P0420

Secondary Air Injection System Relay "B" Circuit Malfunction

Powertrain Emission Controls Secondary Air Injection 🟡 Moderate — Fix within a week ⚠️ Drive with Care
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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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How Your ECU Detects This

Technical sensor logic and voltage thresholds

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

ParameterNormal RangeFault 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.
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When to See a Professional Mechanic

Not all fault codes are safe to DIY

Code P0420 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.

Safety note: OBD-II codes identify the system or circuit where a fault was detected — they do not always identify the exact failed component. A professional mechanic using live sensor data will diagnose the root cause more accurately than replacing parts based on the code alone.
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How to Clear Code P0420

What happens after you fix the fault

Once the fault is repaired, P0420 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.

The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.

✅ Safe to Clear When
  • Fault has been diagnosed and repaired
  • You want to confirm the repair worked
  • Code appeared after a sensor was cleaned
⚠️ Do Not Clear When
  • Preparing for an emissions/PUC test
  • Root cause is still undiagnosed
  • Check engine light is flashing
Emissions test note: Clearing codes resets OBD readiness monitors. Most vehicles need 50–100 km of mixed driving before monitors complete. Do not clear codes immediately before an emissions or PUC inspection.