What Does P1127 Mean?
P1127 indicates that the Bank 1 Sensor 2 oxygen sensor heater was not active during the Key On Engine Running (KOER) self-test. The ECM commands the downstream O2 sensor heater on during the self-test and monitors for proper current draw or voltage response. A failure means the heater did not energize, which prevents the post-catalyst O2 sensor from reaching operating temperature for accurate catalyst efficiency monitoring.
Common Causes
30%
O2 sensor heater element burned out (open circuit)
25%
Blown O2 sensor heater fuse or relay
20%
Open circuit in the heater power supply or ground wire
13%
ECM heater driver circuit failure not providing ground command
12%
Corroded O2 sensor connector preventing heater circuit continuity
Diagnostic Steps
1
Step 1: Check the O2 sensor heater fuse and relay. A blown fuse is a common cause. If the fuse is blown, check for shorts in the circuit before replacing.
2
Step 2: Disconnect Bank 1 Sensor 2 and measure heater resistance across the heater pins. Typical specification is 2–30 ohms. An open (infinite resistance) reading confirms a burned-out heater element.
3
Step 3: With the sensor connected and key on engine running, use a current clamp on the heater power wire. Normal current draw is 0.5–2A depending on the sensor type. Zero current confirms the heater is not operating.
4
Step 4: Verify battery voltage is present at the heater power pin and that the ECM is providing a ground on the control pin during the KOER self-test. Missing power or ground isolates the fault.
5
Step 5: Replace the Bank 1 Sensor 2 O2 sensor if the heater element is open. Clear codes and re-run the KOER self-test to verify the heater activates and the code does not return.
Estimated Repair Cost
$80 - $350
Parts + labor, varies by vehicle and location
P1127 indicates that the Bank 1 Sensor 2 oxygen sensor heater was not active during the Key On Engine Running (KOER) self-test. The ECM commands the downstream O2 sensor heater on during the self-test and monitors for proper current draw or voltage response. A failure means the heater did not energi...
The most common cause of P1127 (HO2S Bank 1 Sensor 2 Heater Not On During Key On Engine Running Self Test) is: O2 sensor heater element burned out (open circuit)
Typical repair costs for P1127 range from $80 to $350, depending on the vehicle, location, and whether you do it yourself or go to a shop.
An inoperative downstream O2 heater primarily affects emissions monitoring and catalyst efficiency testing. The vehicle remains safe to drive, but will fail emissions testing and the check engine light will remain illuminated.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P1127 to identify the root cause.
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Quick Info
Category
Powertrain
System
Emissions / Oxygen Sensor Heating
Difficulty
Type
Manufacturer
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