The ECU has detected a problem with the MAF sensor signal — it's outside the expected range.
P0100 — also written PO1OO, P 0100, 0100 — is the OBD-II code for mass air flow (maf) circuit malfunction. Same code, same fix: this page covers every spelling of it.
MAF (Mass Air Flow) sensor circuit malfunction — the hot-wire element is either fouled, unplugged or getting no reference voltage. On UK diesels a proper £8 MAF clean fixes this six times out of ten before you buy any parts. On petrol turbos a P0100 alongside oil-mist inside the intake usually means the crankcase breather (PCV) is stuffing oil onto the hot-wire — clean the sensor without fixing the PCV and the code returns inside a fortnight.
P0100 is a sensors fault code, typically costing £20–£300 to put right in a UK garage.
What P0100 actually feels like to drive
- Black smoke on diesel acceleration (over-fuelling because ECU can't trust the airflow reading)
- Rough or 'hunting' idle at traffic lights — RPM wanders between 700 and 950
- Hesitation off the mark, then a sudden shove of power as the ECU falls back to a default air-map
- MPG drops 5–10 in mixed driving — the ECU can't run closed-loop trims without valid MAF data
- Engine surges at steady 50–60 mph motorway cruise
- Sometimes resets itself after key-off / key-on and drives normally for a day
- Often stored alongside P0101 (range) or P0102 (low input) — same root cause
What causes P0100?
- Dirty MAF sensor (very common)
- Failed MAF sensor
- Wiring damage to sensor connector
- Air leak between MAF and throttle body
- Blocked air filter
UK vehicles that throw P0100 most often
- Ford Focus / Fiesta / C-Max 1.6 & 1.8 TDCi (PSA DV6) — oil-fouled MAF from tired EGR
- Vauxhall Astra J / Corsa D / Insignia 1.7 & 2.0 CDTi (A17DTR / A20DTH) — carbon-caked hot-wire
- VW Golf / Audi A3 / Skoda Octavia 1.9 & 2.0 TDI (BKD / CBAB / CAGA) — Bosch HFM6 sensor drift
- Nissan Qashqai / Juke 1.5 & 1.6 dCi — MAF genuinely fails around 90k, oil-fouled from PCV
- Citroën / Peugeot 207 / 308 / 3008 1.6 HDi (DV6) — DV6 pattern MAF fouling
- Mercedes C220 / E220 / Sprinter 2.1 CDI (OM651) — MAF plug corrosion, water ingress
- BMW 118d / 320d N47 — MAF loom chafed on exhaust manifold heat shield
How P0100 is fixed
- Clean MAF with dedicated cleaner spray
- Replace MAF sensor
- Inspect wiring and connector
- Replace air filter
The P0100 misdiagnosis that wastes money
P0100 is the code where owners get sold a £180 MAF sensor when the real fault is upstream. Bob's 32 years on the tools rule for P0100: NEVER fit a new MAF sensor until you've done three tests in this order. One — pull the MAF connector, key on engine off, back-probe the 5V reference wire with a multimeter. Missing or low voltage means chafed loom, blown ECU fuse, or corroded ECU pin — the sensor is fine and a new one won't fix it. On BMW N47 the loom chafes on the exhaust manifold heat shield within reach of the sensor plug; that's a £45 loom repair, not a £180 sensor. Two — pull the sensor out and inspect the hot-wire. Any oil film, any dust bridge, any carbon on the platinum wire means the crankcase breather / PCV is dumping oil upstream. Clean the sensor with proper CRC MAF cleaner (never brake cleaner, never carb cleaner — either kills the hot-wire instantly), then investigate the PCV. On 1.6 HDi the PCV is inside the rocker cover and cleaning the MAF without a new rocker cover assembly (£240–£380) means the code returns. Three — inspect the entire intake tract for splits, torn boots, and loose worm-drive clamps between the air filter and the turbo. Any leak downstream of the MAF causes P0100 because unmetered air enters the engine.
What a UK garage should charge for P0100
UK independent garage: diagnostic + live MAF g/s reading + reference voltage test £45–£80. MAF sensor DIY clean (a can of CRC MAF cleaner £8, 15 minutes on the driveway) — worth trying first on any diesel over 60k. Workshop MAF clean £25–£50. MAF connector clean and pin-straighten £25–£70. MAF harness repair (splice, heat-shrink, reroute away from exhaust) £45–£120. Genuine Bosch or Denso MAF sensor £90–£220 fitted (never fit an unbranded eBay MAF — reads inaccurate from day one). Air filter £15–£40 fitted (always replace at the same time). EGR clean or EGR software recalibration £120–£280. Full 'EGR + MAF + intake' clean on diesel £180–£320 — clears the majority of high-mileage diesel P0100 without any new sensor. Rocker cover / PCV assembly (1.6 HDi, 1.4 TSI) £240–£420 fitted.
P0100 repair cost in the UK
Most P0100 repairs land between £20–£300 including parts and labour. The spread is wide because the cheap fix (a sensor, a hose, a leak) and the expensive fix (a major component) throw the same code — which is why a proper diagnosis before parts are bought saves the most money.
Bob's tip on P0100
Bob's real workshop shortcut for P0100 in 20 minutes: on any UK diesel over 60,000 miles, unplug the battery for 15 minutes to reset adaptations, then pull the MAF (single Torx or plastic clip, five minutes), spray the platinum hot-wire GENTLY with a proper MAF cleaner from a distance of 6 inches — never scrub, never touch the wire — and let it air-dry for 20 minutes before refitting. Six out of ten UK diesels with P0100 clear permanently from this alone. Never use brake cleaner, carb cleaner, contact cleaner or WD-40 — the hot-wire element is fragile and the wrong chemical destroys it instantly. On a Ford Focus 1.6 TDCi that stores P0100 alongside low MPG and slight black smoke, the fault is nearly always the swirl-flap motor and a partially caked intake manifold — cleaning the MAF alone gets you a week of relief. On a VAG BKD/CBAB engine, if the MAF reads under 300 mg/stroke at 2,500 RPM full load, the sensor is genuinely worn and needs replacing with a Genuine or Bosch part. And if the code returns within 500 miles of a clean or replacement, the fault is not the sensor at all — it's upstream (torn boot, leaking intercooler, split turbo hose) or the PCV is oiling the sensor. If you're stuck between 'clean vs replace vs deeper investigation', <a href="/ask-a-mechanic-uk?code=P0100">Ask Bob about your P0100</a> for £14.99 with your make, model, mileage and any paired codes — you'll get a straight answer before you spend £180 on the wrong part.
Can you drive with P0100?
Yes, but you'll see rough running, poor MPG and sometimes stalling.
Urgency: medium.