P2178 fault code: System Too Rich Off Idle

P2178 means “System Too Rich Off Idle” — an extended powertrain range fault in the powertrain. We do not yet publish verified symptoms, causes or repair steps for this code — treat an engine management light seriously and book a garage if you are unsure.

Can I DIY?

Not from this page. P2178 has no verified safety grade here yet, so we publish no DIY checks, repair steps or tool guidance for it. Treat an engine management light seriously and have the fault confirmed by a qualified garage.

Stop when it is safe to do so and arrange recovery if the engine management light is flashing, the car judders, overheats or loses power, or you can smell fuel or exhaust fumes inside the car.

P21xxExtended powertrain range
MOT fail riskif the warning light stays on

What does P2178 mean?

P2178 is a powertrain code from the extended SAE range reported by your car’s engine control unit. It sits in the P21xx block, which covers extended powertrain range. The stored description is “System Too Rich Off Idle”. The code itself tells you which circuit or system tripped the fault — not always the exact part that failed.

Will P2178 fail an MOT?

An illuminated engine management light is a Major defect at MOT since 2018 — an automatic fail until the fault is repaired and the light stays off. Clearing the code without fixing the cause is not a repair.

Before booking your test, check what usually fails on your car: see our MOT pass rates by make and model, built from 33.1 million DVSA tests.

Related guides

Car diagnostic test cost: compare the scopeUK booking marketplaces currently publish diagnostic averages of about £52–£65. Engine management light on: steady or flashing?A steady engine management light means the computer has stored a fault. Drive with caution and get the code read. A flashing light is different — stop when it is safe.
P2100Throttle Actuator Control Motor Circuit/Open P2101Throttle Actuator Control Motor Circuit Range/Performance P2102Throttle Actuator Control Motor Circuit Low P2103Throttle Actuator Control Motor Circuit High P2104Throttle Actuator Control System - Forced Idle P2105Throttle Actuator Control System - Forced Engine Shutdown P2106Throttle Actuator Control System - Forced Limited Power P2107Throttle Actuator Control Module Processor

P2178 — quick answers

Will P2178 fail my MOT?

An illuminated engine management light is a Major defect at MOT since 2018 — an automatic fail until the fault is repaired and the light stays off. Clearing the code without fixing the cause is not a repair.

Fault-code identifiers are standard diagnostic facts. Their explanations and diagnostic guidance on this site are original editorial summaries, not reproduced SAE wording. MOT defect rules reflect the DVSA MOT inspection manual (2026). This page is general guidance for UK drivers, not a substitute for a professional diagnosis.