Code
P0056
HUMMER
P — Powertrain
Heated Oxygen Sensor (HO2S) Heater Circuit Bank 2 Sensor 2
Views:
UK: 13
EN: 32
RU: 29
AI status
Completed
Completed
100%
Causes
- Open or short in the heater wiring harness (Bank 2 Sensor 2)
- Failed HO2S heater element (sensor internal failure)
- Blown fuse or faulty relay supplying heater circuit
- Poor connector contact, corrosion or water intrusion at sensor connector
- High resistance in harness (corrosion, damaged terminals, repair splice)
- Faulty PCM or driver circuit (less common)
Symptoms
- Malfunction Indicator Lamp (MIL) illuminated (Check Engine light)
- Stored P0056 freeze frame/test data in PCM
- Possible failed emissions test (exhaust monitoring)
- Occasional rough idle or drivability issues (less common — usually related to downstream sensor monitoring)
- Poor fuel trim or closed-loop readiness incomplete for some readiness monitors
What to check
- Use a scan tool to confirm P0056 and check freeze frame and readiness status
- Attempt to command the HO2S heater on with a capable scan tool and observe status/outputs
- Visually inspect bank 2 sensor 2 connector and wiring for damage, corrosion, or water intrusion
- Check related fuses and relays that supply heater circuits
- Back-probe the sensor heater pins to check for battery voltage and/or PCM command ground while commanding heater on
- Measure heater resistance at the sensor connector with the sensor unplugged and compare to spec
Signal parameters
- Typical HO2S heater resistance (typical range): ~2–20 ohms (refer to vehicle-specific OEM spec)
- Heater supply voltage (when commanded/powered): approximately vehicle battery voltage (~11–14 V)
- Heater current draw (typical): roughly 0.5–3 A depending on sensor design
- PCM controls heater by switching ground or 12 V depending on design — confirm with vehicle wiring diagram
- When commanded ON by PCM, heater circuit should show voltage/current; when OFF it should show open/no drive
Diagnostic algorithm
- Confirm DTC P0056 is current and note freeze frame data with scan tool.
- Verify corresponding fuses/relays for heater circuits; replace if blown and retest.
- Visually inspect Bank 2 Sensor 2 connector and wiring harness for damage, corrosion, melted insulation, or exhaust damage.
- With ignition ON (engine off) and sensor connected: back-probe heater power pin to verify battery voltage present when heater is commanded; if no power, trace to fuse/relay/power source.
- Command heater ON with scan tool (if supported) and measure voltage and current at the heater pins. If commanded and no voltage/current, suspect open supply or PCM driver.
- Unplug sensor and measure heater element resistance at sensor terminals; compare to OEM spec. If resistance is open or out of range, replace sensor.
- If sensor resistance is within spec, check continuity from sensor heater pins to PCM connector pins. Repair any opens/shorts or high resistance connections.
- If wiring and sensor check good, check PCM driver for shorts to ground or signs of failure. Replace PCM only after exhausting wiring and sensor checks and following OEM procedures.
- After repair or replacement, clear codes, perform functional test by commanding heater and/or performing a drive cycle, and verify code does not return.
Likely causes
- Broken or chafed wire to the sensor heater
- Corroded or oil-contaminated connector pins at the oxygen sensor
- Sensor heater element has failed (internal open)
- Blown inline fuse or failed power supply/relay for heater
- Short to ground or short to battery in the heater circuit
- PCM driver fault (check after wiring/sensor verified)
Fault status
Status
P0056 — HO2S Heater Circuit Bank 2 Sensor 2. Heater circuit for downstream oxygen sensor on bank 2 is malfunctioning (open, short, high resistance, or failed heater element).
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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