Rick's Free Auto Repair Advice

P0172 — system too rich

P0172 System too Rich

How to diagnose a P0172 system too rich condition

Start by checking live data short and long term fuel trims. A high long-term fuel trim, above 10%, means the computer is adding a lot of fuel. A negative long-term fuel trim means the computer is subtracting fuel and still can’t keep the air/fuel mixture in balance.

Why the ECM might have to add fuel

1) Vacuum leak or un-metered air is entering the engine causing a lean condition in the exhaust.

The ECM see this lean condition from the upstream oxygen sensor and adds more fuel to compensate.

Most common causes:
Poor connection of air duct to air filter box or throttle body
Tear in the flexible air duct between the filter box and throttle body
Disconnected vacuum line
Cracked or broken vacuum line
Vacuum leak around intake manifold gasket

2) Exhaust Leak(s)

Unmetered air entering the exhaust will cause the upstream oxygen sensor to read lean. The ECM will respond by adding fuel. Long term fuel trim will be high.

Most common causes

Gasket deterioration at clamp joints
Failed flex pipe or rust through in exhaust pipes ahead of the upstream oxygen sensor

3) Incorrect Mass Airflow Reading is overstating the amount of air entering the engine.

To compensate, the ECM adds fuel.

Most common causes

Dirty MAF sensor. Use MAF sensor cleaner. Monitor MAF sensor readings after cleaning to see if they’re within acceptable range for the engine size.

4) Fuel pressure too high or fuel injector is leaking

In this case, the ECM would show a negative long term fuel trim as it’s trying to cut fuel.

Most common causes

Bad fuel pressure regulator
Leaking fuel injector(s)

5) Worn spark plugs/wires causing incomplete combustion

In this case, the upstream oxygen sensor is showing rich and the ECM responds by cutting fuel. To set a P0172, the ECM would show a negative long term fuel trim. However, this would normally show misfires codes.

Most common causes

Worn spark plug or wires

6) Fault in cooling system

A stuck open thermostat or constantly running radiator fans would cause the engine to run cold. The engine coolant temp sensor would report the cold engine and the ECM would provide a rich mixture based on the cold engine. In late model vehicles, this would most likely set a P0128 thermostat operation code.

Common mistakes when diagnosing the P0172 code

A technician may not inspect the coolant temperature sensor and cooling system. When the engine is cold, more fuel is added to help warm it up to a desirable temperature. If the coolant temperature sensor is not sending the proper signal there is a possibility that the engine ECU may assume that it is always running cold.

How serious is the P0172 code and can you keep driving with this code?

• A constant P0172 can overheat the catalytic converter, causing permanent and expensive damage.
• Your gas mileage will suffer
• The rich mixture and cause carbon buildup problems in the cylinder.

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat

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