PACCAR MX P0135 is the fault code page for “Lambda sensor heater – Current too low or open circuit on ECU D420 pin C46” in the 2013 PACCAR MX Diagnostic Service Manual. PACCAR publishes it for engines in the EPA 2013 emissions family, and it is worked with the DAVIE diagnostic software.
What this fault code means
Lambda sensor F834 measures oxygen concentration in the exhaust after the BPV valve, and the PCI ECU turns that into the air/fuel ratio of the combustion process, with typical values between 18 and 45. The reading controls the smoke limit, confirms target EGR for NOx control, and governs regeneration of the diesel particulate filter.
Set and reset conditions
The sensor carries an integrated heater the PCI ECU drives in two stages, first to evaporate condensate and then to about 750 C once the dew point is reached. The diagnostic starts five minutes after the engine starts and runs continuously.
PACCAR MX P0135 wiring and technical data
Six circuits run between ECU D420 and the sensor; the heater element is the one this code is about. The fault code information block records it as 1 trip MIL, 3 drive cycle recovery, readiness group none, freeze frame type Exhaust gas.
| D420 pin | F834 pin | Function |
|---|---|---|
| C23 | 6 | Signal, nernst sensor |
| C45 | 1 | Signal, pump cell current |
| C46 | 3 | Ground, heater element |
| – | 4 to 3 | 2.4 to 4.0 ohm heater at 20 C |
Possible causes and working notes
- Faulty wiring
- Faulty connector
- Faulty sensor
A cold sensor cannot report oxygen correctly, so a heater fault often shows up first as poor regeneration behaviour. Measure the heater element resistance before condemning the sensor.
Document details and format
| Field | Value |
|---|---|
| Brand | PACCAR |
| System group | DTCs, MX Engines (EPA2013) |
| Source publication | 2013 PACCAR MX Diagnostic Service Manual |
| Manual page | 156 |
| Pages | 5 |
| File size | 0.6 MB |
| Format | PDF, English |
Applies to PACCAR MX P0135 repairs
The page sits in the DTCs section for MX engines built to the EPA 2013 emissions standard, so it covers trucks running that engine family with the PCI engine control unit. Coverage and dealer support are published on the PACCAR corporate site.
Who needs this PACCAR MX P0135 page
Technicians working DPF regeneration or EGR complaints on a PACCAR MX engine who need the heater resistance and ECU pin.
The full PDF page is in the DiagTechPro manuals library; a free account downloads it. Keep the PACCAR MX P0135 sheet open next to DAVIE.
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