P2178 is the rich-side limit of the bank 1 load adaptation: above idle the long-term trim has to take away more fuel than allowed. GM's Bosch-based V6 engines of 2005 and 2008 set it below a factor of 0.78, the 2023 and 2024 tables below 0.77, each after two trips. Saab's 9-5 2.0T uses the same window as for the lean code, 6.9 to 41.7 g/s, 18 to 80 % load and 1280 to 4000 rpm, and wants 4 s without break or 50 s in total. VW's 2014 CC 2.0 TSI accepts down to -21 %. A rich error that grows with load usually means an air mass sensor reading too high, excessive fuel pressure or injectors delivering too much. Mercedes saw it on the M254 with fuel boiling in the low-pressure side; Jaguar Land Rover lists P2178 with P0191 on Ingenium petrol engines and cured it with new PCM calibration.
Medium severity. Can I keep driving?You can drive in moderation for a short time. A very lean or very rich mixture can damage the engine or the catalytic converter.
Bank 1 is the side of the engine with cylinder 1. Inline engines only have this bank.
Symptoms
- Check engine lamp on after two trips
- Higher fuel consumption, fuel smell
Most likely causes
- Air mass sensor reading too high
- Fuel pressure too high or injectors over-delivering
- Calibration error fixed by a software update
- Fuel vapour from purge or boiling fuel in the low-pressure system
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: medium
- It affects emissions.
What each character of P2178 means
- P: Powertrain: engine and transmission
- 2: Generic code: same on every make
- 1: Air and fuel
- 78: Specific fault in that system
Frequently asked questions
Can I keep driving with code P2178?
You can drive in moderation for a short time. A very lean or very rich mixture can damage the engine or the catalytic converter.
What causes code P2178?
The most common causes are: air mass sensor reading too high; fuel pressure too high or injectors over-delivering; calibration error fixed by a software update.
Related codes
- P2177System Too Lean Off Idle (Bank 1)
- P2180System Too Rich Off Idle (Bank 2)
- P2188System Too Rich at Idle (Bank 1)
- P0172System Too Rich Bank 1
- P0101Mass or Volume Air Flow Sensor A Circuit Range/Performance
Sources
- saabwisonline.com: dtc p2178 or p2188
- gsi.ext.gm.com: 08 GRP02 LLT Engine
- gsi.ext.gm.com: 24OBDG07
- static.nhtsa.gov: MC 10119356 9999
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.