Drive with caution — fix soon.
P0401 indicates the EGR system isn't recirculating enough exhaust gas back into the engine to lower combustion temperatures and reduce emissions. The most common reason is carbon buildup clogging the EGR valve or its passages. It can cause pinging or rough running and, left unaddressed, the higher temperatures can contribute to engine wear over time.
$150 – $500
Varies by vehicle and root cause.
You can usually drive short distances, but it's not ideal to keep driving long-term. You may notice knocking or rough running, and the higher combustion temperatures can stress the engine over time, so get it diagnosed soon.
If it just needs the EGR valve and passages cleaned, it can be on the lower end, around $150-250. A new EGR valve plus labor typically runs $250-500 depending on the vehicle.
It's considered moderate. It won't usually strand you immediately, but ignoring it can lead to engine knock and increased wear from higher combustion temperatures, so it shouldn't be put off.
The most common cause by far is carbon buildup clogging the EGR valve or its passages. A stuck valve, clogged tube, faulty sensor, or vacuum leak can also be to blame.