P0173 Code: Fuel Trim Malfunction (Bank 2)
Fuel correction on bank 2, the side without cylinder 1, is out of range or unstable, lean or rich.
Safety first · Consult a professional
- Fuel vapor is flammable. No smoking or open flames; work outdoors and fix fuel leaks before driving.
At a glance
Severity: Moderate
- Meaning
- Fuel Trim Malfunction (Bank 2)
- System
- Fuel & Air Metering · Powertrain
- Safe to drive?
- With caution
- Typical repair cost
- $15–$900
- DIY difficulty
- Moderate
P0173 is the bank 2 counterpart of P0170. The PCM has seen the combined short-term and long-term fuel trim on the cylinder bank that does not contain cylinder 1 leave its allowed window, or swing back and forth, without logging a plain lean (P0174) or rich (P0175) fault. It only shows up on engines that monitor two banks separately: V6, V8 and V10 engines plus a few inline-sixes with two upstream sensors. The most useful piece of information is what bank 1 is doing at the same moment. If P0170 is stored too and both banks drift the same way, the cause is shared, such as airflow metering, fuel pressure or crankcase ventilation. If bank 1 is steady and only bank 2 wanders, look only at hardware that bank 2 has to itself.
Can I keep driving? It is normally fine to drive gently to a shop. Skip towing and long climbs until you know whether bank 2 is lean or rich; a flashing check engine light, misfire codes on bank 2 cylinders or a raw fuel smell mean park it.
How to read P0173
- P Powertrain
- 0 Generic (SAE)
- 1 Fuel & air metering
- 73 Specific fault
Symptoms
- Check engine light on, often with P0170, P0174 or P0175 stored alongside
- Idle that feels lopey, as if one side of the engine is working harder than the other
- Slight shake at stoplights that smooths out above about 2,000 rpm
- Ticking from one exhaust manifold at cold start when a leak is involved
- Misfire codes on bank 2 cylinders in more advanced cases
- Fuel economy down by a few mpg
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Exhaust leak between the bank 2 cylinder head and its upstream sensor: manifold gasket, cracked manifold, snapped studs or a loose flange
- Intake gasket or vacuum fitting leaking on the bank 2 side only
- Shared fault with P0170 also stored: skewed MAF, unstable fuel pressure or a cracked crankcase breather hose
- Bank 2 upstream O2 or air-fuel sensor that has aged, slowed down or been contaminated
- Bank 2 camshaft timing off target: sticking oil control valve, worn phaser or a stretched chain on that bank
- One or more bank 2 injectors partly clogged or dripping
- Bank 1 and bank 2 sensor plugs swapped after exhaust or transmission work
Diagnosis step by step
Pin down which side is bank 2
Bank 2 never contains cylinder 1. On a transverse V6 it can be the front bank near the radiator or the rear bank against the firewall depending on the maker, and on longitudinal V8s it varies too. Look up the cylinder numbering for your engine code before you buy a single part.
Tools: Flashlight
Log both banks side by side
With the engine fully warm, record short-term and long-term trim for bank 1 and bank 2 at idle, at 2,500 rpm in park and at a steady 50 mph. A gap of more than about 10 percentage points between the banks at the same operating point means a bank-specific fault. Both banks off by a similar amount means a shared cause, so work through MAF, fuel pressure and breather hoses first.
Tools: OBD-II scanner
Make sure the sensors report the right bank
With the engine idling, briefly unplug the sensor you believe is bank 2 and watch which bank's sensor reading drops out in live data. If bank 1 data goes flat instead, the plugs are crossed and the PCM has been correcting the wrong side, which by itself produces this code.
Tools: OBD-II scanner
Hunt for an exhaust leak on bank 2
On a cold engine, listen at the bank 2 manifold for a tick or puff that fades as the metal expands. Look for black soot streaks at the gasket and for studs with missing heads. A smoke machine fed into the tailpipe, with the engine off, shows small leaks that your ears miss. Even a small leak ahead of the sensor lets outside air in between exhaust pulses and makes the bank look lean.
Tools: Flashlight, Smoke machine
Smoke-test the intake with the focus on bank 2 runners
Fill the intake with smoke and follow the gasket line where the manifold meets the bank 2 head, plus the valve cover and any vacuum ports on that side. On many V engines the rear runners hide under the plenum, so use a mirror and a strong light.
Tools: Smoke machine, Flashlight
Compare cam timing between the banks
Read actual versus desired cam angle for both banks at idle and at 2,500 rpm. Bank 2 lagging its target by more than a few degrees while bank 1 tracks changes how much exhaust gas stays in those cylinders and upsets the trim. Check the oil level and grade before blaming the oil control valve.
Tools: OBD-II scanner
Judge the bank 2 sensor last
Hold 2,500 rpm and compare how quickly the two upstream sensors react when you snap the throttle. A narrowband sensor should cross 0.45 V several times a second; a wideband sensor should follow a throttle snap within about a second. If both sensors have the same part number, swap them: if the fault moves to bank 1, replace the sensor.
Tools: OBD-II scanner, O2 sensor socket
Where the parts are
- 1Ignition coil
- 2Spark plug
- 3Fuel injector
- 4Camshaft sensor
- 5Crankshaft sensor
- 6Engine computer (ECM)
- 7Intake manifold
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Replace bank 2 exhaust manifold gasket or extract broken studs | No | $250–$900 |
| Replace intake manifold gasket on a V engine | No | $350–$900 |
| Replace bank 2 upstream O2 or air-fuel sensor | Yes | $180–$450 |
| Replace bank 2 camshaft oil control valve | Yes | $150–$450 |
| Clean or replace MAF sensor when both banks drift | Yes | $15–$400 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Notes by brand
Mercedes-Benz
On Mercedes-Benz M112 V6 and M113 V8 engines, P0173 usually arrives together with P0170. That pairing means a shared cause, most often the hot-film MAF or brittle breather hoses under the intake manifold, so bank 2 parts are rarely the answer when both codes are present.
Emissions / smog test impact
Will fail
An active P0173 with the check engine light on fails an OBD-II smog inspection. After the repair, allow a few days of mixed city and highway driving so the O2 sensor and catalyst monitors report ready before you go in.
Frequently asked questions
How is P0173 different from P0174 and P0175?
P0174 means bank 2 is lean and P0175 means it is rich. P0173 only says the bank 2 correction is out of range or erratic. Your live data has to tell you the direction, and if it flips between lean and rich, think of leaks that open with heat or a sensor that has become slow.
I have P0170 and P0173 at the same time. Where do I start?
With the parts both banks share. On a V engine both banks breathe through one MAF and one throttle and are fed by one pump, so a skewed MAF, a breather leak at the plenum or unstable fuel pressure upsets both sides together. Bank-specific parts rarely fail as a matching pair.
Can an exhaust leak really trigger a fuel trim code?
Yes. Between exhaust pulses a leak ahead of the sensor draws in a little outside air, and the sensor reads that oxygen as a lean mixture. The PCM adds fuel, so the bank actually runs rich while the sensor still says lean, and the trims end up out of range.
Should I replace both upstream sensors?
Not as a first move. Swap them if they share a part number, or compare their response in live data. If the bank 2 sensor is just as quick as bank 1, the sensor is probably reporting the problem honestly rather than causing it.