voltage between ~0.5–4.5 V (varies by design).
- Low condition: signal often below ~0.2–0.5 V or nearly 0 V (indicates 'low' circuit).
- Open/short checks: sensor harness continuity should be low resistance (typically
- Resistance-type senders: resistance varies with level — consult vehicle specs (do not assume universal ohm values).
Diagnostic algorithm
- Retrieve codes and freeze‑frame; note conditions when code set (fuel level, voltage).
- With key ON (engine OFF), monitor live sensor PID. Confirm it reads very low or near 0 V. Move tank/float if possible and observe change.
- Inspect connector and wiring for corrosion, damage, or water ingress. Repair or clean any issues.
- Backprobe sensor connector: verify PCM reference voltage (≈5 V) and ground presence. If reference missing, trace to PCM and associated fuses/relays.
- Measure signal wire voltage at connector while lifting/lowering float. No change + correct reference/ground → suspect sensor.
- Check continuity of signal wire to PCM input and check for short to ground (resistance to ground should be high when sensor disconnected).
- With sensor removed, measure sender resistance across its terminals while moving float and compare to service data. An open or out‑of‑range reading indicates a bad sender.
- Repair wiring/connectors or replace sensor as indicated. After repair, clear codes and perform a drive/operational cycle to verify the fault does not return.
- If wiring and sensor test good but code persists, consider PCM input test or replacement per manufacturer procedures.
Likely causes
- Corroded/loose connector at the sensor or in-line connector
- Damaged wiring insulation contacting chassis ground causing low signal
- Failed sender element (open circuit or internal short)
- Broken sensor float or mechanical obstruction preventing proper movement
- Faulty ground at module or body ground near tank assembly
Fault status
Status
Alternative Fuel Tank A Level Sensor Circuit Low — voltage/resistance below expected range (possible open/short to ground or sensor failure).
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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