P2178: Mixture too rich away from idle (bank 1)

What P2178 means

Above idle, the ECU has to keep taking fuel away on bank 1 to hold the right mixture. When that correction reaches its limit and there is still too much fuel, it stores this code. Bank 1 is the side of the engine that has cylinder 1.

Most common causes

• Dirty or faulty mass air flow sensor reading less air than is really going in
• An injector that drips or does not close fully
• Fuel pressure too high because of a faulty regulator
• Upstream oxygen sensor giving a false reading
• Evaporative purge valve (fuel vapour canister) stuck open
• Coolant temperature sensor that always reads cold

Symptoms

• Engine warning light on
• Higher fuel consumption than usual
• Fuel smell from the exhaust
• Hesitation or flat response when accelerating

Can you drive with P2178?

With care The car can be driven, but running rich burns more fuel and can damage the catalytic converter over time. Get it checked soon.

How to check it

Watch bank 1 long term fuel trim (LTFT) live at around 2,500 rpm: a strongly negative figure means the ECU is pulling a lot of fuel. Note the air flow reading too, it helps the workshop.

How to read and clear P2178 with Final OBD

Plug an OBD2 scanner in under the steering wheel, switch the ignition on and open Final OBD. Under Faults you’ll see P2178 with this same explanation and whether it’s confirmed, pending or permanent.

With Free you read this code with its short description for nothing, and clear it if the engine stores it over standard OBD. With Pro you get the full explanation, how the car was running when it was logged (revs, temperature, load) and clearing in every control unit.

Clearing the code only turns the light off: if the cause is still there, it comes back. Clear it once it’s fixed, and not right before the ITV. It’s all in how to read and clear fault codes and in the ITV and OBD.

Similar codes

• P2135 Throttle or pedal position sensors A and B voltage mismatch
• P2138 Throttle or pedal position sensors D and E voltage mismatch
• P2146 Fuel injector group A supply voltage circuit or open
• P2177 System too lean off idle (bank 1)
• P2179 Mixture too lean away from idle (bank 2)
• P2187 Mixture too lean at idle (bank 1)
• P2188 Mixture too rich at idle (bank 1)
• P2189 Mixture too lean at idle (bank 2)
• P2100 Throttle actuator A control motor circuit open
• P2101 Throttle actuator A control motor circuit range or performance
• P2102 Throttle actuator A control motor circuit low
• P2104 Throttle control system: forced idle

P2178 is a generic code, so it means the same on any make, for example on Abarth, Cupra, Hyundai, Jeep, Mini, Seat. See what Final OBD does on your car in makes and models.