P0420
Secondary Air Injection System Relay "B" Circuit Malfunction
Powertrain Emission Controls Secondary Air Injection 🟡 Moderate — Fix within a week
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What This Actually Means
In plain language — no jargon

The secondary air injection (SAI) relay that controls air pumping into the exhaust isn't working properly—like a switch that's stuck or broken, preventing fresh air from reaching the catalytic converter. This disrupts the engine's emissions control system and can cause the check engine light to illuminate.

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Symptoms You May Notice
3 known symptoms for this code
Check engine light illuminated
Failed emissions test or high emissions
Possible rough idle or slight performance loss
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Embedded Systems Insight
What the ECU/ECM is actually computing

The ECU monitors the secondary air injection relay circuit by checking for proper voltage and current draw when the relay is commanded on. It expects to see a complete circuit with correct relay resistance and activation response during cold start and catalyst warm-up phases.

Voltage & Parameter Thresholds

Parameter Normal Range Fault Condition
Relay Circuit Voltage 12V when activated, <0.5V when de-energized No voltage rise or stuck voltage indication
Relay Current Draw 0.3-0.8 amps during activation Open circuit (0A) or short circuit (>2A)
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Diagnostic & DIY Fix Guide
Check these in order — from cheapest to most complex
1
SAI relay (12V automotive relay)
Locate relay in engine bay fuse/relay box and swap with identical relay to test, then replace if faulty.
2
Wiring harness connector and pins
Inspect relay socket pins for corrosion or damage and clean with contact cleaner or replace connector.
3
Secondary air injection pump and hoses
Check pump operation by listening for activation on cold start and inspect hoses for cracks or disconnection.