What's the difference between P0173, P0174, and P0175?

All three are bank 2 fuel trim codes, but they describe different specific conditions. P0174 sets when bank 2 is running lean — the PCM has been adding fuel and can't compensate any further. P0175 sets when bank 2 is running rich — the PCM has been subtracting fuel and can't compensate any further. P0173 is the generic 'fuel trim malfunction' code that some platforms use instead of P0174/P0175, or set when trim is bouncing erratically without committing to one direction. Diagnostically, P0173 forces you to do an extra step that P0174 and P0175 spare you: you have to read the live trim data to figure out which direction the engine is actually trimming, and then follow either the lean or rich diagnostic path.

Where is bank 2 on my engine?

Bank 2 is whichever bank does NOT contain cylinder 1. The bank that contains cylinder 1 is bank 1; the other is bank 2. On transverse V6 engines (Honda J35, Toyota 2GR-FE, Nissan VQ35) bank 2 is usually the rear bank against the firewall — harder to access. On longitudinal V6 and V8 engines (Ford Modular, Chrysler Hemi, GM LS) bank 2 is usually the passenger side. The location varies enough by engine family that you should always verify with a wiring diagram or service information specific to your engine before assuming. Replacing a bank 1 sensor when bank 2 is the actual fault is the most common mistake on bank-specific codes, and it's an expensive mistake.

Can I keep driving with P0173?

Yes, in most cases. P0173 is medium severity — the engine still runs, and most drivers don't notice major drivability problems. The longer-term concerns are fuel economy loss, possible damage to the catalytic converter if the underlying fault produces unburned fuel in the exhaust, and emissions inspection failure. If the cause turns out to be a serious vacuum leak or fuel pressure problem, that fault may progress to misfire codes or cause other issues. The right approach is to drive normally for a few days while you arrange diagnosis, but don't ignore the code for months. Get the live trim data read and the root cause identified within a reasonable window.

How much does it cost to fix P0173?

Depends entirely on what the root cause turns out to be — and the cost spread is wide. A vacuum leak fix is $100-700. MAF cleaning is essentially free as a DIY job or $50-100 at a shop. Upstream O2 sensor replacement on bank 2 is $150-400. Fuel pressure regulator is $200-500. Injector repair starts around $300 and climbs with the number of injectors affected. Intake manifold gasket on a V6 or V8 is $400-1200. The investment that pays off is diagnostic time — spending $100-200 to actually identify the root cause before replacing parts almost always costs less than guessing wrong on a $400 part and still having the code afterward.

OBD-II trouble code

P0173: Fuel Trim Malfunction (Bank 2)

The PCM has decided fuel trim on bank 2 is out of acceptable range — but unlike P0174 (lean) or P0175 (rich), this code doesn't commit to which direction. The first diagnostic question is whether the trim is being pulled lean, pushed rich, or oscillating between them.

Quick facts

System: Powertrain
Category: Fuel & Air
Severity: Medium severity
Drivable: Usually safe to drive short-term
Repair cost range: $30 – $1,500
DIY difficulty: Intermediate DIY

What does P0173 mean?

P0173 is the generic 'fuel trim malfunction bank 2' code — the partner to P0170 on bank 1. It's set on platforms that report fuel trim trouble without specifically calling it lean or rich, or when the PCM has detected trim values that oscillate erratically between extremes faster than P0174 or P0175 would catch. Most modern vehicles use the more specific P0174 (lean) and P0175 (rich) codes, but P0173 still appears regularly on older Ford and Chrysler platforms, on certain Toyota and Lexus V6/V8 engines, and on some European applications.

Before diving into bank 2 specifically: bank 2 is whichever bank does NOT contain cylinder 1. On most transverse V6 layouts (Honda J35, Toyota 2GR-FE, Nissan VQ35/VQ37) bank 2 is the rear bank closest to the firewall. On most longitudinal RWD V6 and V8 layouts (Ford Mustang, Chrysler Hemi, GM LS, BMW inline-6 oriented as V) bank 2 is the passenger side. Always verify with the wiring diagram for your specific engine before chasing a bank-specific code — the location varies enough that assumption-based diagnosis leads to wrong-side sensor replacements.

The diagnostic challenge with P0173 is the same as P0170: the code itself doesn't tell you whether the engine is running lean or rich on that bank. You need to look at the live fuel trim data — short-term fuel trim (STFT) and long-term fuel trim (LTFT) — to see which direction the PCM has been pulling. STFT and LTFT positive numbers mean the PCM is adding fuel (compensating for lean). Negative numbers mean the PCM is subtracting fuel (compensating for rich). Out-of-spec in either direction can trigger P0173 on platforms that use this generic code.

If trim is positive (lean side), you're looking at the same causes as P0174 — vacuum leak, MAF contamination, fuel delivery problem, exhaust leak ahead of the upstream O2 sensor, or a bad upstream O2 sensor itself. If trim is negative (rich side), you're looking at the same causes as P0175 — failing fuel pressure regulator, leaky injector, restricted air filter, dirty MAF reading high, or a contaminated O2 sensor reading lean. The split is the entire diagnostic question. Don't replace parts before reading the trim direction.

Common causes

Vacuum leak on bank 2's side of the intake manifold (positive trim direction)
Bank-specific exhaust leak ahead of the upstream O2 sensor (positive trim)
Failed bank 2 upstream O2 sensor reporting falsely lean or rich
Failing fuel pressure regulator affecting both banks equally (rich trim direction)
Leaky injector on a bank 2 cylinder (rich trim direction)
MAF sensor contamination or drift affecting global fuel calculation
Bank 2-specific injector wiring issue causing erratic fueling
Cracked or disconnected PCV hose on the bank 2 side
Bank 2 intake manifold gasket failure
Restricted fuel injector on bank 2 (lean trim direction)

Symptoms

Check Engine Light on, often illuminating after extended driving
Possible rough idle if the underlying issue is severe
Reduced fuel economy
Mild surge or hesitation under light load
May feel like nothing is wrong in many cases — fuel trim codes often set without obvious drivability symptoms
Possible exhaust smell (rich) or hesitation under acceleration (lean)

Diagnostic steps

1.Pull all codes and capture freeze frame data showing the conditions when P0173 set.
2.With a scan tool capable of live data, watch bank 2 STFT and LTFT at idle, at 2500 RPM no load, and during a road test. Note the direction (positive or negative) and the magnitude.
3.If trim is positive (lean side), follow the lean diagnostic path: smoke test the intake for vacuum leaks, check the PCV system, inspect the bank 2 exhaust ahead of the upstream O2 sensor, and look at fuel pressure under load.
4.If trim is negative (rich side), follow the rich diagnostic path: check fuel pressure for over-pressure conditions, look for leaky injectors, inspect the air filter, clean the MAF, and check upstream O2 sensor health.
5.Compare bank 1 trim to bank 2 trim. If both are similarly out of range, the cause is global (fuel pressure, MAF, fuel pump). If only bank 2 is out of range, the cause is bank-specific (intake gasket on bank 2 side, bank 2 injector, bank 2 O2 sensor).
6.Identify bank 2 correctly for your specific engine — the wrong-bank repair is the single most common diagnostic mistake on fuel trim codes.
7.Address the actual root cause rather than replacing the upstream O2 sensor reflexively. O2 sensors are often the symptom carrier, not the actual fault.
8.After repair, drive the vehicle through a full warm-up cycle and confirm bank 2 trims return to within ±10%.

Repair cost

$30 – $1,500

Vacuum leak repair is $100-700 depending on what's leaking. MAF cleaning is $10-50; MAF replacement is $150-450. Upstream O2 sensor replacement is $150-400 (slightly higher on bank 2 due to access on some platforms). Fuel pressure regulator replacement is $200-500. Leaky injector repair runs $300-800 per injector replaced. Intake manifold gasket replacement is $400-1200 depending on engine. The cost range is wide because the root causes are so varied — diagnostic time up front is worth the investment to avoid wrong-part replacements.

Estimate your repair

Run the numbers for your vehicle

Open the Repair Cost Estimator with vacuum leak repair preselected. Adjust labor rate and vehicle category to fit your situation.

Estimate your repair cost

DIY vs shop

This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.