What Is Fuel System 1 and Fuel System 2 Error Code? A Full Guide

Fuel systems are lauded as the blood of any car model, delivering power and fuel to foster smooth control.

Only total novices will ignore their malfunction signals and error codes; unless you wish premature death for the car, immediate treatments are – and should always be – among your top priorities.

So what is fuel system 1 and fuel system 2‘s most common error code, and how should you troubleshoot them?

This burning question will be addressed properly in Bryan’s insightful article. Keep scrolling.

What Is The Fuel System 1 and Fuel System 2 Common Error Codes?

Fuel System 1 and Fuel System 2

P0172 is the most widespread code for Fuel System 1, often translated as “Systems Too Rich.” In short, more gasoline than the required level is detected in your car’s exhaust gasses.

The fuel-air ratio suffers from misbalance risks, with too little oxygen and too much gas.

And how does the P0172 fault code show up?  

An ECU uses instruments such as MAF (mass airflow sensors), MAP (manifold absolute pressures), and oxygen sensors to track an engine’s air/fuel ratio.

In most passenger cars, the sensors take charge of all measurements, calculating carbon monoxide and oxygen levels in your exhaust gasses.

14.7:1 is likely the best ratio for automobiles to date, delivering the highest output on absolutely low fuel consumption.

Of course, it might slightly alter when “richness” issues occur; however, once the compensation margin goes beyond the scope limit, a P01272 error code will be set.

On the other hand, Code P0174 (Systems Too Lean) in your Fuel System 2 indicates severe lean conditions detected by ECM – the Engine Controls Module.

Either a lack of fuel or air in the fuel-air ratio has been the major culprit behind this dilemma.

Some might argue the ECMs are already armed with adjustment capabilities to return “lean” to “normal.”

While these people are not wrong, remember that the adjustable amounts are mostly small/mediocre.

So if your unbalanced ratio demands more compensation than the ECM can manage, code P0174 is inevitable.

What Is Fuel System 1 and Fuel System 2 Error Causes? How to Fix Them 

Faulty sensors, regulators, valves, and stuck thermostats are common triggers behind the P0172 code for Fuel sys 1 closed .

On the other hand, vacuum leaks and loose caps prompt P0174 to occur on Fuel system 2.

Fuel System 1 Status CL (P0172)

fuel system 1 status cl
Fuel System 1

Faulty or Dirty MAP/ MAF Sensor

MAP and MAF sensors – often installed near the intake manifold – are critical to the electronic injection system.

Their focal goal is to help your ECM determine the necessary fuel deliveries to achieve a golden ratio (14.7:1).

Unfortunately, these two are vulnerable to varied contamination sources, such as sensor debris, air, or dirt. Their operation caused them to overstate the air calculation and trigger P0172.

Aside from the code itself, keep an eye on other symptoms: 

  • Flickering check engine lights
  • Black exhaust smokes 
  • Bad fuel lines
  • Bad acceleration
  • Rough idle

Replacing the sensors (if they are faulty) or cleaning their grime buildup should fix the issue.

Also, keep in mind some things to do after the MAF sensor replacement to ensure its utmost performance.

Bad Fuel Pressure Regulators 

A pressure regulator circulates fuel pressure (about 35 to 65 PSI) to and from the injectors, ensuring the vehicle maintains peak fuel economy and power.

Once these regulators go wrong, fuel pressure is pushed beyond the recommended limit and leaves a rich fuel-air ratio.

Two common root causes behind faulty regulators are ruptured diaphragms and stuck caps, manifesting in the following signals:

P0172 code (or a fuse of P0089 and P0172) 

  • Bad engine performance
  • Soot on the spark plugs 
  • Black smoke
  • Fuel spillage in the vacuum line 

The solution is simple: buy a new regulator.

Bad Oxygen Sensors 

Another place to investigate your P0172 code is the oxygen sensors (or upstreams), a device designed to maintain balanced engine ratios to foster optimal emissions, fuel economy, and power.

Defective O2 sensors – instead of doing their intended job – will detect wrong air amounts and return them to the ECUs.

The fuel-air ratio fails to comply with the standard, leading to too-rich systems.

Not want to risk further car damage? Watch out for these pointers: 

  • P0172 and P0130/P0135 (faulty oxygen sensor codes)
  • Blinking check engine light
  • “Lazy” voltage chart
  • Poor mileage
  • Smoke and smell from the exhausts.

Again, replacements are the most straightforward (and most effective) strategy here.

Stuck-Open Exhaust Recirculation Valves 

Lesser-known than other compartments, but EGR is still among the most important elements for a fuel system status cl.

It reduces NOx (automotive nitrous oxide) emissions, taking small exhaust gas quantities through your intake manifold to the combustion chambers.

After some time, the valve might become stuck open due to thickened buildups of carbon deposits.

The exhaust gasses take over the air, dramatically decreasing oxygen in the fuel-air ratio. Equilibrium is officially broken!  

Accidents and power loss will be on the horizon with a stuck EGR, so double-check your car to see whether it experiences: 

  • P0402 and P0172 codes triggered
  • Rough idle
  • Poor performance 
  • Exhaust gas smell 
  • Increased energy consumption 
  • More emissions
  • Engine knock

Purchasing a new EGR valve should not cost you too much. Still, for tight-budget drivers, another acceptable alternative is to eliminate the internal build-up.

Malfunctioning Coolant Temp Sensors

As the name suggests quite clearly, these sensors assess and calculate the coolant’s temperature.

Inaccurate data (due to faulty sensors) gives off the wrong impression that your engine bay is still freezing, prompting the PCMs to keep their rich mixtures even after proper operating temperatures have already been reached.

Such issues often go hand in hand with: 

  • P0118/P0117/P0116 and P0172 codes activated
  • Check engine lights turned on
  • Overheating issues in the engine
  • Poor engine performances
  • Bad fuel efficiency
  • Smoke from exhaust pipes 

To confirm and settle the matter, you can also perform a quick test to compare the temperature readings between the sensors and the thermal gun.

If the gun number differs from your scan tool (more than 190ºF), troubles are clearly plaguing your coolant sensors.

Again, new sensors are often super cheap, so do not hesitate to kill off the codes for good by buying replacements.

Stuck-Open Thermostats 

How can an engine lower its temperature? That’s right; here’s where its thermostats enter.

They close and open continuously to let coolant enter the radiators, keeping your engine from excessive heat.

But can you imagine what would happen if the thermostat refuses to close up?

Instead of overheating, the engine tank will stay extremely cold this time and take more hours to get warmer.

Hence, proper operating temps are out of reach, forcing the system to increase fuel in its fuel-air ratio.

To protect your car from further havoc, inspect whether it has:

  • A code combination of P0128 and P0172
  • Longer warm-up hours
  • No heat
  • Bad gas mileage

Unsticking a stuck thermostat is possible but quite time-consuming. Just buy a new one to save time and effort; it is not pricey anyways.

Fuel System 2 Status CL (P0174)

fuel system 1 cl
Fuel System 2

Faulty or Loose Gas Cap

Checking your gas cap should be the very first step.

A loose or broken cap causes low pressure and reduces fuel levels injected in the chamber, which eventually pops up the P0174 lean condition code.

If it’s the issue, tighten or fix the cap. Otherwise, move on to the second possible cause: 

Vacuum Leaks

Even a beginner should grasp how dangerous vacuum leaks could be: they literally invite unmetered air to the engine, destroying both banks 1 and 2!

That’s why in worsened cases, drivers might see a simultaneous appearance of both P0171 and P0174 (sometimes even P0106 joins the mess, too).

Other indicators to watch out for: 

  • Hissing sounds
  • Rough idling
  • LTFT (Long-term fuels trim) > 10%
  • Check engine light flickering
  • Poor mileage
  • Stalled engine

And how to spot this deadly leak? Start the car engine first. Next, splash some water on the vacuum hose; huge leakages (if there is one) will suck the water back into that hose.

Common reasons behind vacuum leaks are loosened clamps or cracked hoses. Depending on your circumstances, tighten the clamp or install new hoses accordingly.

How Much Does It Cost to Fix Error Codes for Fuel Sys 1 CL and Fuel Sys 2 CL?

Fixing these codes might involve troubleshooting other undetected problems.

Hence, the cost varies – though experts have outlined general anticipated prices for affected components as below: 

For P0172 codes (Fuel system 1 CL)

  • Coolant temp sensors: 150$ – 200$
  • Air/fuel filter: 20$
  • Fuel pressure regulators: 200$ – 400$
  • Thermostat: 200$ – 300$
  • Oxygen sensors: 200$ – 300$
  • MAF sensors: 100$

For P0174 codes (Fuel System CL 2)

  • Vacuum leaks: 1000$
  • Extra fuel pumps: 1300$ – 1700$
  • Fuel injectors: 200$ – 400$
  • Repairing exhaust leaks: 100$ – 200$
  • Oxygen/fuel sensors: 200$ – 300$
  • Resetting MAFs: 100$ (or do it by yourself)
  • Replacing MAFs: 300$
  • Replacing spark plugs: 150$
  • Replacing PCV valves: 75$ – 100$
  • Exhaust gas leaks: 300$ – 400$
  • O2 sensors: 325$ – 400$

Again, keep in mind these are estimated rates. You cannot know how much to pay until your actual visit to the repair shop.

Is It Safe to Ignore These Codes?


These fuel system cl fault signals point to internal engine defects, and your refusal to treat them on time will put both you and your vehicle at severe risk of accidents/ collisions.


What is fuel system 1 and fuel system 2 common error code? Our exhaustive research has addressed the issues for you, proposing numerous alternatives to resolve them once and for all.

Do not put off your visit to mechanics any longer – and feel free to ask us for more help on the fuel system closed loop fault.

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