What this code means
P0135 may indicate a problem in the heated oxygen sensor heater circuit that helps the sensor reach operating temperature.
What the vehicle may do
- The vehicle may turn on the check engine light.
- The engine may seem to run normally in some cases.
- Oxygen sensor activity may be delayed during warm-up.
Possible fault areas
- Possible heater power feed issue.
- Possible control circuit issue between the sensor and engine control module.
- Possible fuse, wiring, terminal, or connector issue.
- Possible heated oxygen sensor heater issue.
- Possible engine control module control or monitoring issue.
Diagnostic path
Opening and code context
On this Silverado, P0135 points us toward a heated oxygen sensor heater circuit problem. In plain language, the engine control module is watching the heater side of the oxygen sensor and may be seeing current that does not match what it expects. The truck may run normally, or it can have a check engine light and delayed oxygen sensor activity during warm-up. The possible fault areas often include the heater power feed, the control circuit back to the engine control module, connectors and terminals, the fuse-fed ignition circuit, or the heated oxygen sensor itself. If other codes are present, check what they mean first, but keep this diagnosis focused on the P0135 heater circuit path.
Start with the basic checks and understand the monitor
Start with the basic system checks, then follow a structured diagnostic approach before getting into circuit testing. The system is simple in concept: ignition power is supplied through a fuse, and the engine control module switches the heater control circuit to ground when it wants the heated oxygen sensor heater on. Once the HO2S heats up and starts working as a sensor, it begins to generate voltage within a range of 40 to 1050 mV. A voltage that increases toward 1000 mV indicates a rich fuel mixture, and a voltage that decreases toward 0 mV indicates a lean fuel mixture. That sensor-voltage behavior is background context; this P0135 path is still a heater circuit diagnosis. For this code, treat the running requirements as monitor gates. The heated oxygen sensor needs to be at normal operating temperature, the heater is inactive, Ignition Voltage = Greater than 10 V, and the monitor runs once after the running conditions are met for greater than 120 s, with 2 Attempts per drive cycle. The diagnostic formula for this code is heater current Less than 0.3 A or Greater than 3.1 A — For greater than 8 s.
Command the heater and watch the scan data
With the ignition on, use the scan tool to command the appropriate HO2S heater increase and decrease. Watch the heater control circuit high voltage, low voltage, and open test status for the sensor involved in this P0135 diagnosis. Those status items should read OK or Not Run. If they do not, move into the circuit testing. If they do, command the heater again and confirm the heated oxygen sensor heater turns on and off. If the heater does not respond, that also sends you into circuit testing.
Check for an intermittent before tearing in
If the heater commands on and off, verify those same high voltage, low voltage, and open test status parameters again. Then work the harness and connectors at the heated oxygen sensor and at the engine control module while watching the parameter. If the status changes while you are moving the wiring, repair the wiring, terminal, or connector issue. If the status stays correct, reproduce the operating conditions, or the captured conditions from the failure data, and confirm P0135 does not reset. If it does reset, go to circuit testing. If it does not reset, the system is currently passing.
Begin circuit testing at the sensor connector
For circuit testing, turn the vehicle and all systems off, then disconnect the appropriate heated oxygen sensor connector. Turn the ignition on or put the vehicle in service mode. First, check the ignition feed with a test lamp between ignition circuit terminal 2 and ground. The lamp should turn on. If it does not turn on and the fuse is OK, turn the vehicle off, remove the test lamp, and test the ignition circuit between terminal 2 and the fuse output. You are looking for less than 2 Ω. If it is 2 Ω or greater, repair the open or high resistance in that circuit. If it is less than 2 Ω, then verify the fuse is OK and that voltage is present at the fuse.
If the fuse is open, isolate the shorted load or circuit
If the test lamp did not turn on and the circuit fuse is open, turn the vehicle off, remove the test lamp, reconnect the heated oxygen sensor, turn the ignition on or put the vehicle in service mode, replace the fuse, and verify the fuse does not open. If the fuse opens with components connected on that circuit, disconnect components one at a time and repeat the earlier check point. If the fuse opens with all components disconnected, repair the short to ground on the circuit. If the fuse stops opening after one component is disconnected, replace the component disconnected last. If the fuse does not open with all components connected, operate each component on the circuit one at a time and confirm the fuse does not open when any component is activated. If it opens when a component is activated, replace the component that caused it. If it never opens, that branch is passing.
Check for an unwanted ground on the control side
If the ignition feed test lamp did turn on between terminal 2 and ground, move to the control side. Check that the test lamp does not turn on between ignition circuit terminal 2 and control circuit terminal 1. If the lamp does turn on there, turn the vehicle off, remove the lamp, disconnect the appropriate engine control module connector, and test for infinite resistance between control circuit terminal 1 at the component harness and ground. If resistance is less than infinite, repair the short to ground on the control circuit. If resistance is infinite, replace the engine control module.
Prove the module can see an open control circuit
If the lamp does not turn on between ignition circuit terminal 2 and control circuit terminal 1, remove the test lamp and run the engine. Now verify the scan tool shows the HO2S Heater Control Circuit Open Test Status as Malfunction. If it does not show that specified state, turn the vehicle off, disconnect the appropriate engine control module connector, turn the ignition on or put the vehicle in service mode, and test for less than 1 V between control circuit terminal 1 at the component harness and ground. If the reading is 1 V or greater, repair the short to voltage on the circuit. If it is less than 1 V, replace the engine control module.
Load the control circuit and check continuity if needed
If the open test status does show Malfunction, connect a 3 A fused jumper wire between ignition circuit terminal 2 and control circuit terminal 1, then run the engine. The scan tool should now show the HO2S Heater Control Circuit High Voltage Test Status as Malfunction. If it does not, turn the vehicle off, remove the jumper wire, disconnect the appropriate engine control module connector, and test the control circuit from terminal 1 at the component harness to the other end of the circuit at the control module harness. You are looking for less than 2 Ω. If it is 2 Ω or greater, repair the open or high resistance in the control circuit. If it is less than 2 Ω, replace the engine control module. Keep in mind that as little as 10 Ω of resistance in a circuit may cause a DTC to set, and this kind of testing may set additional DTCs.
Finish at the sensor, then verify the repair
If the high voltage test status does show Malfunction with the fused jumper in place, test or replace the heated oxygen sensor. After the repair is complete, keep verification separate from the testing path: verify the repair, reproduce the conditions needed for the monitor, and confirm P0135 stays gone. The takeaway is to prove power, prove the control circuit, watch the scan tool response, and only condemn a component after the circuit path supports it. For more diagnostic training, visit stepdiagnostics.com.
Final check
P0135 is often best approached as an electrical heater-circuit diagnosis: verify the command, prove the feed and control side, then confirm the code stays gone.
For more guided automotive diagnostics, visit STEP Diagnostics.
