Vacuum or intake air leaks, such as cracked hoses, leaking intake manifold gaskets or a torn air intake hose (both sources). Faulty mass air flow (MAF) sensor (both sources); OBD-Codes.com calls this the most common cause, especially on Mercedes-Benz and Volkswagen, and warns that many aftermarket MAF sensors cause problems. Damaged MAF sensor wiring or connections (RepairPal), including oil getting into the MAF or oxygen sensor connectors (OBD-Codes.com). Leaking turbocharger charge-air hoses, if equipped (OBD-Codes.com). Faulty fuel pressure regulator (OBD-Codes.com). Faulty oxygen sensor (OBD-Codes.com); RepairPal warns the oxygen sensor is often replaced when the real problem is a vacuum leak or MAF sensor. Engine oil diluted with fuel (OBD-Codes.com). On Mercedes-Benz: leaking camshaft adjuster solenoids, or a cracked crankcase vent hose under the intake manifold, which OBD-Codes.com says is covered by a service recall on some models. The sources differ on the list: RepairPal names only vacuum leaks and the MAF sensor and its wiring, while OBD-Codes.com adds fuel pressure, oxygen sensors, turbo hoses and the Mercedes-specific items.

Repair vacuum or intake air leaks, including hoses and intake manifold gaskets (both sources). Replace the MAF sensor if it reads out of spec, or repair its wiring and connector (both sources); OBD-Codes.com recommends a sensor that meets original-equipment specs. Repair leaking turbo charge-air hoses (OBD-Codes.com). Repair fuel pressure or fuel pressure regulator problems found in testing (OBD-Codes.com). Fix the source of any oil leak into the wiring and replace an oil-fouled oxygen sensor (OBD-Codes.com); on Mercedes-Benz, OBD-Codes.com says a new oxygen sensor may need an adaptation with a compatible scan tool.

P0170⛽ Fuel system / Fuel trim (air-fuel mixture control)Stop and fix promptly

P0170: Fuel Trim Malfunction Bank 1 (Meaning, Causes, Fixes)

Q: What does P0170 mean?

P0170 means the engine computer has pushed its fuel trim adjustments on bank 1 to their limit and still can't get the air-fuel mixture right. Vacuum or intake air leaks and a faulty mass air flow (MAF) sensor are the most common causes. Fuel trim is how the engine computer (PCM) keeps fine-tuning the amount of fuel it injects so the mixture stays close to the ideal 14.7 parts air to 1 part fuel. RepairPal explains that the computer uses short-term and long-term fuel trim to make these corrections, and sets P0170 when it has reached its operational limit with them. OBD-Codes.com says P0170 means a malfunction in the computer's control of the air-fuel ratio, where the fuel trims reached their limit of adding fuel. The OBD-Codes.com article isn't consistent about which way: one paragraph says the computer was compensating for a real or perceived rich condition, while its later explanation says the computer was adding fuel to make up for a lean condition, real or perceived. Adding fuel normally corrects a lean (too much air) condition, which matches the vacuum-leak and MAF causes both sources list. Bank 1 is the side of the engine with cylinder number 1; P0173 is the same fault on bank 2. OBD-Codes.com notes this code shows up most on Mercedes-Benz and Volkswagen and other European cars, often alongside misfire or other fuel trim codes, while American cars more often set P0171, P0172, P0174 or P0175 for similar problems. Searches also show it on Hyundai, Honda, Toyota and Ford. It is a generic OBD-II code, but the exact setting conditions vary by make.

Q: Can I keep driving with P0170?

Stop and fix promptly. RepairPal rates P0170 Urgent, meaning it should be looked at right away. OBD-Codes.com doesn't give a rating, but lists symptoms that can include stalling right after starting, black smoke and misfires. Many cars show no obvious symptoms, but an air-fuel mixture problem can get worse and lead to other faults, so get it diagnosed soon. If the engine stalls or misfires badly, avoid driving it until it's checked.

Reviewed 2026-10-25 · Based on 2 cited source(s)

What P0170 means

Fuel trim is how the engine computer (PCM) keeps fine-tuning the amount of fuel it injects so the mixture stays close to the ideal 14.7 parts air to 1 part fuel. RepairPal explains that the computer uses short-term and long-term fuel trim to make these corrections, and sets P0170 when it has reached its operational limit with them. OBD-Codes.com says P0170 means a malfunction in the computer's control of the air-fuel ratio, where the fuel trims reached their limit of adding fuel. The OBD-Codes.com article isn't consistent about which way: one paragraph says the computer was compensating for a real or perceived rich condition, while its later explanation says the computer was adding fuel to make up for a lean condition, real or perceived. Adding fuel normally corrects a lean (too much air) condition, which matches the vacuum-leak and MAF causes both sources list. Bank 1 is the side of the engine with cylinder number 1; P0173 is the same fault on bank 2. OBD-Codes.com notes this code shows up most on Mercedes-Benz and Volkswagen and other European cars, often alongside misfire or other fuel trim codes, while American cars more often set P0171, P0172, P0174 or P0175 for similar problems. Searches also show it on Hyundai, Honda, Toyota and Ford. It is a generic OBD-II code, but the exact setting conditions vary by make.

Severity: can I keep driving?

Stop and fix promptly

RepairPal rates P0170 Urgent, meaning it should be looked at right away. OBD-Codes.com doesn't give a rating, but lists symptoms that can include stalling right after starting, black smoke and misfires. Many cars show no obvious symptoms, but an air-fuel mixture problem can get worse and lead to other faults, so get it diagnosed soon. If the engine stalls or misfires badly, avoid driving it until it's checked.

Common symptoms

Check engine light on (both sources)
Often no other noticeable symptoms (RepairPal)
Poor fuel economy (both sources)
Hesitation when accelerating (RepairPal), or hesitation or misfire at idle or under load (OBD-Codes.com)
Engine starts and then stalls (OBD-Codes.com)
Black smoke from the tailpipe (OBD-Codes.com)

Likely causes

Roughly most to least common, based on the sources below. Your vehicle may differ.

Vacuum or intake air leaks, such as cracked hoses, leaking intake manifold gaskets or a torn air intake hose (both sources)
Faulty mass air flow (MAF) sensor (both sources); OBD-Codes.com calls this the most common cause, especially on Mercedes-Benz and Volkswagen, and warns that many aftermarket MAF sensors cause problems
Damaged MAF sensor wiring or connections (RepairPal), including oil getting into the MAF or oxygen sensor connectors (OBD-Codes.com)
Leaking turbocharger charge-air hoses, if equipped (OBD-Codes.com)
Faulty fuel pressure regulator (OBD-Codes.com)
Faulty oxygen sensor (OBD-Codes.com); RepairPal warns the oxygen sensor is often replaced when the real problem is a vacuum leak or MAF sensor
Engine oil diluted with fuel (OBD-Codes.com)
On Mercedes-Benz: leaking camshaft adjuster solenoids, or a cracked crankcase vent hose under the intake manifold, which OBD-Codes.com says is covered by a service recall on some models
The sources differ on the list: RepairPal names only vacuum leaks and the MAF sensor and its wiring, while OBD-Codes.com adds fuel pressure, oxygen sensors, turbo hoses and the Mercedes-specific items

Diagnostic steps you can do

⚠ Before you start

Work on a cool engine, parked on level ground with the parking brake set. Never work under a car supported only by a jack.

Read all stored codes and freeze-frame data. Note any misfire codes or other fuel trim codes (P0171 to P0175), and fix those along with P0170.
Check for recalls and technical service bulletins for your vehicle. On some Mercedes-Benz models, OBD-Codes.com says there's a recall for the crankcase vent hose under the intake manifold.
Inspect all vacuum hoses, the intake air hose and, on turbo engines, the charge-air hoses for cracks, tears or loose connections (both sources point to air leaks).
Check the MAF sensor connector for oil or corrosion. With a scan tool, compare the MAF reading in grams per second against the spec for your engine; OBD-Codes.com gives example figures for a 1.8-liter Mercedes of about 3.5 to 5 g/s at idle and 9 to 12 g/s at 2,500 rpm with no load.
If the MAF and hoses check out, have the fuel pressure tested and check the regulator for leaks (OBD-Codes.com).
Inspect the oxygen sensor connectors for oil intrusion before replacing an oxygen sensor; RepairPal says replacing the oxygen sensor when the real cause is a vacuum leak or MAF sensor is a common mistake.

Typical fixes

Repair vacuum or intake air leaks, including hoses and intake manifold gaskets (both sources)
Replace the MAF sensor if it reads out of spec, or repair its wiring and connector (both sources); OBD-Codes.com recommends a sensor that meets original-equipment specs
Repair leaking turbo charge-air hoses (OBD-Codes.com)
Repair fuel pressure or fuel pressure regulator problems found in testing (OBD-Codes.com)
Fix the source of any oil leak into the wiring and replace an oil-fouled oxygen sensor (OBD-Codes.com); on Mercedes-Benz, OBD-Codes.com says a new oxygen sensor may need an adaptation with a compatible scan tool

Repair costs vary widely by vehicle, region, parts brand and shop, so we don't quote prices. Get a written estimate after diagnosis.

🛠 When to call a pro

You can't find a vacuum leak by looking; a shop can use a smoke test to locate it
The fuel pressure needs testing, which means working on the pressurized fuel system
The code comes with misfire codes, stalling or black smoke
A Mercedes-Benz needs a brand-compatible scan tool for sensor adaptations or recall work

Where to buy

Plain search links. Always match parts to your exact year, make, model and engine.

OBD2 scanners on Amazon Parts at Advance Auto Parts catalog at RockAuto

FAQs

Is P0170 serious?

RepairPal rates it Urgent. Many cars run normally with it, but it means the engine computer can't control the air-fuel mixture properly, and OBD-Codes.com lists stalling, misfires and black smoke as possible symptoms, so get it diagnosed soon.

What's the difference between P0170 and P0171?

Both involve fuel trim on bank 1. P0170 is a general fuel trim malfunction, set when the computer reaches its correction limit, while P0171 specifically means the system is too lean. OBD-Codes.com says the setting conditions are very similar and that European cars tend to set P0170 where American cars set P0171.

Should I replace the oxygen sensor for P0170?

Not first. RepairPal says a common mistake is replacing the oxygen sensor when the real problem is a vacuum leak or a bad MAF sensor. Check for air leaks and test the MAF sensor before replacing an oxygen sensor.

How much does it cost to fix P0170?

Neither source gives a repair cost. It depends on the cause, from a cracked hose to a MAF sensor or fuel system repair, so get a diagnosis or a written estimate first.

Sources

Last reviewed 2026-10-25. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.