P2178

System Too Rich Off Idle (Bank 1)

Car Fault Codes Fuel & air

Can you still drive? With care, short trips only

Who can fix it? Needs a workshop

Typical repair time 30 minutes – 3 hours

Typical cost SAR 100 to 2,500

What it means

The engine computer has found that bank 1 runs rich while driving at higher RPM, and it has cut fuel as far as it can without correcting the mixture. This usually points to excess fuel or the MAF sensor over-reading.

Symptoms4

Check engine light on.
Noticeably higher fuel consumption.
Fuel smell from the exhaust or sometimes black smoke.
Weakness or stumbling on acceleration.

Possible causes6

Mass airflow (MAF) sensor over-reading because it is contaminated or faulty.
High fuel pressure from a faulty pressure regulator.
One or more leaking injectors.
EVAP purge valve stuck open.
Upstream oxygen sensor biased toward lean, so the computer adds extra fuel.
Faulty coolant temperature sensor making the computer think the engine is cold.

How to diagnose5

Watch short- and long-term fuel trims while driving; large negative values under load confirm the rich condition.
Compare the MAF reading with the expected value for the engine size.
Measure fuel pressure and check that it holds after shutdown.
Test the EVAP purge valve and confirm it does not pass vacuum when closed.
Compare the coolant temperature sensor reading with the actual engine temperature.

How to fix5

Clean or replace the MAF sensor if it over-reads.
Replace the EVAP purge valve if it is stuck open.
Replace the coolant temperature sensor or oxygen sensor if proven faulty.
Repair the fuel pressure regulator or the leaking injector.
Clear the codes and confirm the trims return to normal while driving.

Repair cost

0–500

500–2,000

2,000–5,000

5,000+

In Saudi riyals; the highlighted part is the expected range for this fault.

The cost is low for a MAF cleaning or purge valve, and rises when injectors or a pressure regulator are needed.

Prices are rough estimates for common cars at independent workshops and vary with the car and the source of the part.
Prices last updated: September 2026

Frequently asked questions

Does a rich mixture harm the engine?

Yes, over time. It raises fuel use, fouls the spark plugs and overheats the catalytic converter, which can destroy it.

Can a clogged air filter cause it?

On modern cars that measure air with a MAF sensor, a clogged filter rarely causes a rich condition, but check it and replace it if it is very dirty.

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This guide is for information only and does not replace an inspection by a qualified technician. If the fault involves brakes, steering, engine temperature, electrics or refrigerant, do not attempt the repair yourself.

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