Poor Fuel Economy – Codes That Hurt MPG

A sudden drop in MPG often points to a rich condition, stuck-open thermostat (engine stays cold), failing O2 sensors, or dragging brakes — but codes help narrow it down.

What To Do Next

Scan for codes, check thermostat operation (engine should reach normal temp), and review long-term fuel trims.

Practical Tips

Compare MPG over a full tank, not one short trip.

Cold thermostat = rich mixture = worse economy.

Soft brake drag can also kill MPG without a code.

Common Related Codes

These codes frequently appear with this symptom. Click for causes, cost estimates and repair steps.

Code	Description	Severity
P0420	Catalyst System Efficiency Below Threshold (Bank 1)	Med
P0128	Coolant Thermostat (Temperature Below Regulating Temperature)	Low
P0172	System Too Rich (Bank 1)	Med
P0442	Evaporative Emission System Leak Detected (Small Leak)	Low
P0135	O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1)	Med
P0507	Idle Air Control System RPM Higher Than Expected	Med

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