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Decoding P0401: Understanding and Fixing the EGR System in Toyota Prius Gen 3

Most of us have been there. A fleeting check engine light or pending DTC P0401 coming and going. Sometimes when hot, other times when cold. So what does this mean?

Pictured: T-SB-0027-16

What is an EGR (Exhaust Gas Recirculation System)?

While this blog post is addressing the Gen 3 Toyota Prius, this information (different TSBs) coincides with just about all Toyota / Lexus EGR systems currently in use.

Before you attempt to diagnose your EGR system, you must learn what components it's comprised of, as well as the diagnostic procedures required to determine if further disassembly is required for a proper visual inspection.

The Exhaust Gas Recirculation (EGR) system is a critical component of a vehicle's emission control system. The primary function of the EGR system is to reduce the formation of Nitrogen Oxides (NOx) which are harmful pollutants. It does so by recirculating a portion of an engine's exhaust gas back to the engine cylinders. This dilutes the O2 in the incoming air stream and provides gases inert to combustion (primarily CO2 and H2O) to act as absorbents of combustion heat to reduce peak in-cylinder temperatures. This reduction of heat inside the cylinders prevents to formation of NOx thus reducing smog conditions.

Exhaust Gas Recirculation Valve (EGR Valve): This component is the heart of the EGR system. It opens and closes to allow exhaust gases to be recirculated back into the engine. The EGR valve on the Prius is electronically controlled, and the car's computer (ECM) determines when and how much it should open based on factors such as engine load, temperature, and speed. When the EGR Valve experiences excessive carbon build up, the sensors have a harder time determining position of the valve during operation, as well as decreased exhaust flow.

EGR Cooler: Due to the high temperature of the exhaust gases, it's necessary to cool them before they can be reintroduced into the engine to avoid causing any damage. This is the function of the EGR cooler. In the Prius, this cooler is a heat exchanger that uses engine coolant to reduce the temperature of the exhaust gases. The primary failure cause is excessive carbon buildup restricting cooling capacity and exhaust flow.

EGR Passage: This is the channel through which the recirculated exhaust gases flow from the exhaust manifold back into the engine's intake manifold. It's designed to withstand high temperatures and pressure, although is susceptible to carbon build up which then restricts flow.

Pictured: Location of EGR components - 2010 Toyota Prius

Pictured: Exploded diagram view of the EGR system - 2010 Prius

Unlike most EGR valves in production that typically require vacuum input as a metric or actuator, the Gen 3 Prius ( as well as other Toyota / Lexus models) uses an entirely electronically controlled stepper motor for accurate opening and closing when demanded. The opening and closing of the EGR Valve is controlled by a specific calibration file (algorithm) that resides within the ECM (Engine Control Module). Early generation 3 Toyota Prius required a calibration file update, as well as potential part replacement / cleaning of the EGR components.

Understanding a Technical Service Bulletin:

No matter what vehicle you're working on, all manufacturers are required to provide a remedy to commonly reported issues while under warranty. These typically result in TSBs or recall campaigns. TSBs are typically remedied at the expense of the customer once the vehicle is out of warranty, however, when a severe issue arises even for vehicles outside of their respective warranties, a recall campaign will be established and performed at no cost to the consumer.

Testing and Inspection:

When you're ready to dive in, grab a dedicated bi-directional scan tool or Toyota Techstream, and a calibrated DVOM (digital voltage ohm meter).

Pictured: On-Vehicle EGR test.

After recording the data values from the in-vehicle test, unplug the EGR valve connector and measure the resistance values. This is best done while the vehicle is cold so as to match the Toyota supplied baseline values as closely as possible (68F).

Pictured: EGR resistance values.

Both tests showed no current fault conditions, now what?

If both diagnostic procedures didn't reproduce the fault, or showed MAP and resistance specifications to be within specification, you can now attempt a recalibration of the engine computer via Toyota Techstream & retest.

*Note* If the vehicle just started showing symptoms and a recalibration was performed, it may take up to a week of normal drive cycles for the ECM to determine if proper EGR flow is occurring. In the event DTC P0401 returns, it's time for disassembly, cleaning, and part replacement.

The Inevitable, DTC P0401 Returns:

Grab some disposable gloves and get ready to get dirty. Start by removing the cowl and wiper assembly. Once out of the way you'll be able to access the exhaust manifold where the EGR cooler bolts up. Remember to take your time and use magnetic extensions to prevent losing nuts and bolts (not fun). You'll need to remove the intake manifold (go ahead and replace that PCV while you're there, its right behind the intake on the side of the engine block) as well as the throttle body in preparation of cleaning the internal EGR ports located within the intake manifold.

Pictured: Intake manifold removed, EGR Valve outlet at the top right, PCV in the center of the engine block

.Pictured: EGR outlet tube with excessive carbon buildup, located between the EGR Valve and the Intake manifold:

Pictured: EGR Valve with excessive carbon buildup.

Pictured: EGR Cooler completely clogged with excessive carbon buildup.

Pictured: Intake manifold at the throttle body, with typical oil pooling in the bottom.

*NOTE* Not pictured, is the intake manifold where it meets the cylinder head. Next to the intake ports are small EGR passages that will need to be cleaned with a solvent, and a bore brush. We recommend using any TDI cleaner (typically used on Diesel & Euro EGR / Turbo systems). In total, you'll clean the EGR passages in the intake manifold, as well as the EGR TUBE. You'll need to replace the EGR Valve and the EGR Cooler.

If the parts "test" good, why can't I just clean them?

You can. It will take a 24 hour solvent deep soak to break up the carbon inside the EGR cooler. You'll then need to burn out the residual carbon with a torch, then re-soak until the desired results are achieved. In practice it takes about 36-48 hours to get the EGR cooler back to a usable level of exhaust flow. We don't recommend this as it's labor intensive, and you're still left with old parts susceptible to failing again.

To ensure the fix, replace the EGR valve and EGR cooler with Toyota OEM, or a reputable aftermarket equivalent. Clean the intake manifold, EGR Tube, Throttle Body and MAF (for good measure) thoroughly, and replace any and all gaskets necessary. Once back together, reset all faults, reset fuel economy, and perform a drive cycle while monitoring all applicable data pids on your bi-directional scan tool. If after a week the condition never returns, you've completed the repair successfully.

If you're having a hard time repairing your vehicle, or you're looking for a top down solution to remedy your vehicle's performance needs, contact Evergreen Auto Solutions at 305-804-9744 or submit your question in the chat box.


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