Code
P0406
HUMMER
P — Powertrain
Exhaust Gas Recirculation Sensor A Circuit High
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UK: 28
EN: 23
RU: 62
AI status
Completed
Completed
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Causes
- Open or shorted wiring between EGR sensor and ECM (short to battery voltage)
- Poor or corroded connector or pin at EGR sensor or ECM
- Failed EGR position/feedback sensor
- Faulty ECM (rare)
- Aftermarket or improper sensor replacement/wiring
- Intermittent harness damage (pinched, melted, water intrusion)
Symptoms
- Malfunction Indicator Lamp (MIL) illuminated
- Reduced engine performance or limp mode in some cases
- Poor idle quality or hesitation
- Failed emissions test or increased NOx emissions
- Possible stored EGR-related codes (flow or position)
What to check
- Retrieve freeze frame and active/continuous data with a scan tool; note EGR sensor voltage/value
- Check for additional stored codes (EGR, wiring, PCM)
- Visually inspect EGR valve/sensor connector and wiring for damage, corrosion, or water
- Backprobe sensor connector and measure signal voltage with key ON/engine OFF then during operation
- Measure reference supply (typically ~5 V) and ground at the sensor connector
- Wiggle test wiring while monitoring signal for intermittent changes
Signal parameters
- Reference supply (Vref) to sensor: approx. 5.0 V (±0.25 V) with key ON
- Sensor signal voltage (typical idle/closed): ~0.5–4.5 V depending on EGR position
- Circuit high condition: signal voltage above expected upper threshold (often >4.5–4.8 V)
- Sensor ground continuity: near 0 ohms to chassis/PCM ground
- Recommended test tool: digital multimeter and ideally an oscilloscope for dynamic signal
Diagnostic algorithm
- Read and record freeze-frame data and all stored DTCs. Note conditions when code set.
- Perform a thorough visual inspection of the EGR valve/sensor, connector, and harness for damage, corrosion, or water intrusion. Repair obvious issues.
- Check connector pins for pulled, bent, or corroded terminals. Clean and secure as needed.
- With ignition ON (engine OFF), backprobe the EGR sensor connector: verify Vref ~5 V, signal voltage level, and sensor ground. If Vref is missing or out of range, suspect ECM or wiring to ECM.
- If Vref is correct and signal is high (>4.5–4.8 V), disconnect the sensor and measure connector signal terminal voltage again. If voltage remains high at harness with sensor disconnected, look for a short to battery voltage in the harness or an upstream source feeding voltage into the signal circuit.
- If harness signal goes to 0 V or open when sensor disconnected, test the sensor on the bench (if removable) or swap with a known-good unit per service manual procedures.
- Perform a continuity check from signal pin to ECM pin to look for short/opens; check resistance to battery positive if suspect short to power.
- If intermittent, perform wiggle tests while monitoring live data or use an oscilloscope to spot transient shorts or spikes.
- After repairs or component replacement, clear codes, perform functional test/drive cycle, and confirm code does not return and sensor values are within expected range.
Likely causes
- Connector contaminated/corroded at the EGR valve/sensor
- Signal wire shorted to battery voltage or another 12V feed
- EGR sensor internal failure (stuck high output)
- Broken ground or bad reference voltage from ECM causing abnormal reading
Fault status
Status
EGR Sensor A Circuit High — the EGR position/feedback sensor signal is higher than the allowed range. Inspect sensor, wiring, connectors, and ECM inputs.
Repair difficulty: Medium
Diagnostic time: 0.5–2.5 hours
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