P0170–P0173 Fuel Trim Bank 1
The ECU has logged a fault on P0170. The definition below is the exact wording the standard uses for this code.
Can you drive it? The engine light will stay on and the car may run in a reduced-power mode. Read the freeze-frame data to see the conditions the fault was logged under.
Which code is which
Same fault, different cylinder or bank — diagnosed the same way. Several set together usually means one shared cause.
| Code | Definition | Severity |
|---|---|---|
P0170 | Fuel Trim Bank 1 | Investigate soon |
P0173 | Fuel Trim Bank 2 | Investigate soon |
The definition
P0170: Fuel Trim Bank 1.
P0170 is a powertrain code (the engine or gearbox), defined by the SAE standard - it means the same thing on any make of car, and any generic OBD-II reader will decode it.
What to check first
Cheapest and fastest checks first:
- Read the freeze-frame data stored with the fault - engine speed, load and temperature at the moment it logged tell you whether this happens cold, hot or under load.
- Check the connector at the part for corrosion, and that it is fully seated.
- Clear the code and see whether it returns, and under what conditions.
What actually causes this on a VAG
Fuel trim is the running correction the ECU applies to the injectors to keep the mixture right, based on what the front lambda sensor is telling it. P0170 means that correction has run out of road on bank 1 - the ECU has adjusted as far as it's allowed, in one direction or the other, and still can't get the mixture where it wants it. So it isn't a fault with one particular part; it's the symptom of something else, usually air getting into the engine that hasn't been measured, or fuel not arriving as expected. You may see it alongside a "system too lean" or "system too rich" code, which is worth noting because it tells you which way the problem runs.
Most likely causes, in order
- Unmetered air getting in (vacuum or breather leak). Split or hardened crankcase breather hoses, a failed PCV valve, a tired intake manifold or throttle body gasket, a perished brake servo hose, or on turbo engines a split intake or charge pipe will all let air in that the ECU never counted. A missing or perished oil filler cap seal or a badly seated dipstick does the same thing and costs nothing to check. Worth knowing that a leaking servo hose is not just a fuelling problem - it weakens the brake assistance too, so that one gets fixed regardless. A smoke test finds these quickly and is money well spent.
- The airflow meter has drifted. Airflow meters on these engines tend to drift out of calibration rather than fail outright, so no MAF-specific code appears while the ECU quietly fuels to a wrong air figure. A garage can compare measured airflow against what your engine should be drawing on a road test - that comparison is what confirms it. A dirty air filter, or an oiled aftermarket one, brings it on sooner.
- An exhaust leak before the front sensor. A blowing manifold gasket, a cracked manifold, a leaking downpipe joint or a split flexi pipe pulls air in ahead of the sensor. The sensor reads lean, so the ECU adds fuel that the engine never needed, and the trims run away. Common on higher-mileage cars and easy to spot by the soot trails and the noise on a cold start.
- Fuel delivery or injector problems. A tired in-tank pump, a blocked filter, or on direct-injection petrol engines a worn high-pressure fuel pump - on some 2.0 petrol engines the cam follower that drives that pump is known to wear through - can't keep rail pressure up. Leaking or partly blocked injectors cause the same complaint from the other direction. This is the expensive end of the list, so work through the cheap checks above first.
Worth knowing: Before touching anything, get someone to read the actual fuel trim figures and note which way they're going. Positive trims mean the ECU is adding fuel because it thinks the engine is lean - chase air leaks, the airflow meter and fuel supply. Negative trims mean it's pulling fuel out - look at leaking injectors and excess fuel pressure. Knowing the direction halves the work.
Reading the code is the easy part
A code names the circuit that complained. What tells you why is watching the sensor behind it while the engine runs — and that is where a code reader stops and a diagnostic tool starts.
VAGPULSE reads every module in a VW, Audi, Skoda or SEAT — not just the engine — shows the live measuring blocks behind this fault, gives you the freeze-frame conditions it was logged under, and clears it once you have fixed it. It also does what a generic OBD reader cannot on a VAG: service resets, coding, adaptations, and Stage 1–3 flashing with your original file saved first.
$249 once. No subscription, unlimited cars. Windows 10/11.
See VAGPULSE — $249 Look up another codeSoftware downloads instantly · cable ships September 30 · cable guarantee — refund if it won't connect
Prefer to work on your phone? The VAGPULSE Android app is a free download and its demo mode runs against a sample car with no hardware at all, so you can see exactly what it reads before spending anything. Reading your car with it needs the VAGPULSE cable, which comes with the licence.
Common questions
What does P0170 mean?
P0170 means Fuel Trim Bank 1. It is logged when the ECU sees this circuit outside its expected operating window.
Can I drive with P0170?
The engine light will stay on and the car may run in a reduced-power mode. Read the freeze-frame data to see the conditions the fault was logged under.