Is your Lexus RX400h showing a P1420 trouble code and the check engine light is on? This guide walks you through diagnosing and fixing the issue, which usually involves the secondary air injection system. Follow our detailed steps to resolve it safely and save on repair costs.
Key Takeaways
- Understand the P1420 Code: It indicates a malfunction in the secondary air injection system, often due to a faulty pump, relay, or clogged air lines.
- Diagnose Before Replacing Parts: Use an OBD2 scanner to confirm the code and check live data to pinpoint the exact cause.
- Inspect the Air Injection Pump: Listen for unusual noises and test the pump’s operation to determine if it needs replacement.
- Check the Air Injection Relay: A failed relay is a common and inexpensive fix—test or swap it to see if the light goes off.
- Examine Air Hoses and Valves: Cracks, leaks, or blockages in the air lines or check valves can trigger the P1420 code.
- Clear the Code After Repair: Use a scanner to reset the check engine light and verify the fix was successful.
- Prevent Future Issues: Regular maintenance and keeping the system clean can help avoid recurrence.
How to Fix Rx400h Check Engine Light P1420
Seeing the check engine light illuminate on your Lexus RX400h can be stressful—especially when you’re not sure what’s causing it. If your OBD2 scanner returns a P1420 trouble code, you’re dealing with an issue in the secondary air injection system. This system helps reduce emissions during cold starts by pumping fresh air into the exhaust stream. While it may sound complex, fixing the P1420 code is often straightforward if you follow the right steps.
In this comprehensive guide, you’ll learn exactly what the P1420 code means, how to diagnose the root cause, and how to fix it yourself—without spending hundreds at the dealership. We’ll walk you through inspecting the air injection pump, testing the relay, checking air lines, and clearing the code. By the end, you’ll have the confidence to tackle this common RX400h issue and keep your hybrid SUV running smoothly.
What Is the P1420 Code on a Lexus RX400h?
The P1420 code stands for “Secondary Air Injection System – Malfunction.” On the Lexus RX400h, this system activates during cold engine starts to reduce harmful emissions. It works by injecting fresh air into the exhaust manifold, helping burn off unburned fuel and lowering hydrocarbon emissions before the catalytic converter warms up.
Visual guide about How to Fix Rx400h Check Engine Light P1420
Image source: warninglights.net
When the engine control module (ECM) detects that the air injection system isn’t working as expected—such as low airflow, a stuck valve, or electrical failure—it triggers the P1420 code and turns on the check engine light. This doesn’t usually affect drivability, but it can cause your vehicle to fail emissions tests and may lead to increased pollution if left unresolved.
Common Causes of P1420 on the RX400h
- Faulty air injection pump: The pump may wear out over time or fail due to moisture or debris.
- Failed air injection relay: This relay controls power to the pump and can burn out or corrode.
- Clogged or cracked air hoses: Air lines can degrade, crack, or become blocked by dirt or water.
- Stuck or leaking check valves: These one-way valves prevent exhaust gases from flowing back into the pump.
- Electrical issues: Damaged wiring, poor connections, or blown fuses can interrupt the system.
Tools and Supplies You’ll Need
Before you begin, gather the following tools and supplies to make the repair process smooth and efficient:
- OBD2 scanner (preferably one that can read live data)
- Multimeter (for testing electrical components)
- Basic hand tools (socket set, wrenches, screwdrivers)
- Safety gloves and eye protection
- Flashlight or work light
- Replacement parts (if needed): air injection pump, relay, hoses, or check valves
- Clean rags and compressed air (for cleaning)
Having these items on hand will help you diagnose and fix the issue without unnecessary trips to the auto parts store.
Step 1: Confirm the P1420 Code
The first step is to confirm that the check engine light is indeed due to the P1420 code. Sometimes, multiple codes can appear, and you want to make sure you’re addressing the right issue.
How to Read the Code
- Locate the OBD2 port under the dashboard, usually near the driver’s side knee panel.
- Plug in your OBD2 scanner and turn the ignition to the “ON” position (do not start the engine).
- Follow the scanner’s instructions to read the trouble codes.
- Look for P1420 in the list. If it appears, note any other codes that may be present.
Check Live Data (Optional but Helpful)
If your scanner supports live data, monitor the secondary air injection system while the engine is cold. You should see the pump activate for a few seconds after startup. If it doesn’t turn on, or if the ECM reports low airflow, this confirms the P1420 issue.
Tip: Clear the code after reading it, then drive the car for a day or two. If the light returns, the problem is still present and needs fixing.
Step 2: Locate the Secondary Air Injection System
The secondary air injection system on the RX400h is located near the front of the engine bay. Here’s how to find the key components:
Air Injection Pump
The pump is usually mounted on the driver’s side of the engine, near the firewall. It’s a small, black or gray box with electrical connectors and air hoses attached. It may be tucked behind other components, so use a flashlight to locate it.
Air Injection Relay
The relay is typically found in the engine compartment fuse box. Refer to your owner’s manual or the fuse box cover diagram to identify the correct relay (often labeled “AIR” or “Secondary Air”).
Air Hoses and Check Valves
Follow the hoses from the pump to the exhaust manifold. You’ll see one or more check valves along the way—these are small, cylindrical components that allow air to flow in only one direction.
Safety Note: Always let the engine cool before working under the hood. Hot components can cause burns.
Step 3: Test the Air Injection Relay
The air injection relay is a common culprit behind the P1420 code. It’s inexpensive and easy to test or replace.
How to Test the Relay
- Turn off the engine and disconnect the negative battery terminal for safety.
- Locate the air injection relay in the fuse box.
- Remove the relay and inspect it for signs of burning, corrosion, or damage.
- Use a multimeter to test for continuity across the relay terminals (refer to the relay’s pinout diagram).
- If the relay fails the test, replace it with a new one of the same type.
Quick Swap Test
If you’re unsure whether the relay is faulty, try swapping it with another identical relay in the fuse box (like the horn or fog light relay). Start the car and see if the pump activates. If it does, the original relay was bad.
Tip: Relays are cheap—usually under $20. Replacing a suspected faulty relay is a quick and low-risk fix.
Step 4: Inspect the Air Injection Pump
If the relay is working, the next step is to check the air injection pump itself.
Listen for Pump Operation
- Have a helper start the engine while you listen near the pump.
- You should hear a brief humming or whirring sound for 5–10 seconds after startup.
- If you don’t hear anything, the pump may not be receiving power or could be seized.
Test Pump Power
- Use a multimeter to check for voltage at the pump’s electrical connector.
- Turn the ignition to “ON” and have someone start the engine.
- You should see 12 volts at the connector for a few seconds.
- If there’s no voltage, the issue may be wiring or the ECM.
- If there is voltage but the pump doesn’t run, the pump is likely faulty.
Check for Physical Damage
Inspect the pump for cracks, water damage, or burnt smell. Moisture can enter the pump and cause internal corrosion. If the pump is damaged, it will need replacement.
Note: Replacement pumps can cost $200–$400, but aftermarket or refurbished units are often available for less.
Step 5: Examine Air Hoses and Check Valves
Even if the pump and relay are working, blocked or leaking air lines can trigger the P1420 code.
Inspect the Hoses
- Follow the air hoses from the pump to the check valves and exhaust manifold.
- Look for cracks, splits, or loose connections.
- Gently squeeze the hoses—if they feel brittle or crumble, they need replacement.
- Use compressed air to blow through the hoses. Air should flow freely in one direction only.
Test the Check Valves
- Remove the check valves from the hoses (they usually snap in and out).
- Blow through each valve. Air should pass in one direction but not the other.
- If air flows both ways or is blocked, the valve is faulty and must be replaced.
- Check valves are inexpensive—typically $10–$20 each.
Pro Tip: Some check valves can be cleaned with carburetor cleaner if they’re just dirty, but replacement is often the best long-term fix.
Step 6: Check for Electrical Issues
Wiring problems can also cause the P1420 code. Inspect the electrical connections for corrosion, fraying, or loose pins.
Inspect Connectors
- Unplug the pump and relay connectors.
- Look for green corrosion, bent pins, or melted plastic.
- Clean any corrosion with electrical contact cleaner and a small brush.
- Reconnect firmly and ensure a snug fit.
Test Fuses
Check the fuses related to the air injection system. A blown fuse can cut power to the pump. Replace any blown fuses and investigate why they failed (e.g., short circuit).
Step 7: Clear the Code and Test Drive
After making repairs, it’s time to clear the check engine light and verify the fix.
Clear the Code
- Plug in your OBD2 scanner.
- Select “Clear Codes” or “Erase DTCs.”
- Turn off the ignition and wait a few seconds.
- Start the engine and check if the light remains off.
Test Drive
Drive the vehicle for at least 10–15 minutes, including a cold start. The ECM will re-test the air injection system during the next cold startup. If the P1420 code returns, the problem may not be fully resolved.
Note: Some scanners allow you to monitor readiness monitors. Wait until the “Catalyst” and “O2 Sensor” monitors are complete before retesting.
Troubleshooting Tips and Common Mistakes
Even with careful diagnosis, you might run into issues. Here are some troubleshooting tips:
- Code keeps returning: Double-check all connections and ensure the pump is actually running. A weak pump may not generate enough airflow.
- Pump runs but light stays on: The check valve may be stuck open, allowing exhaust gases to backflow. Replace the valve.
- No power to pump: Trace the wiring back to the relay and fuse. Use a wiring diagram if needed.
- Aftermarket parts fail quickly: Use OEM or high-quality replacement parts. Cheap pumps may not last.
- Water in the system: If you live in a rainy area, consider installing a moisture trap or rerouting hoses to prevent water ingress.
Warning: Never bypass the air injection system. It’s essential for emissions compliance and can affect your vehicle’s performance over time.
When to See a Mechanic
While many P1420 issues can be fixed at home, some situations require professional help:
- You’re unsure about electrical testing or wiring.
- The ECM or sensor issues are suspected (requires advanced diagnostics).
- Multiple systems are affected, indicating a larger problem.
- You’ve replaced parts but the code persists.
A certified Lexus technician can perform a smoke test to detect leaks or use factory-level tools to diagnose complex issues.
Preventing Future P1420 Codes
Once you’ve fixed the issue, take steps to prevent it from happening again:
- Inspect the system annually: Check hoses, valves, and pump during routine maintenance.
- Keep the engine bay clean: Dirt and debris can clog air lines or damage components.
- Avoid deep water crossings: Water can enter the pump and cause failure.
- Use quality parts: Invest in reliable replacements to avoid repeat failures.
- Monitor the check engine light: Address any new codes promptly to prevent cascading issues.
Conclusion
Fixing the P1420 check engine light on your Lexus RX400h doesn’t have to be intimidating. By understanding the secondary air injection system and following this step-by-step guide, you can diagnose and resolve the issue yourself—saving time and money. Start by confirming the code, then test the relay, pump, hoses, and valves. Clear the code and test drive to ensure the fix holds.
With regular maintenance and attention to detail, you can keep your RX400h running cleanly and efficiently for years to come. Remember, most P1420 problems are mechanical or electrical—not computer-related—so a hands-on approach often pays off. Stay safe, take your time, and don’t hesitate to seek help if needed.