P0401 code: insufficient EGR flow

What each part of the code means

P: Powertrain
0: Generic code, defined by SAE
4: Auxiliary emissions controls
01: The specific fault

P0401 and the related codes

P0400: Exhaust Gas Recirculation Flow Malfunction
P0401: Exhaust Gas Recirculation Flow Insufficient Detected

How serious is it?

DeepDiag rates a stored P0400 or P0401 as major, and a pending one as pending. The rating is a starting point, not a safety check.

Possible causes

Exhaust gas recirculation (EGR) sends a small, controlled share of the exhaust back into the engine's intake. That lowers the peak temperature in the cylinders, which cuts the nitrogen oxides (NOx) the engine makes. With too little EGR, NOx goes up, and the engine can knock or ping under load, such as when accelerating.

P0401 means the engine computer called for EGR flow but didn't detect as much as it expected. Makers judge the flow in different ways, such as how much the intake manifold pressure changes when the valve opens, a rise in temperature in the EGR passage, or, on Ford engines with a DPFE sensor, the pressure difference across a small orifice in the EGR tube.

California's OBD rules require cars that have EGR to check it for both too little and too much flow. A loss of flow has to be caught before it would push emissions over the limit, so a system can lose some flow before P0401 sets.

In no particular order: which one is likeliest depends on your car and its data.

Carbon in the passages: carbon builds up in the EGR passages and ports and narrows or blocks them.
The EGR valve: a valve that has failed, or that sticks and doesn't open properly.
The vacuum supply: on valves worked by engine vacuum, a damaged, loose or misrouted hose, or a failed solenoid that controls the vacuum.
Ford's DPFE sensor: on Ford engines that measure EGR flow with a DPFE sensor, a faulty sensor can set P0401.
The valve position sensor: a faulty or sticking sensor gives the computer the wrong valve position.
Wiring: a corroded or loose connector, a loose wire or a poor ground in the EGR circuit.

On engines that feed EGR to each cylinder through its own runner, carbon can block some runners and send all the recirculated exhaust to the cylinders that are still open. Those cylinders can misfire, so a misfire code may appear alongside P0401.

What to check in the data

EGR monitor tests: results for the EGR system on each bank, where the car reports them.
Commanded EGR and EGR error: live readings, where the car supports them.
Intake manifold pressure: live readings, where the car supports them.
Emissions readiness: whether the EGR or EGR/VVT system monitor has completed its self-test since codes were last cleared.
Freeze frame: the sensor snapshot from the moment the fault was recorded.
On-board EGR test: DeepDiag can request an EGR test, and asks you to confirm first because the command moves real vehicle components. Most cars don't support these tests, and the app tells you when yours doesn't.
Other codes: a failure in one component can set a code in another, so read every stored and pending code.

Before you clear it or buy parts

Clearing erases stored and pending codes and resets the emissions readiness checks, so the car may fail an emissions test until it has been driven enough for them to complete again. If a stored code comes straight back, that points to an active fault.

Rank the possible causes by likelihood, then run the simplest, cheapest test that would confirm the cause before you buy a part. Read what to do before you clear codes.

Check the passages before you buy a valve: the code doesn't prove the valve has failed. A valve that opens normally still can't pass enough exhaust if carbon has blocked the passages behind it.
Try a vacuum test on an older valve: on some older valves worked by engine vacuum, you can open the valve by hand with a vacuum pump at a warm idle. The engine should stumble badly or stall. If nothing much changes and the valve did open, the passages are blocked. An electronic valve can be commanded open with a scan tool that has bidirectional controls instead.
Look for a service bulletin: some makers, Honda among them, have issued bulletins for P0401 on particular models. Search NHTSA's recalls page by year, make and model. The results include manufacturer communications, such as the bulletins makers send to dealers.

How DeepDiag helps

DeepDiag reads P0401 and the data above, then explains in plain language what the data shows, what the AI suspects and how confident it is. It also suggests checks to run before you buy parts, and says whether it's a job you can do yourself.

DeepDiag gives an AI-assisted reading of the data your car reports. It isn't a professional inspection, a safety check or a guarantee, and the AI can be wrong. Every feature needs an active Premium subscription.

Coming soon

Get told when it's ready See a sample report

Sources

The possible causes, definitions and repair tips on this page come from these sources, checked in November 2026. What DeepDiag reads and how it rates a code come from the app itself.

P0401: insufficient EGR flow.