P1140

Code

CADILLAC P — Powertrain

HO2S Transition Time Ratio Bank 1 Sensor 2

Brand: CADILLAC

Type: P — Powertrain

Views: UK: 14 EN: 37 RU: 27

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Page language: EN

Causes

Contaminated or aged downstream O2 sensor (Bank 1 Sensor 2)
Open/short or high resistance in sensor heater circuit or signal wiring
Exhaust leaks between the engine and the downstream sensor
Failing catalytic converter or reduced catalyst efficiency on Bank 1
Engine fueling or air metering faults (excessive fuel trim, MAF/TP sensor faults)
PCM/ECM software anomaly or internal fault (less common)

Symptoms

Malfunction Indicator Lamp (MIL) illuminated with stored P1140
Possible poor fuel economy or drivability issues in some cases
Failed emissions test or readiness monitor failure for downstream O2/catalyst
Noisy or unstable O2 sensor voltage on live data, slow switching between lean/rich states

What to check

Scan tool: read freeze frame, pending counts and live data for Bank 1 Sensor 2 and Bank 1 Sensor 1
Compare downstream sensor voltage waveform to upstream (Bank 1 Sensor 1) while engine is at normal operating temperature
Check O2 sensor heater circuit with meter for resistance and power/ground (with key on engine off)
Visual inspection of sensor and wiring for damage, corrosion, burns or oil/coolant contamination
Perform smoke or visual check for exhaust leaks between exhaust manifold and sensor
Check fuel pressure and basic fuel/air systems (MAF, MAP, intake leaks) if upstream O2 also abnormal

Signal parameters

Typical zirconia HO2S voltage range: ~0.1 V (lean) to ~0.9 V (rich) when switching; downstream sensors normally show less aggressive switching than upstream
Heater resistance (typical ranges vary by sensor): often a few ohms to tens of ohms — consult vehicle-specific data before condemning
When warm and under dynamic conditions, upstream sensor should switch quickly; downstream sensor should show lower amplitude and slower changes but still respond within a reasonable time (switching frequency typically >~0.5–1 Hz under rich/lean pulses)
Transition time ratio is a PCM-calculated parameter (no universal numeric threshold); code indicates measured ratio/time outside the expected range for this vehicle

Diagnostic algorithm

Verify DTC and record freeze-frame and pending codes with a scan tool. Note conditions when fault set (temp, rpm, load).
Warm engine to normal operating temperature. Monitor live data: Bank1 Sensor1 (upstream) and Bank1 Sensor2 (downstream) voltages and heater status. Command a rich/lean change (snap throttle) and observe responses.
If downstream sensor is stuck, slow, or shows no change while upstream responds, inspect sensor connector and wiring for damage, corrosion, continuity, shorts to ground or battery, and proper reference ground. Wiggle test wiring while monitoring live data.
Check heater circuit: with key on (engine off) measure heater circuit voltage and sensor heater resistance to vehicle specification. Verify heater relay/fuse and PCM drive. Replace sensor if heater open or not powered.
Perform exhaust system inspection for leaks between the engine and the downstream sensor. Repair leaks and retest.
If wiring and heater are good, evaluate catalytic converter function (temperature delta across converter, increased backpressure, or P0420 present). A failing catalyst can alter downstream O2 behavior.
If wiring/heater/catalyst appear OK, consider replacing the downstream HO2S (Bank 1 Sensor 2) with an OE-equivalent sensor. Clear codes and test-drive to confirm repair. If fault returns, escalate to PCM/vehicle-specific diagnostics including software updates.

Likely causes

Contaminated/slow-response downstream oxygen sensor
Wiring or connector fault in sensor signal or heater circuit
Catalyst degradation reducing downstream sensor switching characteristics
Exhaust leak affecting sensor readings

Fault status

⚠️ Status

HO2S Transition Time Ratio Bank 1 Sensor 2 — downstream oxygen sensor switching/transition behavior outside expected range; stored when PCM detects abnormal transition time/ratio.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1-3 hours

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P1140 — HO2S Transition Time Ratio Bank 1 Sensor 2