Honda Insight P0135 (2010 ZE2): A/F Sensor 1 Heater Circuit Malfunction — Diagnostic Guide

Honda-specific code — factory diagnostic data

P0135 means the PCM cannot verify the A/F sensor (Sensor 1) heater is drawing current correctly. The A/F sensor heater warms the sensor element fast enough after a cold start to get closed-loop fueling online quickly; the PCM monitors the heater circuit for proper current flow and sets P0135 when the circuit is open, shorted, or stuck outside expected behavior. Honda’s diagnostic path walks a specific order: 10 A fuse first, A/F sensor relay next, then the sensor heater’s own resistance, then the signal-vs-heater isolation inside the sensor, and only then wiring and PCM-side control. A cheap blown fuse or a failed relay is much more common than either the sensor or the PCM — don’t skip ahead.

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P0135 Honda Insight Quick Answer

Check the No. 26 A/F SENSOR (10 A) fuse first — if blown, look for a short between the relay and the sensor before replacing it. Test the A/F sensor relay with a battery and jumper wires (or swap with a known-good relay of the same type). With the sensor disconnected, measure 1.9–2.7 Ω between sensor connector pins 3 and 4 (heater circuit); out of spec = replace the sensor. Also check there’s no continuity between pin 2 (signal) and pins 3 or 4 (heater) — any continuity means an internal sensor short, also a replacement. Wiring checks into PCM pins C9 and A21 are the last step before the PCM itself.

The Diagnostic Procedure

Tools: Honda HDS, DMM (0.1 Ω resolution), wiring diagram for your Insight trim (LHD/RHD under-dash fuse box differs), relay test harness or a known-good relay for swap-test. Warm the engine first (3,000 RPM unloaded until radiator fan cycles) to make sure the P0135 is current, not a cold-start intermittent.

1. Confirm P0135 is current. Clear the DTC with HDS. Start the engine, hold 3,000 RPM unloaded (P or N) until the radiator fan comes on, then idle. Re-check for pending or confirmed DTCs.
   → P0135 returns: go to step 2.
   → P0135 doesn’t return: intermittent. Check terminal tension and seating at the A/F sensor connector, the A/F sensor relay socket, and PCM connectors A and C. Re-seat and monitor.
2. Check the No. 26 A/F SENSOR (10 A) fuse in the under-dash fuse/relay box.
   → Fuse OK: go to step 3.
   → Fuse blown: something downstream pulled the fuse. Remove the A/F sensor relay and disconnect the A/F sensor 4P connector. Check continuity between sensor pin 4 and body ground.
      → Continuity to ground: short in the wire between the A/F sensor relay and the sensor. Repair and replace the fuse. Go to step 9.
      → No continuity: the under-dash fuse/relay box itself is faulty. Replace it.
3. Test the A/F sensor relay. Remove the relay from the under-dash fuse box. Bench-test it with 12 V on the coil terminals and a continuity check across the contacts, OR swap with a known-good relay of the same spec.
   → Relay good: go to step 4.
   → Relay bad: replace the relay. Go to step 9.
4. Measure A/F sensor heater resistance. Disconnect the A/F sensor (Sensor 1) 4P connector. At the sensor side, measure resistance between pins 3 and 4. Honda spec: 1.9 to 2.7 Ω at room temperature.
   → In spec: heater element is good, go to step 5.
   → Out of spec: replace the A/F sensor. Go to step 9.
5. Check for internal sensor short. Still at the sensor side, check continuity between pin 2 (signal) and pin 3 individually, then between pin 2 and pin 4 individually.
   → Any continuity on either pair: the sensor has an internal short between the signal and heater circuits. Replace the A/F sensor. Go to step 9.
   → No continuity on either pair: sensor is electrically isolated correctly. Go to step 6.
6. Check heater control wire to PCM. Jump the SCS line with HDS (mandatory before PCM probing). Disconnect PCM connector C (44P). Check continuity between sensor 4P connector pin 3 and PCM terminal C9.
   → Continuity: wire is good, go to step 7.
   → No continuity: open in the C9-to-sensor-pin-3 wire. Repair and retest.
7. Check heater relay feed wire. With the A/F sensor relay removed from the fuse box, jumper sensor 4P pin 4 to body ground with a wire. Check continuity between A/F sensor relay 4P connector pin 1 and body ground.
   → Continuity: wire between relay and sensor pin 4 is good, go to step 8.
   → No continuity: open between the relay and the sensor. Repair and retest.
8. Check relay ground-side control from PCM. Disconnect PCM connector A (44P). Jumper PCM terminal A21 to body ground. Check continuity between A/F sensor relay 4P pin 3 and body ground.
   → Continuity: relay-control wire is good. Proceed to PCM software update / substitution (step 10).
   → No continuity: open between PCM A21 and the relay. Repair and retest.
9. Clear, reset, and verify. Reconnect all connectors. Turn ignition ON. Reset the PCM with HDS. Perform the PCM idle learn procedure. Check for pending/confirmed DTCs, then monitor OBD STATUS for P0135.
   → PASSED: repair verified.
   → FAILED: recheck connector tension and move to step 10 (PCM path).
10. PCM software update, then substitute. Reconnect all connectors. Update the PCM to the latest Honda software. Start the engine and re-monitor OBD STATUS.
    → Clears after update: done.
    → Still failing: substitute a known-good PCM. If the substitute clears the fault, replace the original PCM.

Professional tip. On the Insight ZE2, the “No. 26 A/F SENSOR” fuse label is hybrid-platform nomenclature — non-hybrid Honda service information calls the equivalent fuse by a different name and position. Use the fuse-box diagram printed on the lid, not a generic “Honda A/F sensor fuse” search. Also worth checking: if you’re swapping in a used A/F sensor from another Honda for diagnosis, confirm the part number matches — heater resistance specs vary across Honda engine families, and a wrong-part sensor will set P0135 or P0137-family codes even when wired correctly.

Common Causes on the 2010 Insight ZE2

Ranked by real-world frequency on the LDA engine, mapped to the step that isolates each:

• Blown No. 26 A/F SENSOR (10 A) fuse — isolated by step 2. Often blown by a chafed wire between the relay and the sensor. Check the wiring before installing a new fuse, or it’ll pop again.
• Failed A/F sensor relay — isolated by step 3. Relays in under-dash positions see heat cycling and vibration; bench-test with a swap from a known-good identical relay.
• A/F sensor heater element open or shorted (resistance out of 1.9–2.7 Ω) — isolated by step 4. Sensor age, thermal shock, or contamination from coolant or oil leaks can damage the heater element.
• Internal A/F sensor short (signal pin 2 shorted to heater pins 3 or 4) — isolated by step 5. Replace the sensor.
• Open in the PCM C9-to-sensor-pin-3 control wire — isolated by step 6. Common failure at harness bulkhead grommets or where the harness passes near heat sources.
• Open in the relay-to-sensor pin-4 feed wire — isolated by step 7.
• Open in the PCM A21-to-relay ground-control wire — isolated by step 8.
• Under-dash fuse/relay box internal fault — isolated by step 2 (NO-continuity branch on a blown fuse). Rare but called out in Honda’s path.
• PCM software or driver fault — last resort. Apply the latest Honda PCM update before substituting a known-good module.

Severity & Driving

P0135 itself is not immediately dangerous — the engine will run, but closed-loop fueling is delayed after every cold start because the A/F sensor takes longer to reach operating temperature without working heater current. Expect noticeably worse cold-start emissions, occasional rough idle during warmup, and slightly reduced fuel economy until the sensor heats up via exhaust heat alone (which takes several minutes versus a few seconds with a working heater). Over time, the delayed closed-loop behavior can also affect how the PCM learns fuel trims, potentially setting P0171/P0172 as secondary codes. Repair within a reasonable timeframe but continued driving to a workshop is safe.

Last updated: April 19, 2026