Code
P1418
HUMMER
P — Powertrain
Secondary Air Injection System Relay A Control Circuit High
AI status
Completed
Completed
100%
Causes
- Short to battery voltage on the relay control/wiring
- Stuck or failed SAI relay (Relay A)
- Corroded, damaged or loose connector at relay or ECM
- Open or shorted wiring between ECM and relay
- Poor or missing ground at relay or ECM
- Faulty ECM (less common)
Symptoms
- MIL (Check Engine Light) illuminated
- Reduced or no secondary air injection during cold start
- Failed emissions test or elevated cold-start emissions
- Possible rough idle or driveability issues during cold start
- Stored trouble codes related to SAI or emissions
What to check
- Read freeze frame and pending/stored codes with a scan tool
- Visual inspection of SAI relay, fuse, wiring harness, and connectors for damage or corrosion
- Verify battery voltage and vehicle charging system health
- Backprobe relay control pin and ECM control pin to measure voltage with key ON and during commanded activation
- Check relay coil resistance and swap relay with known-good if available
- Inspect and test grounds for continuity and low resistance
Signal parameters
- Battery/relay feed voltage: ~12.0–14.5 V (engine running)
- Control circuit expected when de-energized: near battery voltage or pulled high depending on design
- Control circuit expected when energized: close to 0 V (ECM typically grounds the relay control) or below 1 V
- Typical relay coil resistance range: ~30–200 ohms (varies by relay — check service data)
Diagnostic algorithm
- Scan and record all stored codes, freeze frame data, and pending codes.
- Perform a visual inspection of harness, connectors, relay socket, and SAI components for damage, corrosion, melted insulation, or repair splices.
- Verify battery voltage and charge; check related fuses and power feeds to the SAI relay.
- With ignition ON (engine OFF), backprobe the relay control terminal in the harness with the relay removed. Note the voltage with the ECM not commanding and while commanding (use a scan tool to request relay on).
- If control circuit reads high when it should be low (ECM commanding), disconnect the relay connector and measure voltage at the harness connector to determine if voltage is present without the ECM connected (identifies short to battery or power feed).
- Measure relay coil resistance; replace relay or swap with identical known-good relay to see if code clears.
- If harness shows no external short and relay is good, backprobe ECM control pin and check for expected switching to ground. If ECM pin remains high or damaged, suspect ECM fault and verify wiring continuity between ECM and relay connector.
- Repair any wiring, connector, or ground issues found. Replace relay if suspect. If ECM output driver is confirmed faulty after wiring and relay checks, consult manufacturer service procedures for ECM testing/replacement.
- Clear codes and perform functional test/cold-start monitor to verify repair. Re-scan after test drive and confirm no return of fault.
Likely causes
- SAI relay contacts or coil internally shorted
- Power feed shorted into control circuit (wiring harness chafed or pinched)
- Connector corrosion allowing high resistance or intermittent high voltage reading
- Ground circuit open so control output floats high
- ECM driver transistor leaking or failed
Fault status
Status
High voltage detected on Secondary Air Injection System Relay A control circuit — possible short to battery, faulty relay, wiring or ECM driver fault.
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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Repair manuals
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138
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