On a V-type engine, each bank gets its own fuel corrections from the engine computer. When the corrections on one side grow too large, it flags a fault for that side. A P0173 code means the fuel trim on bank 2 went beyond the limits the engine computer allows, either rich or lean.
This guide explains how fuel trim works, how to compare the two banks and what usually causes the problem.
P0173 Code Breakdown
Every OBD-II code follows the same five-character pattern set by the SAE J2012 standard. Each character narrows down which system has the fault. This table shows what each part of the code tells you.
| Character | Meaning | What It Tells You |
|---|---|---|
| P | Powertrain | The fault is in the engine or transmission systems. |
| 0 | Generic (SAE) code | The definition is the same on every OBD-II vehicle. |
| 1 | Fuel and air metering | The fault involves the sensors that measure air and fuel. |
| 73 | Specific fault | Fuel Trim Malfunction (Bank 2) |
What the P0173 Code Means
The SAE J2012 standard defines the P0173 code as Fuel Trim Malfunction (Bank 2). Bank 2 is the side of the engine that does not contain cylinder 1. You see this code only on V-type and flat engines.
The powertrain control module (PCM) adjusts fuel for each bank using that bank’s front oxygen sensor. Short-term fuel trim reacts within seconds, while long-term fuel trim learns and stores the correction. Positive numbers mean the PCM adds fuel, and negative numbers mean it pulls fuel.
When bank 2’s combined trim reaches the PCM’s limit, often around plus or minus 25 percent, the code sets. Most newer vehicles use the more specific lean code P0174 or rich code P0175 instead.
Symptoms of a Bank 2 Fuel Trim Fault
Symptoms depend on whether bank 2 runs lean or rich. Common signs of a P0173 code include:
- Check engine light on
- Rough idle, often worse when cold
- Hesitation or misfires
- Poor fuel economy
- Fuel smell if the bank runs rich
- Failed emissions test
What Causes the P0173 Code?
The cause may be on bank 2 alone or shared by both banks. These are the common causes:
- Intake gasket leak on bank 2. A leak near one side’s ports leans out only that bank.
- Clogged or leaking injectors on bank 2. Fuel delivery changes on that side only.
- Exhaust manifold leak on bank 2. Outside air fools the front sensor into reading lean.
- Faulty bank 2 front oxygen sensor. A biased sensor pushes trims the wrong way.
- Dirty MAF sensor. This usually affects both banks.
- Fuel pressure problems. These also tend to affect both banks.
How to Diagnose the P0173 Code
You need a scan tool with fuel trim data for both banks. A smoke machine and fuel pressure gauge help.
Compare Bank 1 and Bank 2 Trims
Warm the engine and read long-term trims for both banks at idle and 2,500 rpm. If bank 2 is far off while bank 1 is normal, the fault is on bank 2 only. If both banks are off in the same direction, look for a shared cause like the MAF sensor or fuel pressure.
Decide Lean or Rich
A large positive total on bank 2 means lean. Look for intake gasket leaks, clogged injectors and manifold leaks on that side. A large negative total means rich. Look for leaking injectors or a biased oxygen sensor on bank 2.
Test the Bank 2 Front Sensor
Do a propane test at idle. The bank 2 front sensor should go rich right away. Then pull a vacuum hose, and it should go lean. A sensor that responds well is probably reporting a real problem rather than causing one.
Repairs That Fix the P0173 Code
Fix the cause, then clear the code and drive normally so the PCM can relearn fuel trims.
- Replace a leaking intake gasket on bank 2.
- Clean or replace faulty injectors on that bank.
- Repair exhaust manifold leaks and broken studs.
- Replace a biased bank 2 front oxygen sensor.
- Clean the MAF sensor or fix fuel pressure if both banks are affected.
Repair Cost Breakdown
The cost to fix a P0173 code varies with the cause. This table shows common ranges.
| Repair | DIY Cost | Typical Shop Cost |
|---|---|---|
| Intake manifold gasket | $30 to $100 | $300 to $800 |
| Fuel injector service | $15 to $200 | $100 to $500 |
| Exhaust manifold gasket or studs | $20 to $80 | $250 to $800 |
| Bank 2 front O2 sensor | $40 to $150 | $150 to $450 |
| Smoke test | Not usually DIY | $75 to $150 |
Is It Safe to Drive with a P0173 Code?
You can usually drive for a short time. A lean bank may misfire and run hot, while a rich bank wastes fuel and fouls spark plugs.
Either one can damage the bank 2 catalytic converter over time. Get the cause found soon, and stop driving if the check engine light starts flashing.
Related Codes
These codes often show up with this one or point to the same system:
| Code | Meaning |
|---|---|
| P0170 | Fuel Trim Malfunction (Bank 1) |
| P0171 | System Too Lean (Bank 1) |
| P0172 | System Too Rich (Bank 1) |
| P0174 | System Too Lean (Bank 2) |
| P0175 | System Too Rich (Bank 2) |
Bringing Bank 2 Fuel Trim Back in Range
A P0173 code means bank 2 fuel trim has hit its limit. Problems on that side, such as intake gasket leaks and injector faults, cause many cases.
- Compare trims on both banks.
- Decide whether bank 2 runs lean or rich.
- Test the bank 2 front sensor’s response.
- Look for leaks and injector problems on that side.
Frequently Asked Questions
Why Would Only One Bank Have a Fuel Trim Problem?
Each bank has its own intake ports, injectors, exhaust manifold and front oxygen sensor. A problem with any of these on bank 2 affects only that side. Shared parts like the MAF sensor and fuel pump usually affect both banks equally.
What Is the Difference Between the P0173 Code and P0174?
The P0173 code is a general fuel trim fault on bank 2 and does not say which direction. P0174 specifically means bank 2 runs lean. Some vehicles, often older ones, use the general code instead of the specific ones.
Can an Intake Gasket Leak Affect Just One Bank?
Yes. On V-type engines, the intake gasket seals each side separately. A leak near the bank 2 ports lets unmetered air into those cylinders only. A smoke test usually finds this kind of leak quickly.
Will Clearing the Code Fix the Problem?
No. Clearing the code resets fuel trim learning and turns off the light for a while. If the cause is still there, bank 2 trim climbs back to its limit and the code returns. Find and fix the cause first.