OBD-II Diagnostic Trouble Code
P0157 — O2 Sensor Circuit Low Voltage (Bank 2 Sensor 2)
P0157 means the bank 2 downstream (sensor 2) oxygen sensor voltage stayed below its expected minimum, usually under about 0.1 to 0.2 V, for a set time. The sensor is reporting a constant lean exhaust after the catalytic converter, or its signal circuit is shorted to ground. This code applies mainly to gasoline V-engine trucks.
Severity 2/5
Less common on commercial trucks
Service soon
Severity
Service soon
O2 Sensor Circuit Low Voltage (Bank 2 Sensor 2)
Code
P0157
Category
Powertrain
Subsystem
Emissions
Truck-relevant
Less common
What this means for truckers
Service this week. Not an immediate stop, but ignoring it usually leads to a more expensive fix downstream.
Before you replace parts — try this first
P0157 on the Emissions system is most often exhaust leak ahead of the downstream sensor that lets outside air reach it — not a dead part. On commercial diesels especially, work these in order before spending money on parts:
- Confirm the symptoms. Match what your truck is doing to the Symptoms section below. If the code is set but the symptoms are not there, clear the code and re-test after a drive cycle — it may not return.
- Clean and dry the sensor connector. Water, road salt, or oil seeping into a connector is the #1 fake sensor failure. Disconnect, look for green corrosion or moisture, spray with electrical contact cleaner, dry, reseat. Many faults clear here for $0.
- Inspect the wiring harness. Trace the wire from the sensor toward the ECM — look for chafing against the engine block, melted insulation near the turbo or exhaust, or pinch points where a clamp has rubbed through.
- Test the sensor before swapping it. Most Common Causes (below) list the expected voltage or resistance range — a $20 multimeter saves a $300 sensor swap that does not fix the problem.
- Only then, replace the part. Use the OEM or a tier-1 aftermarket. Cheap sensors throw the same code in two weeks.
Symptoms
Check-engine light is on, and drivability is usually normal. The catalyst monitor may fail or not run. If an exhaust leak is pulling in fresh air, you may hear ticking or puffing near the converter.
Common causes
- CommonExhaust leak ahead of the downstream sensor that lets outside air reach it ($150–$600)
- CommonSignal wire shorted to ground or chafed against the frame ($100–$350)
- CommonFailed or contaminated downstream oxygen sensor ($110–$300)
- OccasionalReal lean condition on bank 2 (check fuel trims) ($100–$800)
How to fix
Check bank 2 fuel trims and the upstream sensor. If both show lean, diagnose the engine lean condition first.
When to use: To rule out a real lean condition before blaming the sensor
Smoke test or inspect the exhaust between the manifold and the sensor for leaks, and repair them.
When to use: When fuel trims are normal but the downstream sensor still reads low
Check the signal circuit for a short to ground, then replace the sensor if the wiring is good.
When to use: When there is no exhaust leak
Affects most
No specific vehicle pattern found in our complaint database. This code can appear on any vehicle that supports OBD-II diagnostics.
Related
- → Browse all DTC trouble codes
- → Decode a VIN to check codes for your truck
- → Owner-operator maintenance guides
- → P0100 — Mass or Volume Air Flow Circuit
- → P0101 — Mass or Volume Air Flow Circuit Range/Performance
- → P0102 — Mass or Volume Air Flow Circuit Low Input
- → P0103 — Mass or Volume Air Flow Circuit High Input
- → P0104 — Mass or Volume Air Flow Circuit Intermittent
Frequently Asked Questions
Can I drive with code P0157?
P0157 is severity 2 — service this week. Not an immediate stop, but ignoring it usually compounds the problem. Most common cause for this code is a worn sensor or minor performance fault that won't strand you today but will get more expensive the longer you wait.
How much does it cost to fix P0157 on a commercial truck?
Total repair cost ranges $100–$800 depending on the root cause. The cheapest fix is usually the first one listed below — Exhaust leak ahead of the downstream sensor that lets outside air reach it — which is also the most common. On commercial trucks the cost is usually higher than passenger vehicles due to part availability and shop labor rates ($120–185/hour at most truck shops, vs $90–130 at general auto shops). DIY-capable fixes (sensors, hoses, filters) are at the low end; major component replacements (turbo actuator, high-pressure fuel pump, transmission solenoid pack) are at the high end. Always diagnose with a scan tool BEFORE replacing parts — guessing wrong on a $1,500 turbo actuator when the actual issue was a $30 boost hose is a common expensive mistake.
Will P0157 fail a DOT inspection?
Yes — P0157 is an emissions-related code, and DOT inspectors check emissions compliance. An active emissions code with a check engine light can result in an out-of-service violation under FMCSA Part 393. The truck typically can't be moved until the code is cleared. Repeated emissions violations escalate to CSA scores.
What's the difference between P0157 and related codes?
P0157 is closely related to: P0100 (Mass or Volume Air Flow Circuit), P0101 (Mass or Volume Air Flow Circuit Range/Performance), P0102 (Mass or Volume Air Flow Circuit Low Input), P0103 (Mass or Volume Air Flow Circuit High Input). These codes share the same subsystem (emissions) and often appear together. If you see this code, scan for the full code list — fixing the root cause usually clears multiple companion codes at once. If only P0157 is set without the companions, the issue is more isolated.
How do I prevent P0157 from coming back after a repair?
For P0157 (O2 Sensor Circuit Low Voltage (Bank 2 Sensor 2)), prevention centers on the emissions system. Keep up with PM on the EGR system — clean the valve at 200K miles. Use OEM-spec air filters. Don't ignore minor codes; they often escalate into bigger emissions failures.