P0400 on Mazda
Exhaust Gas Recirculation (EGR) Flow Malfunction
P0400 on Mazda vehicles indicates exhaust gas recirculation (egr) flow malfunction. Au7o has documented this code across 1 Mazda model — most commonly on MX-6. P0400 is a generic powertrain code indicating the engine computer detected a problem with exhaust gas recirculation (EGR) flow — either too much, too little, or incorrect flow compared to what was commanded. The EGR system routes a small amount of exhaust back into the intake to lower combustion temperatures and reduce emissions. A fault can cause rough idle, pinging/knock, increased emissions, and failed emissions testing, though it is generally not an immediate driving hazard. Typical repair costs on Mazda range from $75 to $350, depending on the specific model and root cause.
Common Causes of P0400
- •Clogged or carbon-blocked EGR passages
- •Faulty EGR valve (stuck open or closed)
- •Faulty EGR position or DPFE/flow sensor
- •Vacuum or electrical control fault to the EGR valve
- •Blocked or cracked EGR tube
- •Wiring/connector issues
- •PCM fault (rare)
P0400 on Mazda by Model
Mazda MX-6(1 issue)
- EGR Valve Carbon Buildup Causing Rough Idle and P0400 Codes1996-1997
The KL-DE V6 EGR system is highly prone to carbon clogging. Soot accumulates inside the EGR valve diaphragm and inside the EGR passages cast into the throttle body and intake. When the valve sticks open, exhaust gas enters at idle and produces a rough, surging, or stalling idle with a CEL. Owners report that on the MX-6, P0400 (EGR flow insufficient) is the most common emissions code, and approximately 95% of the time it is a clogged valve or completely blocked EGR ports in the throttle body, not a failed component. Access is poor: the upper intake plenum and throttle body must usually come off to clean it properly.
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View P0400 across all makes →Frequently Asked Questions
What does P0400 mean on Mazda?▼
P0400 stands for "Exhaust Gas Recirculation (EGR) Flow Malfunction." P0400 is a generic powertrain code indicating the engine computer detected a problem with exhaust gas recirculation (EGR) flow — either too much, too little, or incorrect flow compared to what was commanded. The EGR system routes a small amount of exhaust back into the intake to lower combustion temperatures and reduce emissions. A fault can cause rough idle, pinging/knock, increased emissions, and failed emissions testing, though it is generally not an immediate driving hazard. On Mazda specifically, this code is documented across 1 model.
What causes P0400 on Mazda vehicles?▼
Common causes on Mazda: Clogged or carbon-blocked EGR passages, Faulty EGR valve (stuck open or closed), Faulty EGR position or DPFE/flow sensor, Vacuum or electrical control fault to the EGR valve, Blocked or cracked EGR tube. Specific causes vary by model and year — see the per-model sections below.
How much does it cost to fix P0400 on a Mazda?▼
Repair costs on Mazda range from $75 to $350, depending on the specific model and root cause.
Which Mazda models have P0400 documented?▼
Au7o has documented P0400 on 1 Mazda model: MX-6.