P0170 — Fuel Trim Bank 1 | ErrorCodeFinder

What Does P0170 Mean?

P0170 is a diagnostic trouble code indicating: Fuel Trim Bank 1. This code relates to the fuel and air metering system. It has been reported in NHTSA complaints across 2 different vehicle models.

Fuel trim is the ECU's real-time adjustment to fuel delivery. Short-term fuel trim (STFT) reacts immediately to O2 sensor feedback, while long-term fuel trim (LTFT) is a learned correction stored in memory. When LTFT exceeds approximately ±25%, the ECU sets a fuel trim code because it's run out of adjustment range. Positive fuel trim means the ECU is adding fuel (lean condition), negative means it's subtracting fuel (rich condition). Understanding fuel trims is key to diagnosing these codes — they tell you the direction and magnitude of the problem.

Fuel trim is the ECU's real-time adjustment to fuel delivery. Short-term fuel trim (STFT) reacts immediately to O2 sensor feedback and fluctuates constantly. Long-term fuel trim (LTFT) is a learned correction stored in memory that compensates for gradual changes like wear and altitude. When LTFT exceeds approximately ±25%, the ECU has run out of adjustment range and sets a fuel trim code. Understanding the difference between STFT and LTFT is key: STFT shows what's happening NOW, LTFT shows the average correction over many drive cycles. At idle, high positive fuel trims suggest vacuum leaks. At cruise, high positive trims suggest fuel delivery issues (pump, filter, pressure). Common affected vehicles: Any vehicle — fuel trim codes are universal and affect all makes equally.

**Real-World Diagnostic Walkthrough:** P0170 is relatively rare compared to P0171/P0172 because it's set when the ECU detects that fuel trim has exceeded its normal range but can't determine the direction. This usually happens during specific test conditions the ECU runs periodically. The diagnostic approach is the same as for lean or rich codes: check fuel trims with a scan tool. STFT and LTFT values tell you the direction. Positive = lean (vacuum leak, weak fuel pump, dirty MAF). Negative = rich (leaking injector, stuck-open purge valve, faulty O2 sensor). If fuel trims are normal when you check, the code may have been set during a transient condition — clear it and monitor.

Symptoms of P0170

⚠️ check engine light
⚠️ poor fuel economy
⚠️ rough idle
⚠️ hesitation
⚠️ possible misfires

Common Causes

🔍 Vacuum leak
🔍 Dirty MAF sensor
🔍 Faulty O2 sensor giving incorrect readings
🔍 Fuel pressure issue

P0170 Reports by Year

2009

1

2011

1

Real Owner Reports

From NHTSA complaint database — actual owner descriptions.

"DURING ALL OF THE 5 INCIDENTS IN 2009 THE ENGINE MISFIRED VIOLENTLY. AT TIMES 35MPH WAS MAX SPEED. CRONIN KIA HAD MY VEHICLE OVER 30 DAYS IN 5 VISITS IN 2009. THEY HAVE REPLACED; TWO CAT. CONVERTERS, THE IGNITION COIL PACK AND SPARK PLUGS, A LEAKING FUEL INJECTOR AND ASSOCIATED WIRING. PLEASE NOTE"
— Kia Sportage owner, 03/07/2009

"AFTER HAVING THE CHECK ENGINE LIGHT COME ON, WE TOOK THE CAR TO THE DEALERSHIP WE PURCHASED IT FROM TO PERFORM A DIAGNOSTIC AFTER RECEIVING CODES P0170 AND P0173 (MB CODES 2092/2096) WHICH MB CLASSIFIES AS "MIXTURE IN RIGHT / LEFT CYLINDER BANK IS TOO RICH WHEN IDLING" AND WERE TOLD WE WOULD NEED TO"
— Mercedes-Benz C-Class owner, 12/21/2011

Source: NHTSA Complaints Database

Most Reported Vehicles for P0170

Based on NHTSA owner complaint data.

#	Vehicle	Reports
1	KIA SPORTAGE
2	MERCEDES-BENZ C-CLASS

Diagnostic Tips

Read STFT and LTFT at idle AND at 2500 RPM. If both are high at idle but normal at higher RPM, suspect a vacuum leak. If both are high everywhere, suspect fuel delivery (pump, filter, pressure).
Smoke test the intake system — this is the fastest way to find vacuum leaks that cause lean fuel trims.
Check fuel pressure with a gauge. Low pressure causes lean codes; high pressure causes rich codes.
If only one bank shows abnormal fuel trims, the cause is likely isolated to that bank (O2 sensor, exhaust leak, injector on that bank).

Common Mistakes to Avoid

❌ Replacing O2 sensors based on fuel trim codes — the sensors are correctly reporting the problem, not causing it.
❌ Not checking fuel trims before starting repairs — fuel trim data tells you exactly which direction to investigate.
❌ Clearing codes without fixing the root cause — LTFT relearns within a few drive cycles and the code returns.

Frequently Asked Questions

What is fuel trim?

Fuel trim is the ECU's adjustment to the base fuel injection amount. Positive (+) means adding fuel (engine running lean). Negative (-) means removing fuel (engine running rich). Normal range is ±5%.

Is P0170 the sensor or the engine?

It's the engine — P0170 means the ECU has been compensating for a rich or lean condition beyond its normal range. The fuel system has a problem that needs fixing.

What are normal fuel trim values?

Normal STFT and LTFT should be within ±5% at idle and cruise. Values up to ±10% are acceptable. Beyond ±15% indicates a problem. Beyond ±25% triggers a fuel trim code.

How do I read fuel trim data?

Use any OBD-II scan tool with live data capability ($20-$100). Look for 'Short Term Fuel Trim' (STFT) and 'Long Term Fuel Trim' (LTFT) for each bank. Positive = adding fuel (lean). Negative = removing fuel (rich).

What To Do Next

Possible Fixes

🔧 Clean MAF sensor
🔧 Repair vacuum leaks
🔧 Replace O2 sensor if faulty

1
Find your vehicle above
Click your make and model for real owner reports and common causes specific to your vehicle.
2
Check for recalls
Go to nhtsa.gov/recalls and enter your VIN. Free repairs if covered.
3
Get a professional diagnosis
A code alone doesn't identify the exact failed part. A diagnostic ($50–$150) pinpoints the root cause.

P0170: Fuel Trim Bank 1